Nmlc ef1 module 2

EXCELLENCE AND COMPETENCY TRAINING CENTER INC.
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NMLC-EF1-Module 1
Function 1:
Marine Engineering at the
Management Level
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COVERAGES:
Module 1 – Manage the operation of propulsion plant machinery
Module 2 – Plan and schedule operations
Module 3 – Operation surveillance, performance assessment and
maintaining safety of propulsion plant and auxiliary machinery
Module 4 – Manage fuel, lubrication and ballast operations
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OBJECTIVES:
Function 1: Marine Engineering at the Management Level
■ Upon successful completion of the training under
this Function, trainees shall be expected to have
gained the minimum knowledge, understanding
and proficiencies needed to carry out and
undertake at the management level the tasks,
duties and responsibilities in marine engineering
on ships powered by main propulsion machinery
of 3,000 kW propulsion power or more.
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Naval Architecture and Ship Construction Including
Damage Control
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❑ Center of Gravity (G): that point at which all the vertically
downward forces of weight of the vessel can be considered to
act; or it is the center of the mass of the vessel.
Basic Stability and Trim
G is the resultant of all vertical downward forces of gravity 5

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❑ Center of Buoyancy (B): that point at which all the vertically
upward forces support (buoyancy) can be considered to act;
or, it is the center of the volume of the immersed portion of
the vessel.
B resultant of all vertical upward forces of
buoyancy
B is the center of gravity of the
immersed portion of the vessel
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❑ Transverse Metacenter (MT): point through which the center of
buoyancy (B), acts vertically upwards as the vessel is inclined
and (B) shifts towards the low side.
The same displacement; the same
angle of inclination but G moves,
transverse metacenter (M) and the
equilibriums
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❑ Metacentric Height (GM): is the measure of the initial stability
of a vessel or it is the vertical distance between the ship’s
center of gravity and the initial transverse metacenter (M).
GM as a Function of GZ
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❑ Heel (roll) is caused by external forces to the ship.
- Ocean waves cause an external force on the ship that start
the ship rolling and keep it rolling.
Effects of Listing and Heeling
❑ List is caused by internal forces.
- may have been created by poor loading policies,
- by shift of cargo or ballast in heavy weather, or
- by unsymmetrical flooding after damage.
In any of these cases, the list will be attributed either to a negative
initial stability (a negative GM) or a condition in which G is off the
centerline. Either way, the ship will oscillate about this angle of list
instead of about the vertical.
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❑ It merely means that the vessel does
not have any initial stability, and that
she will incline to an angle where B
has moved far enough toward the
low side of the vessel to be once
more in the same vertical line as G.
Dangers of a Ship Having Negative GM
❑ If the center of gravity lies above the transverse metacenter
(G above M), the vessel is in state of unstable equilibrium, that
is, she possesses a negative GM.
❑ There is no tendency for the vessel to right herself at small
angles of inclination.
❑ An upsetting moment is formed, and the vessel will incline from
the erect position.
❑ A negative GM does not mean that
the vessel will capsize.
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❑ Thorough knowledge of the effects of free surface on
transverse stability since an excessive amount of free surface
can easily change a vessel with a positive GM into one with a
negative GM.
Effects of Amount of Liquid in Tank
Free Surface
❑ Whenever the surface of either a liquid or a movable dry bulk
cargo within a vessel is free to move, a condition known as
free surface is present.
❑ It is even possible to cause capsizing, especially in damaged
condition.
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Effects of Amount of Liquid in Tank (Continuation)
Effect of Surface Dimensions
❑ When a vessel rolls in a seaway the
liquid in the tank moves from side
to side, the center of gravity of the
liquid is, in effect, no longer in its
original position. It is somewhere
above the liquid.
❑ The phenomenon is known as a
virtual rise of the center of gravity.
❑ The liquid is “revolving”, for small
angles; in the arc of a circle
having m at its center. The
weight of the liquid is in effect
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❑ Filling up of double bottom
tanks will increase the GM.
Effects of Amount of Liquid in Tank (Continuation)
Effect of Surface Dimensions
❑ Depending on the amount of liquid in the tank, there is a
change in the center of gravity of the vessel. Free surface
effects causes, the G to rise, reducing metacentric height by
the distance of rise.
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❑ Individual tanks also reduce the effects of the free surface
on the stability of the ship.
Longitudinal and Transverse Bulkheads
❑ It divides the cargo carrying section of the vessel into a
number of tanks.
❑ Separation of different types of cargo.
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❑ The initial and final trims are then compared and change of
trim found according to the following rules;
- If the trims are both by the head or both by the astern,
subtract the lesser from the greater.
- If trims are different, that is one by the head and other
by the astern, add the two to produce change of trim.
Trim
❑ Difference between the drafts forward and aft.
❑ Change of trim is found by noting the trim of the vessel before
loading or discharging and the trim of the vessel after loading
and discharging.
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❑ It is usual to consider a ship displacing salt water of density
1.025 t/m3, however, fresh water values of displacement
(1.000 t/m3) are often quoted in ship’s hydrostatic data.
Law of Flotation
❑ States that every floating body displaces its own mass of the
liquid in which it floats.
❑ The displacement of a ship (or any floating object) is defined
as the number of tons of water it displaces.
Center of Flotation - The center of gravity of the water plane; the
point around which a vessel trims.
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❑ If the ship is pitching (in light condition) the propeller will tend
to race. This accompanied with increased vibration may cause
propeller shafts damage.
- Rudder efficiency will be intermittent as the ship pitches.
- Ballast suctions are sited at the aft end of tanks, a head
trim will make these impossible to empty completely.
Effects of Trim
A Trim by the Head Should be Avoided for the Following Reasons:
❑ Rudder will be immersed less making the ship difficult to steer.
❑ More water is likely to be shipped forward.
❑ Reduced propeller immersion will lessen propulsion efficiency.
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❑ Pitching may be excessive in heavy weather causing excessive
panting and pounding (this will be evident regardless of trim if
the forward draught is too small).
Effects of Trim
Excessive Trim by the Stern Should also be Avoided because:
❑ The large wind area forward and too deep immersion of the
stern will make the ship difficult to steer.
❑ A large blind area will exist forward, especially with an aft
bridge, hindering pilotage and reducing lookout effectiveness.
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❑ If a ship is trimmed by the stern, the soundings obtained will
indicate a greater depth of liquid than is actually contained in
the tank.
Effects of Trim on Tank Soundings
❑ It is desirable to find the head of liquid required in the
surrounding pipe which will indicate that the tank is full.
❑ A tank sounding pipe is usually situated at the after end of the
tank and will therefore only indicate the depth of the liquid at
that end of the tank.
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X t head when full trim
--- = --- or ----------------- = -----------------
l L Length of tank length of ship
Effects of Trim on Tank Soundings (Continuation)
In figure ‘t’ represents the trim of the ship, ‘L’ the length of the
ship, ‘l’ the length of the double bottom tank, and ‘x’ the head of
liquid when the tank is full. In triangles ABC and DEF, using the
property of similar triangles:
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Head when full = 1.5 x 12
X or AB ----------- = 0.18m
100
Depth of the tank = 1.5m
Sounding when full = 1.5m + 0.18m = 1.68m
Effects of Trim on Tank Soundings (Continuation)
Example:
A ship 100m long is trimmed 1.5m by the stern. A double bottom
tank 12m x 10m x 1.5m has the sounding pipe at the after end.
Find the sounding which will indicate that the tank is full.
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Dry-docking and Grounding
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❑ It is important to note the draught of the ship so as to
estimate the tide at which she should enter the dock.
Preparation for Dry-dock
❑ Docking of any ship depends on the ship's draught.
❑ The draughts of container ships are usually 5-7m and for
tankers about 3m.
U.S. Navy submarine USS
Greeneville in dry dock following
collision with a fishing boat.
Holland America Line‘s passenger cruise
ship MS Zaandam dry-docked at Grand
Bahama Shipyard, Freeport in January
2003.
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Sequence of Events when Dry-Docking (1)
❑ The ship enters the dry dock with a small trim by the astern
and is floated into position.
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❑ The gates are closed and water is pumped out of the dock until
the ship touches the blocks aft.
❑ Immediately the ship touches the blocks aft this denotes the
start of the critical period (it is now that the ship will start to
experience a loss of stability, hence the term).
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❑ As more water is pumped out of the dock the true mean
draught will start to reduce as the ship experiences more and
more support at the astern.
❑ The up-thrust afforded by the blocks at the stem is termed the
‘P force’, this continue to increase as buoyancy force reduces.
❑ Throughout the docking process the ship will displace a
progressively lessening volume of water as the true mean
draught reduces and the ‘P force’ increases to provide more
support for the ship.
❑ At this stage the aft draught will be reducing at a greater rate
than what the forward draught is increasing, the ship will be
trimming by the head as overall true mean draught reduces.
❑ Loss of stability will also be increasing as the P force increases.
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❑ Eventually the ship will come to rest on the blocks along its
entire length, the critical instant denotes the end of the critical
period, since for a flat bottomed ship the problem of stability
loss is no longer of concern.
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❑ After setting on the blocks forward and aft water continues to
be pumped from the dock and the draught reduces at the
same rate forward and aft.
❑ The up-thrust P becomes uniformly distributed along the ship’s
length and continues to increase as the effective buoyancy
force reduces.
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❑ When the dock becomes nearly empty and the ship is fully dry
the up-thrust will be equal to the ship’s displacement having
now replaced all the up-thrust afforded by the buoyancy force.
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❑ The maximum loss of GM of concern occurs instant
immediately prior to the ship setting on the blocks forward and
aft – this time being termed the critical instant.
Loss of Stability during Dry-docking
❑ It commences as soon as the ship touches the blocks and
continues to worsen as the value of the P force increases.
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❑ For ships that have a relatively small percentage of flat
bottoms area additional measures must also be taken such as
using side shores to support the ship in upright condition when
in the dry dock.
❑ Once the ship is flat on the blocks it will be in a safe condition
as the risk of heeling over as result of becoming unstable will
have passed (most ships having a substantial area of flat
bottom).
Loss of Stability during Dry-docking (Continuation)
❑ In the case when the ship is entering the dock in a damaged
state, the required draughts and trim may not be attainable.
This case would require the ship to be docked in a floating
dock.
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❑ If a vessel grounds on a level
or nearly level bottom, stability
is not a consideration, at least
not while the vessel is
grounded.
Effects of Grounding on Stability
❑ A ‘bilged’ ship is one that has suffered a breach of the hull
through grounding, collision or other means and water has
been admitted into the hull, whenever a ship suffers damage
and flooding of compartments takes place there will always be
an increase in the draught.
❑ It does not always follow that
the ship’s initial stability will
be worsened; in some
instances stability is improved.
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The reason: the upward force in tons on the ship’s bottom caused
by the grounding is equivalent to the removal of the same
number of tons from this area.
Effects of Grounding on Stability (Continuation)
❑ But if the vessel grounds on a pinnacle of any type and it are
free to heel or trim, stability may be affected considerably.
❑ If the grounding pressure becomes sufficiently great, the
ship’s center of gravity appears to rise above the metacenter,
causing it to list. The list may worsen, causing the ship to
capsize, either immediately or at a later time.
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❑ The vessel must be prepared before entering the dry dock.
Correct Condition for Dry-Docking
❑ A dry-dock list of new items is created with specification sheets
describing individual jobs.
❑ These sheets are compiled into a dry dock file which some time
before the due date of the docking is submitted to several dry
docks for pricing. The jobs are priced individually and as a
whole. This allows the ship managers to streamline the jobs
provided maximum value for money.
❑ Structural loading must be taken into account as the vessel is
to be point supported on blocks.
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❑ This allows the dry dock ship managers to place the blocks on
which the vessel will sit.
Correct Condition for Dry-Docking (Continuation)
❑ Special attention should be made when planning this for any
tanks whose contents may be varied due to repair or
housekeeping requirements.
❑ A docking plan of the ships which shows such things as drain
plugs, sea boxes, underwater attachments etc is sent to the
dry dock. Added to this are indications where full repairs are
required.
❑ The vessel must be trimmed so as to be equal draught with
zero lists.
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Damage Control
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❑ Cargo separation.
Purpose of Bulkheads
❑ It divides the ship into watertight compartments giving a
buoyancy reserve in the event of hull being breached.
❑ The number of compartments is governed by regulation and
type of vessel.
Types of Bulkheads: Watertight bulkheads, Non-watertight
bulkheads and Oil-tight or tank bulkheads.
❑ Longitudinal deck girders and deck longitudinal are supported
by transverse watertight bulkheads which act as pillars.
❑ Increased transverse strength, they act like ends of a box.
❑ They restrict the passage of flame.
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3. A bulkhead at each end of the machinery space; the after
bulkhead may, for an aft engine room, be the after peak
bulkhead.
Significant Factors of Subdivision
❑ Transverse watertight provides considerable structural strength
as support for the decks and to resist deformation caused by
broadside waves (racking).
❑ Spacing of watertight bulkheads or the watertight subdivision
of the ship is governed by rules, dependent upon ship type,
size, etc. all ships must have:
1. A collision or fore peak bulkhead which is positioned not less
than 0.05 X length of the ship, not more than 0.08 X length of
the ship from the forward end of the load waterline.
2. An after peak bulkhead which encloses the stern tube and
rudder trunk in watertight compartment.
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❑ The purpose of watertight subdivision and the spacing of the
bulkheads are to provide an arrangement such that if one
compartment is flooded between bulkheads the ship’s waterline
will not rise above the margin line.
Significant Factors of Subdivision (Continuation)
❑ The subdivision of passenger ships is regulated by statutory
requirements which are in excess of classification society rules
for cargo ships, but the objects of confining flooding and
avoiding sinking are the same.
❑ The margin line is a line drawn parallel to and 76 mm below
the upper surface of the bulkhead deck at ship’s side.
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❑ As a result the transverse stability of a vessel usually increases,
the trim of the vessel will change, and there may be a loss of
longitudinal hull strength.
Damage Condition
Damage Stability – is the stability of a vessel after flooding. The
unspecified term stability includes both transverse stability as
well as longitudinal stability.
❑ Overall characteristics of an intact vessel can be considerably
changed in the damaged condition.
❑ A modern merchant vessel will experience a gain in draft after
damaged has occurred.
❑ This is true because of the ship’s natural tendency to seek an
equilibrium condition.
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❑ Ice accumulation.
Equilibrium – the vessel is in a state where there is no movement:
The G (Center of Gravity) must be in the same
vertical line with B (Center of Buoyancy).
Damaged Condition may be Caused by any of the Following:
❑ Collision – high energy, moderate energy, or low energy.
❑ Grounding or stranding.
❑ Flooding due to: fire fighting operations, internal damage (i.e.,
a broken pipe or skin valve), and hull plating failure.
❑ Cargo shifting.
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❑ The loss or gain in transverse stability.
Damage of Compartments that may Cause Ship to Sink:
A vessel in a flooded condition may actually increase her initial
transverse stability, but this would be of no value if the vessel
foundered due to loss of reserve buoyancy or hull failure.
A study of the effects of damage due to a moderate energy collision
involves:
❑ The loss of longitudinal hull strength.
❑ The investigation of loss of reserve buoyancy.
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❑ The flooded compartment can be considered as a sieve offering
no buoyancy to the vessel and no free surface effects.
Effects of Flooding on Transverse Stability – two methods used:
Lost Buoyancy Method
Since buoyancy has been lost, it must be regained by an increase
in draft. The vessel will sink until the volume of the newly
immersed portions equal the volume of the flooded compartment.
With increase of draft the center of buoyancy will rise, increasing
KB (height of the center of buoyancy above the keel).
❑ Assuming flooded compartments has free communication with
the sea.
❑ Only those intact portions of the vessel on either side of the
flooded compartment are contributing to the buoyancy.
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Effects of Flooding on Transverse Stability
❑ In this method, the center of gravity of the vessel is assumed to
remain in its original position before flooding.
Lost Buoyancy Method (Continuation)
Permeability of flooded surface the percentage of the total surface
area of the flooded compartment which can be occupied by water.
Intact buoyancy is a term which is used to describe spaces within
the flooded compartment which exclude water. Thus, if a hold is
breached and flooded and the double bottoms under the hold are
still intact, there would be considerable intact buoyancy present.
❑ KM increase or decrease will, therefore, directly affect the
value of GM, or initial stability.
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Effects of Flooding on Transverse Stability (Continuation)
❑ The water which enters the vessel is considered as added
weight, thus affecting the position of the center of gravity.
Added Weight Method
❑ If the compartment does not have free communication with
the sea, i.e., if water has entered the vessel and the breach
has been repaired, or flooded due to fire fighting, the only
possible method of approaching the problem is through this
method.
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❑ There is an important difference between this and the effect of
a flooded wing compartment when the compartment is in free
communication with the sea.
Dangerous Effect of Flooded Wing Compartment
❑ In addition to the effects on stability, that when the vessel
rolls, the water in the compartment will flood in and out, thus
shifting the position of the ship’s center of gravity back and
forth approximately in the arc of a circle.
❑ The effect of flooding on stability has been confined to the
flooding of the centerline compartments.
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❑ The many shifting of G may be replaced with one virtual position
G’. The correction GG’ (virtual rise in G) is apt to represent a
serious loss of initial stability.
Dangerous Effect of Flooded Wing Compartment (Continuation)
❑ The flooding of wing compartments can take place in a variety
of complex situations:
- with the compartments above or below the waterline,
- with the compartments empty, filled, or slack, with a
small, moderate, or extreme angle list, and so on.
❑ If damage is severe and a list develops, water, of course, may
continue to flood until the compartment is filled, causing an
increase in the angle of list.
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❑ G might rise owing to free surface.
One thing must be stressed that free communication flooding of
any type in a wing compartment is very dangerous. Every effort
should be made to close the rupture in the hull as soon as possible.
GM might be affected in four ways:
❑ M might move owing to an increase in displacement.
❑ G might shift owing to addition of weight, with both vertical
and transverse shifts involved.
❑ G might rise owing to free communication.
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❑ As weight is added to a vessel, this volume decreases.
Reserve Buoyancy
❑ It is the volume of intact space remaining above the waterline.
❑ If any reserve buoyancy whatsoever is present the vessel will
float.
❑ When no reserve buoyancy remains, the vessel will
immediately sink.
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❑ When the hull of a vessel is opened up and one or more
compartments are flooded, it is possible, as seen, for the
vessel to lose its transverse stability and capsize if longitudinal
bulkhead is present, but in the case of contemporary ships
flooding one or more compartments could actually make a
vessel more stable.
Effects of Flooding on Reserve Buoyancy
❑ It is much more likely that the vessel will founder because of
loss of reserve buoyancy.
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❑ The ship should be checked for loss of longitudinal strength
members to help determine the loss of longitudinal hull strength.
• Damage near the ship’s neutral axis will allow great flooding but
cause minimal loss of hull strength.
• Damaged to the deck and bottom (extreme fibers of the ship’s
hull girder) will cause severe loss of hull strength.
Longitudinal Hull Strength and Damaged Condition
In addition to ascertaining if ship will capsize or founder due to loss
of transverse stability and reserve buoyancy respectively should
also consider that while the ship is indeed afloat and stable it could
break-up due to loss of longitudinal hull strength or excessive
bending moments due to grounding.
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❑ The best policy is prevention obtained by always arranging the
stowage so that the ship is operating with a minimum amount
of hull stress.
Longitudinal Hull Strength and Damaged Condition (Continuation)
❑ A ship can capsize in a matter of seconds once sufficient
transverse stability is lost. Once capsize occurs water will
enter through non-watertight openings and it can be expected
that it will eventually sink.
❑ If the ship sinks, due to progressive flooding it need not capsize.
The quickest a vessel could sink is due to a major hull failure
caused by excessive bending moments and shear stresses.
❑ In this case the vessel suffers severe changes in hydrostatic
properties by breaking in two. The forward half due to its finer
lines could immediately capsize while the after broader end
could sink by progressive flooding.
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❑ Damage control is based on the premise that the safety and life
of a ship depends on watertight integrity.
Emergency Action Following Hull Damage
❑ The procedures described are emergency measures taken by
the damage control team to maintain watertight integrity of
the ship in the event of accident, collision, or grounding.
❑ An emergency procedure, in the event the ship’s hull has been
punctured and watertight integrity has been lost.
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Emergency Action Following Hull Damage (Continuation)
Damage Control Program
❑ This team may consist of the chief officer, an engineer, bosun,
two or more seamen and engineman.
❑ No such thing as “little leak”, any size leak is a cause of alarm.
❑ Through damage control, this “leak” may be either stopped or
reduced to a point where the ship’s pumps can control any
excess water.
Damage Control Team
❑ Along with other emergency duties (fire and lifeboat), certain
crew members are also assigned.
❑ There should be sufficient skills among the team members to
perform the tasks required.
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Purpose of Damage Control Team
❑ Assist in maintaining the watertight integrity of the ship.
❑ Damage control also consists of either shoring up decks that
are weakened or strengthening bulkheads between flooded
compartments.
❑ When plugging leaks, the ultimate aim is to stop the leak
permanently.
Shoring
Involves two phases:
❑ Stopping or reducing the inflow of water.
❑ Bracing or shoring up the damaged or weakened members of
the ship’s structure by transferring and spreading the pressures
to other portions of the ship.
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Damage Control Procedures
❑ Furnish fire protection and extinguish fires.
Procedures that helps to reduce the harmful effect of impairment to
the ship:
❑ Preserve the watertight integrity of the ship.
❑ Maintain the stability and maneuverability of the ship.
❑ Make rapid repairs to damage gear on structures.
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Ship’s Systems and Equipment Included in Preparation for
Emergencies:
❑ Drainage and Flooding System
❑ Fire Main and Sprinkling System
❑ Ventilation System
❑ Fuel and Fresh Water System
❑ Communication System
❑ Compressed Air System
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Ship Compartmentation - the ship is divided into compartments to:
Compartments are designated and identified by symbols that are
made up of letters and numbers. Symbols are stenciled on
bulkheads. Port compartments have even numbers while starboard
compartments carry odd numbers.
❑ Control flooding
❑ Restrict chemical agents and gases
❑ Segregate activities of personnel
❑ Provide underwater protection by means of tanks and voids
❑ Strengthen the structure of the ship
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Emergency Equipment/Actions to Control Flooding:
❑ Forming a bucket brigade (if other methods fail).
❑ Using Submersible Pumps
❑ Jettisoning Equipment or Cargo
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Holes in Hull above the Waterline
❑ Therefore, plug those holes at once. Give
high priority to holes near the waterline.
❑ That reduces stability; and because the water almost invariably
presents a large, free surface (it shifts with ship movement), it
becomes doubly dangerous.
❑ Holes in the hull or just above the waterline
may not appear to be very dangerous, but
they are.
❑ They destroy reserve buoyancy; and if your ship rolls in a
heavy sea or loses buoyancy, those holes become submerged
and admit water at a very dangerous level—above the center
of gravity.
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❑ Never paint these plugs because unpainted wood absorbs
water and grips better than painted wood.
Methods Used to Control Flooding
❑ Several readily available methods can be used to plug or patch
holes to control flooding.
❑ The simplest method of repairing a fairly small hole is to insert
some kind of plug.
❑ If the inflow of water can be reduced by as little as 50 percent,
flooding may be controllable with portable pumps.
❑ Each repair locker has a large assortment of conical, square-
ended and wedge-shaped wooden plugs.
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Methods Used to Control Flooding (Continuation)
❑ Ordinary galvanized buckets can be used in a variety of ways to
stop leaks; for example, you can push them into a hole to form
a metal plug and held in place by shores.
❑ Wrap plugs with lightweight cloth to help them grip better.
❑ Roll up pillows and mattresses and shove them into holes but
this action should be backed up with some type of patch or
shoring.
❑ Plate patches are commonly used types of patches. They are
made from tables; doors; deck plates; or any relatively strong,
flat material.
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Submersible Pump
❑ The whole assembly is submerged
in the fluid to be pumped.
❑ Pump which has a hermetically
sealed motor close-coupled to the
pump body.
❑ Advantage is that it can provide a
significant lifting force as it does
not rely on external air pressure to
lift the fluid.
This makes Electric Submersible Pumping (ESP) a form of "artificial
lift" (as opposed to natural flow) along with gas lift, beam pumping,
plunger lift and progressive cavity pump.
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Possible Repairs to Hull Damage:
❑ Pillows, mattresses, and blankets can be rolled up and shoved
into holes. They can be rolled around a wooden plug or a
timber to increase their size and to provide rigidity. Such plugs
cannot be relied upon, as they have a tendency to be torn out
of the holes by action of the sea.
❑ Patching used to cover larger holes with sections of improvised
or prefabricated material. This only describes the procedures
for applying a soft patch because in damage control, stopping
or controlling the inflow of water is the primary concern.
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❑ Hinged plate patch – this is a circular plate, cut in two, and so
hinged that can be folded and pushed through a hole from
inside the vessel. The plate should be fitted with a gasket, and
also a line for securing to the vessel. Using diving equipment,
this patch can be applied over a submerged hole. Used over
small holes, as it has no vertical support to hold it in place.
❑ Bucket patch – an ordinary galvanized bucket can be used in a
variety of ways to stop leaks. It can be pushed into a hole,
bottom first, to form a metal plug, or it can be stuffed with
rags and put over a hole. It can be held in place by shoring or
by using a hook bolt.
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❑ Hook bolt for securing a patch – is a long bolt having the head
end shaped so that the bolt can be hooked to plating through
which it has been inserted. The head end of the bolt is
inserted through a hole and the bolt rotated until it cannot be
pulled back through the hole. A pad or gasket, backed by a
plank or strong back, is then slid over the bolt and the patch
secured in place by taking up on the nut.
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Rudders, Resistance, Powering and Fuel
Consumption
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Types of Rudder:
❑ When more than 25% of the
rudder area is forward of the
turning axis there is no torque
on the rudder stock at certain
angles.
Balanced Rudder
❑ The object of balanced is to
achieve a reduction in torque
since the center of lateral
pressure is brought nearer the
turning axis.
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Types of Rudder:
Semi-Balanced Rudder
turning axis.
❑ A rudder with a small part of
its area forward of the
turning axis.
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Types of Rudder:
Unbalanced Rudder
turning axis.
❑ A rudder with all its area aft
of the turning axis.
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Construction
❑ A special lifting bar with eye plates is used to lift the rudder.
❑ A lifting hole is provided in the rudder to enable a vertical in-
line lift of the rudder when it is being fitted or removed.
❑ The upper face of the rudder is formed into a usually horizontal
flat palm which acts as the coupling point for the rudder stock.
❑ Modern rudders are of steel plate sides welded to an
internal webbed framework.
❑ Integral with the internal framework may be heavy
forgings which form gudgeons or bearing housings of the
rudder.
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Construction (Continuation)
❑ A drain hole is provided at the bottom of the rudder to check
for water entry when the ship is examined in dry dock.
❑ The internal surfaces are usually coated with bitumen or some
similar coating to protect the metal should the plating leak.
❑ On the unbalanced and semi-balanced rudders are eddy
plates installed at the forward edge.
❑ This is welded in place after the rudder is fitted to provide
a streamlined water flow into the rudder.
❑ Every rudder is air tested to a pressure equivalent to a
head of 2.54m above thru top of the rudder to ensure its
watertight integrity.
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Considerations which Govern the Size and Shape of a Rudder
❑ A rudder width between 20 and 40% of its area forward of the
stock is balanced since there will be some angle at which the
resultant moment on the stock due to the water force will be
zero.
❑ If the rudder has its entire area aft of the rudder stock then it
is unbalanced.
❑ The ratio of the depth to width of a rudder is known as aspect
ratio and is usually in the region of 2.
❑ The turning action of the rudder is largely dependent on its
area, which is usually of the order of one-sixtieth to one-
seventieth of the length X depth of the ship.
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Considerations which Govern the Size and Shape of a Rudder
❑ Most modern rudders are of the semi-balanced design.
❑ This means that a certain proportion of the water force
acting on the after part of the rudder is counter acted by
the force acting on the forward half of the rudder; hence,
the steering gear can be lighter and smaller.
❑ A rudder may lift due to the buoyancy effect; the amount
of lift is limited by the jumper bar fitted to the stern frame.
❑ The jumper/rudder clearance must be less than the
steering gear cross head clearance to prevent damage.
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Maximum Rudder Angle Limited to 35O
❑ There are various methods of preventing this from occurring
and they all involve feeding energy into the stream of fluid
adjacent to the rudder or aerofoil surface. This is called
boundary layer control.
❑ A normal rudder is effective up to angles of about 35O,
after which the flow over the rudder stalls in a manner
similar to that over an aero plane wing at high angles of
incidence.
❑ The major advantage of putting a rudder over to such a
high angle is that the flow from the main engines may be
deflected through a much larger angle than with a
conventional rudder, and static side thrusts of over 50
percent of the bollard pull have been measured.
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Maximum Rudder Angle Limited to 35O (Continuation)
❑ Another main advantage is that its effect is independent of
forward speed and works as effectively at zero as at full
speed.
❑ At slow speeds and confined areas 35O is less effective for
maneuvering purposes.
❑ At any stage, up to the designed maximum angle, the
rudder retains smooth flow across both the faces and this
creates positive pressure on one side and equally
important, negative pressure on the opposite side. This
gives its lateral lift.
❑ At any angle exceeding the maximum angle the water flow
across the rudder, particularly on low pressure side
becomes more turbulent and rudder becomes ineffective.
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6.4 Rudders
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❑ The significance of this is that for ram systems excessive wear
can lead to bending moments on the rams.
Rudder Inspection and Measurement
❑ Refers to the measurements taken generally during a docking
period to indicate excessive wear in the steering gear system
particularly the rudder carrier.
❑ The readings taken, are offered for recording by the
classification society.
❑ For rotary vane systems it can lead to vane edge loading.
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❑ The trammel is manufactured to suit these marks as the
carrier wears the upper pointer will fall below the centre
punch mark by an amount equal to the wear down.
Trammel
❑ 'L' shape bar construction.
❑ Distinct centre punch mark is placed onto the rudder stock and
onto a suitable location on the vessels structure, here given as
a girder which is typical.
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❑ A clearance is given (referred to as the jumping clearance).
Rudder Clearance
❑ Pads are welded to the hull and rudder.
❑ As the carrier wears this clearance will increase.
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Resistance, Powering and Fuel Consumption
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2. Resistance to steady forward motion.
a) Frictional resistance – friction between the water and hull
surfaces is dependent upon:
- Water density and viscosity
- Area of the hull in contact with water
- Speed of water relative to the ship
(normally square of the speed)
- Friction coefficient
Various Resistances that Affects the Speed
1. Hydrodynamics – this is the dynamic interactions of the hull
with water. Dynamic interactions govern resistance of the hull to
steady forward motion. The choice of propulsive power is
dependent upon the resistance.
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b) Residuary resistance
- Wave making resistance
- Eddy making resistance
Various Resistances that Affects the Speed (Continuation)
c) Wave making resistance – energy expended in creating the
wave system caused by the hull. Wave making resistance
increases rapidly as the ship’s speed. Requires more power
to overcome than is practicable to build into ship.
3. Ship-generated wave – a significant feature of waves generated
by the passage of a ship is that they travel at the same speed
as the ship and their speed is proportional to the square root of
their length. Ship-generated waves originate at different parts
of the hull (bow wave, stern wave, shoulder waves, etc.).
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❑ This is the main reason for the dramatic increase in the total
resistance as the speed increases.
Various Resistances that Affects the Fuel Consumption
Flow of water – transverse wave system travels at appropriately
the same speed as the ship, since the ship producing the wave.
❑ At slow speeds, the waves are short and several crests are
seen along the ship’s length.
❑ As the ship speeds up, the length of the transverse waves
increases the wave making resistance increases very rapidly.
❑ The ship in effect must power itself through this wave, at this
point, the energy expenditure increases more rapidly than the
increase in speed.
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❑ Wave making increases so rapidly as ship speed increases that
it eventually requires more power to overcome than is
practicable to build into ship.
Various Resistances that Affects the Fuel Consumption (Continuation)
❑ If hull speed is to be exceeded, then the power will have to be
increased which becomes uneconomical as the fuel
consumption also increases at an alarmingly high rate.
❑ Even a trivial increase in speed, beyond 1.3 X hull speed
requires a virtually infinite increase in power to fulfill the energy
demand of the wave system.
Residuary Resistance
Hull Speed
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Determination of Propulsion Power - SHIP DRIVE TRAIN
Engine Reduction
Gear
Bearing Seals
Screw
Strut
BHP SHP
DHP
THP
EHP
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Determination of Propulsion Power - SHIP DRIVE TRAIN
❑ Effective Horsepower (EHP) – horsepower required to move
the ship to a given speed in the absence of propeller action.
❑ Brake Horsepower (BHP) – power output at the shaft coming
out of the engine before the reduction gears. Most cases Shaft
Horsepower (SHP) is used instead.
❑ Shaft Horsepower (SHP) – power output after any reduction
gears. They are necessary to convert the high rpm of the
engine to slower rpm required for screw propeller operation.
SHP being always smaller value than BHP.
❑ Thrust Horsepower (THP) – power actually produced in the
propeller in its normal environment.
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❑ The effect of all the separate efficiencies down the ship drive
train, are often merge into one called the propulsive coefficient
(PC). It is the ratio of EHP to SHP.
Propulsive Coefficient (PC)
❑ Link between THP and EHP is the hull efficiency (CH), it is now
possible to establish the BHP requirement for a ship from the
magnitude of EHP obtained from the power curve.
❑ A well-designed propeller and drive train would produce a
propulsive coefficient of about 0.6.
❑ Provided the power curve and the propulsive coefficient for a
ship are known, it is possible for the prime mover to size at an
early stage in the ship design process.
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Power Curve
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❑ Also when the engine is not running in the optimum operating
point this value will be higher.
Estimation of Fuel Consumption (Continuation)
❑ Generator engines on board ships may have their optimum at
70% load as these engines are probably averaging this load in
operation.
❑ In practice the fuel consumption will be higher because of the
more unfavorable ambient conditions, lower heat value of the
fuel and wear of engine components.
❑ Main engines will have the optimum designed maximum
efficiency and so the minimum specific fuel consumption at full
load or close to this.
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❑ The consumed fuel can be measured in the following ways:
• In case a day tank is used, the time for the consumption of
the whole tank content will be suitable.
• If flow meter is used a minimum of 1 hour is recommended.
Estimation of Fuel Consumption Guidelines
❑ Perform the measurements under calm weather conditions.
❑ Calculation of the specific fuel oil consumption, the engine
output and consumed fuel oil amount are known for a certain
period of time. The output calculated from indicator diagrams.
❑ The oil amount should be measured during a suitable long
period to achieve a reasonable measuring accuracy.
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❑ On the bunker sheet the density will be given at 15 °C/60 °F.
Estimation of Fuel Consumption Guidelines (Continuation)
❑ Necessary to know the oil density to convert it to weight units.
❑ The density is to correspond to the temperature at the
measuring point i.e. in the day tank or at the flow meter.
❑ The specific gravity can be determined by means of a
hydrometer immersed in a sample taken at the measuring
point, but the density can also be calculated on the basis of
bunker specifications.
❑ The consumed oil quantity in kg is obtained by multiplying the
measured volume by the density.
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❑ Corrected fuel consumption can be calculated by multiplying
the measured consumption by (LCV of fuel used/42,707).
Estimation of Fuel Consumption Guidelines (Continuation)
❑ The ambient conditions (blower inlet temperature and pressure
and scavenge air coolant temperature) will also influence the
fuel consumption.
❑ To compare the specific fuel consumption for various types of
fuel, allowances must be made for the differences in the lower
calorific value of the fuel concerned.
❑ The specific fuel consumption given in the test report is based
on a LCV of 42,707 acc. to ISO.
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Propulsion and Propellers
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❑ When rotated it ‘screws’ or thrusts its
way through the water by giving
momentum of water passing through it.
Delivered Horsepower (DHP)
❑ Power output after passage of the shaft through bearings,
seals and struts if any, where losses and lesser horsepower is
delivered to the propeller.
Principle of Propeller
❑ Consists of a boss with several blades
of helicoidal form attached to it.
❑ The thrust is transmitted along the shafting to the thrust block
and finally to the ship’s structure.
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❑ A propeller which turns clockwise when viewed from aft is
considered right-handed and most single-screw ships have
right-handed propellers.
Principle of Propeller
❑ A solid fixed-pitch propeller or usually described as fixed, the
pitch does vary with increasing radius from the boss.
❑ The pitch at any point is fixed, however, and for calculation
purposes a mean or average value is used.
❑ A twin-screw ship will
usually have a right-handed
starboard propeller and a
left-handed port propeller.
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❑ The ratio of effective horsepower to thrust horsepower is called
the hull efficiency (ῃH), and is defined as:
EHP
ῃH = -------------
THP
Hull Efficiency
❑ Once the ship’s effective horsepower has been determined, it
is now necessary to relate EHP to the power produced by the
drive train.
❑ This is done by relating the power required to tow the ship
through the water (EHP) to the power produced by the
propeller (THP).
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Thrust force X ship speed
Propeller Efficiency = ------------------------------
Shaft power
Propeller Efficiency
❑ The engine shaft power is transmitted to the propeller with
only minor transmission losses.
❑ The operation of the propeller results in a forward thrust on
the thrust block and the propulsion of the ship at some
particular speed.
❑ The propeller efficiency is a measure of effectiveness of the
power conversion by the propeller.
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❑ The power conversion by the propeller is a result of its rotating
action and the geometry of the blades.
Propeller Efficiency (Continuation)
❑ The slip will vary at various points along the blade out to its tip
but an average value is used in calculations.
❑ This is the distance that a blade would move forward in one
revolution if it did not slip with respect to the water.
❑ The principal geometrical feature is the pitch.
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❑ The slip of the propeller is measured as a ratio or percentage
as follows:
Theoretical speed/distance moved – actual speed/distance moved
Propeller Slip = --------------------------------------------------------------------------------
Theoretical speed or distance moved
Propeller Efficiency (Continuation)
❑ The theoretical speed is a product of pitch and the number of
revolutions turned in a unit time.
❑ The actual speed is the ship speed.
❑ It is possible to have a negative value of slip if, for example, a
strong current or wind were assisting the ship’s forward motion.
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❑ A propeller blade works in the same manner as an aircraft wing.
Propeller Action
❑ Water velocity over the suction back of the blade is greater
than the velocity across the high-pressure face of the blade.
❑ A propeller blade, has a shape similar to an aircraft wing.
❑ Water flow over the propeller blade creates a pressure
differential across the blade which creates a lifting or thrust
force that propels the ship through the water.
❑ Using Bernoulli’s equation, this velocity differential across the
blade results in a pressure differential across the blade.
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❑ The resultant lifting force can be resolved into thrust and
resistance vectors.
Forces acting on a propeller blade
Propeller Action (Continuation)
❑ Is the resistance vectors that pushes the ship through the water.
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❑ Blade tip – furthest point
on the blade from the hub.
Propeller Parts
❑ Propeller radius (R) – distance from the propeller axis to the
blade tip.
❑ Hub – connection between the blades and the propeller shaft.
❑ Blade root – point where
the blade joins the hub.
❑ Tip circle – described by
the blade tips as the
propeller rotates.
❑ Propeller disc – area
described by the tip circle
(propeller area).
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❑ Pressure face – high pressure side of the propeller blade. Astern
side of the blade when moving the ship forward.
Propeller Parts (Continuation)
❑ Leading edge – first portion of the blade to encounter the water.
❑ Trailing edge – last portion of the blade to encounter the water.
❑ Suction back – low pressure side of the blade. Where pressure
difference developed across the blade occurs on the LP side.
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❑ It is the formation and subsequent
collapse of vapor bubbles in regions
on propeller blades where pressure
has fallen below the vapor pressure
of water.
Cavitations
❑ Tip – blade tip cavitations is the most common form of
cavitations. It forms because the blades are moving the fastest
and therefore experience the greatest dynamic pressure.
❑ Cavitations occur on propellers that
are heavily loaded, or experiencing a
high thrust loading coefficient.
Types of Cavitations
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Types of Cavitations (Continuation)
❑ Sheet – cavitations refers to a large and stable region of
cavitations on a propeller, not necessarily covering the entire
face of a blade. The suction face of the propeller is susceptible
to sheet cavitations because of the low pressure there.
Additionally, if the angle of attack of the blade is set
incorrectly (on a controllable pitch propeller, for instance) it is
possible to cause sheet cavitations on the pressure face.
❑ Spot – cavitations occurs at sites on the blade where there
is a scratch or some other surface imperfection.
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Types of Cavitations (Continuation)
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❑ Erosion of the propeller blades. As cavitations bubbles form
and collapse on the tip and face of the propeller blade,
pressure wave formed causes a small amount of metal to be
eroded away. Excessive cavitations can erode baled tips and
cause other imperfections on the blade’s surface.
Consequences of Cavitations
❑ Reduction in the thrust produced by the propeller.
❑ Increase in ship’s radiated noise signature.
❑ Vibration in the propeller shafting.
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❑ Speed – every ship has a cavitations inception speed where tip
cavitations begin to form.
Preventing Cavitations
❑ Fouling – propeller must be kept un-fouled by marine
organisms and free of nick and scratches. It causes reduction
in propeller efficiency.
❑ Thrust – speed must not be increased too quickly.
❑ Pitch – operators of ships with controllable pitch propellers
must take care that propeller pitch is increased or
decreased in a smooth manner.
❑ Depth – since cavitations is a function of hydrostatic
pressure, increasing hydrostatic pressure (i.e. depth) will
reduce the likelihood of cavitations.
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❑ Singing is produced if the vortices develop into a well behaved
system (e.g., a continuous steady "train" of eddies) and the
frequency of this "train" is in the audible range.
Singing Propellers
❑ Some propellers produce an audible high-pitched tone which
has come to be known as "singing".
❑ The most frequent cure for a singing propeller is the popular
"anti-singing edge". This is a chamfer applied to the trailing-
edge to promote separation of the vortices.
❑ Most opinion is that the tone is produced by alternating
vortices which roll off of the trailing edge of the blade.
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❑ Contract terms usually require the speed to be achieved under
specified conditions of draft and deadweight, a requirement met
by runs made over a measured course.
Ship Speed Trial
- adequate depth of water;
- freedom from sea traffic;
- sheltered, rather than exposed, waters; and
- clear marking posts to show the distance.
❑ It is usual to conduct a series of progressive speed trials, when
the vessel's performance over a range of speeds is measured.
❑ Formal speed trials, necessary to fulfill contract terms, are
often preceded by a builder's trial.
The essential requirements for a satisfactory measured course are:
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❑ The ship should be run on trial in the fully loaded condition; but
this is difficult to achieve with most dry-cargo ships.
Ship Speed Trial (Continuation)
❑ Whenever possible, good weather conditions are sought.
❑ Large vessels with a low displacement–power ratio must cover
a considerable distance before steady speed can be attained;
hence they need to make a long run before entering upon the
measured distance.
❑ With a hull recently docked, cleaned, and painted, sea-trial
performance can provide a valuable yardstick for assessing
performance in service.
❑ It is, however, comparatively simple to arrange in oil tankers,
by filling the cargo tanks with seawater.
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❑ The average speed obtained using the maximum power over a
measured course in calm weather with a clean hull and specified
load condition.
Ship Speed Trial (Continuation)
❑ The average speed at sea with normal service power and
loading under average weather conditions.
Service Speed
Trial Speed
❑ This speed may be a knot or so more than the service speed.
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❑ The effective stroke of the fuel pumps are properly adjusted
according the values in the setting table.
Ship Power Calculation
❑ The mean effective pressure developed under service condition
is approximately the same as the mean effective pressure
established on the test bed at the same load indicator position
(fuel pump index).
❑ The engine is in good condition and properly supplied with air
(i.e. turbochargers are in good order and the air and exhaust
lines have low additional resistance).
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Pe = shaft power in kW per cylinder;
P/engine = Pe/cyl x number of cylinders
Constant factor = c = ¼ x π x d2 x s x 1/60 x 100
(for a 2-stroke engine)
Ship Power Calculation (Continuation)
The output for a specific engine is:
Pe/cyl = constant factor x pe x n (kW)
D = cylinder bore in meters
S = stroke in meters
N = revolutions per minute
The MEP can now be obtained from test bed protocols, the constant
factor be calculated and the rpm read from the tachometer.
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❑ In practice the fuel consumption will be higher because of the
more unfavorable ambient conditions, lower heat value of the
fuel and wear of engine components.
Ship Fuel Consumption
❑ Generator engines on board ships may have their optimum at
70% load as these engines are probably averaging this load in
operation.
❑ Main engines will have the optimum designed maximum
efficiency and so the minimum specific fuel consumption at full
load or close to this.
❑ Also when the engine is not running in the optimum operating
point this value will be higher.
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❑ In the performance curve for a certain engine the load at which
the engine has the lowest fuel consumption can be found.
Ship Fuel Consumption (Continuation)
❑ The optimizing point is the rating at which the turbocharger is
matched and at which the engine timing and compression
ratio are adjusted.
❑ As can be seen in the figure the fuel consumption in kg/s
increases with the load but in this example the design has
been made in such a way that the overall efficiency is highest
at 100% load and so the spec fuel consumption is lowest at
that load.
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Propulsion and Propellers
% BRAKE POWER
0 50% 100%
Spec. Fuel Cons.: g/kWh
Brake Thermal Efficiency

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Hull Dimensions

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❖ Definition of Terms:
▪ Forward Perpendicular (FP) – perpendicular drawn to the waterline at
the point where the foreside of the stem meets the summer load
line.
▪ After Perpendicular (AP) – perpendicular drawn to the waterline at
the point where the aft side of the rudder posts meets the summer
load line.
▪ Length Between Perpendicular (LBP) – length between the forward
and aft perpendicular measured along the summer load line.
▪ Length Overall (LOA) – length of vessel taken over all extremities.
▪ Amidships – a point midway between the after and forward
perpendiculars.
▪ Breadth – measured at amidships, this is the maximum breadth to
the moulded line.

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▪ Depth (D) – vertical distances between the base line and the
uppermost watertight deck and is the sum of freeboard and draft.
▪ Draught – vertical distance from the waterline to that point of the
hull which is the deepest in the water.
▪ Sheer – curvature of decks in the longitudinal direction, measured as
the height of deck at side at any point above the height of deck at
side amidships.
▪ Flare – the outward curvature of the side shell above the waterline
promotes dryness and is therefore associated with the fore end of
the ship.
▪ Camber (round of beam) – curvature of decks in the transverse
direction, measured as the height of deck at center above the
height
of deck at side.

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▪ Rise of Floor (Deadrise) – the rise of the bottom shell plating line
above the base line, measured at the line of moulded beam.
▪ Trim – difference between drafts forward and aft.
▪ List – definite attitude of transverse inclination of a static nature.
▪ Heel – temporary inclination generally involving motion.
▪ Water planes – the plane defined by the intersection of the water
in which a vessel is floating with the vessel’s sides.
▪ Freeboard (F) – vertical distance measured at the ship’s side
between the summer load line (or service draft) and the freeboard
deck.
▪ Load Line (Plimsoll Mark) – lines painted along the ship’s side.
▪ Center of Flotation (F) – geometric center of the waterline plane,
point which the ship inclines or trims in the fore-and-aft
direction.

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▪ Center of Buoyancy (B) – geometric center of the submerged hull,
when ship is at rest, with or without a list, the center of buoyancy
is usually directly below the center of gravity.
▪ Center of Gravity (G) – weight of the ship distributed throughout
the ship, considered to act through a single point.
▪ Tons per centimeter – the mass which must be added to, or
deducted from a ship to change its mean draught by 1 cm.
▪ Lightweight or Mass - measures the actual weight of the ship with
no fuel, passengers, cargo, water, etc. on board.
▪ Load Displacement (W) – weight of the water of the displaced
volume of the ship; for static equilibrium, it is the same as the
weight of the ship and all cargo on board.

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▪ Deadweight - is the displacement at any loaded condition minus
the lightship weight. It includes the crew, passengers, cargo, fuel,
water, and stores.
▪ Deadweight Scale (table) - simply cross-references three factors:
cargo, draft, and displacement. An empty ship has light
displacement. In terms of displacement, the difference between
an empty ship and a full ship is weight of cargo. Weight of cargo
is called deadweight tonnage, which is why the table mentioned
above is called a deadweight table.
1. Scantlings - the measurements of the various frame-work parts of
the vessels structure, as frames, beams, floors, stringers, plating,
etc.

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▪ Coefficients of form
• are dimensionless numbers that describe hull fineness and overall
shape characteristics.
• are ratios of areas or volumes for actual hull form compared to
prisms or rectangles defined by ship’s length, breadth, and draft.
• Since length and breadth on the waterline as well as draft vary with
displacement, coefficients of form also vary with displacement.
• Tabulated coefficients are usually based on the molded breadth
and draft at designed displacement.
• Length between perpendiculars is most often used, although some
designers prefer length on the waterline.

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▪ Coefficients of form
• Length between perpendiculars is most often used, although some
designers prefer length on the waterline.
• Coefficients of form can be used to simplify area and volume
calculations for stability or strength analyses.
• As hull form approaches that of a rectangular barge, the
coefficients approach their maximum value of 1.0.

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Load Lines – Freeboard Draught

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❖ Forces and Distortion
Static Forces
Are due to the differences in weight and buoyancy which
occur at various points along the length of the ship. This is known
as the still water bending moment (SWBM).
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Dynamic Forces
Result from the ship’s motion in the sea and the action of
the wind and waves.
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These static and dynamic forces create longitudinal,
transverse and local stresses in the ship’s structure.

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Longitudinal Stresses
Longitudinal stresses are greatest in magnitude and result in
bending of the ship along its length.
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Hogging – ship where the buoyancy amidships exceeds the weight.

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Sagging – ship where the weight amidships exceeds the buoyancy.

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Transverse Stresses
A transverse section of the ship is subjected to static pressure from
the surrounding water in addition to the loading resulting from the
weight of the structure, cargo, etc. although transverse stresses are
lesser of magnitude than longitudinal stresses, considerable distortion
of the structure could occur, in the absence of adequate stiffening.
!
Localized Stresses
The movement of the ship in a seaway results in forces being
generated which are largely of a local nature. These factors are,
however, liable to cause the structure to vibrate and thus transmit
stresses to other parts of the structure.

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Stress Variations at Different Depths of the Structure
▪ If the ship is now considered to be moving among waves, the
distribution of weight will still be the same.
▪ The distribution of buoyancy will vary as a result of the waves.
▪ The movement of the ship will also introduce dynamic forces.
▪ The traditional approach is to convert the dynamic situation into
an equivalent static one.
▪ To do this, the ship is assumed to be balanced on a static wave
of trochoidal form and length equal to the ship.
▪ The profile of the wave at sea is considered to be trochoid. This
gives waves where the crests are sharper than the troughs.

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▪ The wave crest is considered initially at amidships and then at
the ends of the ship.
▪ The maximum hogging and sagging moments will thus occur in
the structure for the particular loaded condition.
▪ The total shear force and bending moment are thus obtained
and these will include the still water bending moment.
▪ If actual loading conditions for the ship are considered which will
make the above conditions worse, e.g. heavy loads amidships
when the wave through is amidships, then maximum bending
moments in normal operating service can be found.

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The Bending of a Ship Causes Stresses to be set up within its
Structure.
▪ When a ship sags, tensile stresses are set up in the bottom shell
plating and compressive stresses that are set up in the deck.
▪ When the ship hogs, tensile stresses occur in the decks and
compressive stresses in the bottom shell.
!
This stressing, whether compressive or tensile, reduces in
magnitude towards a position known as the neutral axis. The
neutral axis in a ship is somewhere below half the depth and is, in
effect, a horizontal line drawn through the center of gravity of the
ship’s section.!!Module 3

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Cause of Deflection of the Beam
➢ The external loading of the beam is the difference between the
upward support of the beam and the downward weight of the
beam with any concentrated weight that may be present.
➢ This causes external vertical shearing stresses on the beam,
which result in the beam having a tendency to bend.
➢ A measure of this tendency is known as bending moment.
➢ Where there is maximum bending moment there will be a
potential for maximum deflection or bending of the beam.

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➢ A force acting through a distance is called a moment (bending
moment).
➢ The reason for constructing structural shapes in the form of “I”
beams is to place as much of the material in the beam as
possible at the upper and lower surfaces or flanges where it will
do the most good in resisting bending.
➢ However, the flanges must be connected by an adequate web.
➢ The tendency of a loaded beam to bend, is a function of the
amount of weight and the distance of the weight (its center of
gravity) from the ends or from the intermediate supports.

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➢ While the external loading of the beam results in bending or a
tendency to bend, and thus internal formation of tension,
compression, and shearing stresses, the tendency to bend also
varies with:
a) The distribution of the weight on the beam, whether uniform or
concentrated and uniform distribution create less bending
tendency.
b) The fixity of the end connections. A beam may merely rest on a
support at its ends (simply supported), or it may be fixed rigidly
in place (fixed-end supported) by the use of brackets. Fixed-
ended supports create less bending tendency.

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Maintain the Integrity of Principal Strength Members
❑ Bottom structure – provides increased safety in the event of
bottom shell damage, and also provides liquid tank space low
down in the ship. Smaller vessels have single bottom
construction and larger vessels other than tankers are fitted with
some form of double bottom.
❑ Shell plating – forms the watertight skin of the ship and at the
same time, contributes to the longitudinal strength and resists
vertical shear forces. Internal strengthening of the shell plating
may be both transverse and longitudinal and is designed to
prevent collapse of the plating under the various loads to
which it is subject.

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Maintain the Integrity of Principal Strength Members
❑ Bulkheads and pillars – it is responsible for carrying the vertical
loading experienced by the ship. The principal bulkheads
subdivide the ship hull into a number of large watertight
compartments. Bulkheads which are of greatest importance are
the main hull transverse and longitudinal bulkheads dividing the
ship into watertight compartments.
❑ Deep tanks – fitted adjacent to the machinery spaces amidships
to provide ballast capacity, improving the draft with little trim,
when the ship is light.

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Structural Deformations Caused by the Following:
❑ Slamming or pounding – in heavy weather, when the ship is
heaving and pitching, the forward end leaves and re-enters the
water with slamming effect. This slamming down of the forward
region on to the water is known as pounding.

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❑ Panting – the movement of waves along a ship causes
fluctuations in water pressure on the plating. This tends to
create an in-and-out movement of the shell plating. The effect is
particularly evident at the bows as the ship pushes its way
through the water.
❑ Racking – when a ship is rolling it is accelerated and
decelerated, resulting in forces in the structure tending to
distort it. Its greatest effect is felt when the ship is in the light
or ballast condition.

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❖ Materials
Mild steel
Is the principal material used in ship construction with 0.15– 0.23%
carbon content.
Properties required of good shipbuilding steel are:
✓ Reasonable cost,
✓ Easily welded with simple techniques and equipment,
✓ Ductility and homogeneity,
✓ Yield point to be a high proportion of ultimate tensile strength,
✓ Chemical composition suitable for flame cutting without
hardening, and
✓ Resistance to corrosion.
These features are provided by the five grades of mild steel designated by
the classification societies.

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❖ Materials
Higher Tensile Steels
➢ Steels having higher strength than that of mild steel are employed
in the more highly stressed regions of large tankers, container ships
and bulk carriers.
➢ Use of higher strength steels allows reductions in thickness of deck,
bottom shell, and framing where fitted in the amidships portion of
larger vessels; it does, however, lead to larger deflections.
➢ The weldability of higher tensile steels is an important consideration
in their application in ship structures and the question of reduced
fatigue life with these steels has been suggested.
➢ Also, the effect of corrosion with lesser thickness of plate and
section may require vigilant inspection.

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❖ Materials
Class Approval Procedures and Identification/Designation on Ship Steel
Steel for a ship classed with Lloyd’s Register is produced by an
approved manufacturer, inspection, and prescribed tests are carried
out at the steel mill before dispatch. All certified materials are marked
with the Society’s brand and other particulars as required by the rules.
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Ship classification societies originally had varying specifications for
steel, the major societies agreed to standardize their requirements in
order to produce the required grades of steel to a minimum.

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❖ Materials
Five different qualities of steel employed in merchant ship
construction:
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➢ Grade A being ordinary mild steel to Lloyd’s Requirements.
➢ Grade B better quality mild steel than grade A and specified
where thicker plates are required in the more critical regions.
➢ Grade C, D, E possess increasing notch-touch characteristics,
being to American Bureau of Shipping requirements

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❖ Materials
Steel Castings
!
➢ Molten steel produced by the open hearth, electric furnace, or
oxygen process is poured into a carefully constructed mould and
allowed to solidify to the shape required.
➢ After removal from the mould a heat treatment is required, for
example annealing, or normalizing and tempering to reduce
brittleness.

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❖ Materials
Steel Forgings
!
➢ Forging is simply a method of shaping a metal by heating it to a
temperature where it becomes more or less plastic and then
hammering or squeezing it to required form.
➢ Forgings are manufactured from killed steel made by the open
hearth, electric furnace, or oxygen process, the steel being in the
form of ingots cast in moulds.
➢ Stern frames, rudder frames, spectacle frames for bossing, and
other structural components may be produced as castings and or
forgings.

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❖ Materials
Principle of Cathodic Protection
!
➢ Only when metals are immersed in an electrolyte can the
possible onset of corrosion be prevented by cathodic protection.
➢ The fundamental principle of cathodic protection is that the
anodic corrosion reactions are suppressed by the application of
an opposing current.
➢ This superimposed direct electric current enters the metal at
every point lowering the potential of the anode metal of the local
corrosion cells so that they become cathodes.

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❖ Materials
Two Main Types of Cathodic Protection Installation:
a) Sacrificial Anode Systems – sacrificial anodes are metals or alloys
attached to the hull which have a more anodic, i.e. less noble,
potential than steel when immersed in sea water. Modern anodes
are based on alloys of zinc, aluminum, or magnesium which have
undergone many tests to examine their suitability.
!
Sacrificial anodes may be fitted within the hull, and often fitted in
ballast tanks. However, magnesium anodes are not used in the cargo
ballast tanks of oil carriers owing to the “spark hazard”. Aluminum
anode systems may be employed in tankers provided they are fitted in
locations where the potential energy is less than 28 kg.m.

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❖ Materials
❑ Impressed Current Systems – these systems are applicable to the
protection of the immersed external hull only.
▪ The principle of the systems is that a voltage difference is
maintained between the hull and fitted anodes, which will protect
the hull against corrosion, but not overprotect it thus wasting
current.
▪ For normal operating conditions the potential difference is
maintained by means of an externally mounted silver/silver
chloride reference cell detecting the voltage difference between
itself and the hull.

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❖ Materials
▪ An amplifier controller is used to amplify the micro-range
reference cell current, and it compares this with the preset
protective potential value which is to be maintained.
▪ Using the amplified DC signal from the controller a saturable
reactor controls a larger current from the ship’s electrical system
which is supplied to the hull anodes.
▪ An AC current from the electrical system would be rectified before
distribution to the anodes.

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❖ Materials

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❖ Ship Types and Ship Construction
❑ Passenger Ships
▪ Early passenger ships did not have the tiers of superstructures,
and had a narrower beam in relation to the length.
▪ The reason was the Merchant Shipping Act 1894 which limited the
number of passengers carried on the upper deck.
▪ An amendment in 1906 removed this restriction and vessels were
then built with several tiers of superstructures.
▪ This produced problems of strength and stability, stability being
improved by an increase in beam.
▪ The transmission of stresses to the superstructure from the main
hull girder created much difference in opinion as to the means of
overcoming the problem.

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❑ Passenger Ships
▪ Both light structures of a discontinuous nature, i.e. fitted with
expansion joints, and superstructures with heavier scantlings able
to contribute to the strength of the main hull girder were
introduced.
▪ Present practice, where the length of the superstructure is
appreciable and has its sides at the ship side, does not require
the fitting of expansion joints.
▪ Where aluminum alloy structures are fitted in modern ships it is
possible to accept greater deformation than would be possible
with steel and no similar problem exists.

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Passenger Ships
▪ The introduction of aluminum alloy superstructures has provided
increased passenger accommodation on the same draft, and/or a
lowering of the lightweight center of gravity with improved stability.
▪ This is brought about by the lighter weight of aluminum structure.

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❑ General Cargo Ships
▪ Called the “maid of all work” operating a worldwide ‘go anywhere’
service of cargo transportation.
▪ Consists of large clear open cargo-carrying space, together with
facilities required for loading and unloading the cargo.
▪ Access to the cargo storage areas or holds is provided by openings in
the deck called ‘hatches’.
▪ Hatches are made as large as strength considerations will allow to
reduce horizontal movement of cargo within the ship.
▪ Hatch covers of wood or steel, as in modern ships, are used to close
the hatch openings when the ship is at sea.

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▪ Hatch covers are made watertight and lie upon coamings around
the hatch which are set some distance from the upper or weather
deck to reduce the risk of flooding in heavy seas.
▪ One or more separate decks are fitted in the cargo holds and are
known as ‘tween decks’.
▪ Greater flexibility in loading and unloading, together with cargo
segregation and improved stability.
▪ Since full cargoes cannot be guaranteed with this type, ballast-
carrying tanks must be fitted.
▪ Ships will always have sufficient draught for stability and total
propeller immersion.

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▪ Fore and aft tanks assist in trimming the ship.
▪ A double bottom is fitted which extends the length of the ship and
is divided into separate tanks, some of which carry fuel oil and fresh
water.
▪ Remaining tanks are used for ballast and deep tanks may be fitted
which can carry liquid cargoes or ballast.

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Double Hull (Typical Mid-ship Section)

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Container Carriers
Box type girders are used extensively. These provide considerable
strength and rigidity and they allow for a large central open
space.

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Container Carriers

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❑ Roll-on-roll-off Ships
▪ They are characterized by the stern and in some cases the bow
or side doors giving access to a vehicle deck above the waterline
but below the upper deck.
▪ Access within the ship may be provided in the form of ramps or
lifts leading from this vehicle deck to upper decks or hold below.
▪ They may be fitted with various patent ramps for loading through
the shell doors when not trading to regular ports where link-span
and other shore side facilities which are designed to suit are
available.
▪ Cargo is carried in vehicles and trailers or in unitized form loaded by
fork lift and other trucks.

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❑ Roll-on-roll-off Ships
▪ In order to permit drive through vehicle deck a restriction is placed
on the height of the machinery space and the ro-ro ship was
among the first to popularize the geared medium speed diesel
engine with a lesser height than its slow speed counterpart.

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❑ Liquefied Gas Tanker
▪ Critical factors in the carriage of gas in liquid form are the
boiling temperature at atmospheric pressure and the critical
tempo (temperature above which the gas cannot be liquefied no
matter what the pressure).
▪ The type of containment vessel used for the cargo will differ
depending upon the desired tempo and pressure.
▪ The tempo must always be below the critical.
▪ In general, low pressures may be used if the tempo is kept low,
alternately higher temperatures may be used but higher
pressures are required (LNG).

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Liquefied Gas Tanker
▪ Tanks are in the form of pressure vessels, cylindrical or spherical.

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❑ Bulk Carriers
▪ The bulk carrier is designed for the carriage of dry cargo such as
grain, iron ore, etc.
▪ Upper ballast hoppers aid stability to prevent cargo shift and the
bottom hoppers aid in the collection of the cargo for discharge.
▪ Relatively low density cargoes such as grain and coal would be
carried in each hold.
▪ Heavy cargoes such as iron ore may be carried in alternate holds.
▪ The internal tank design for bulk carriers is a clean one.
▪ The floor is absent of framing allowing ease of cargo discharge
and cleaning.

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Bulk Carriers

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❑ Duct Keel Construction for Transversely Framed Hull

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❑ Longitudinally Framed Hull (Tanker) - The longitudinal framing
is much better able to resist buckling when the hull is hogging.

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❑ Longitudinal framing (Dry Cargo)

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