1. RaetsMarine Insurance B.V.
Draft Survey calculation sheet
Date
/
1a
Draft forward
1b
Draft aft mean
1c = 1a + 1b / 2
Draft fore & aft mean
2a
Draft midship PS
2b
Draft midship SB
2c = 2a + 2b / 2
Draft midship mean
3 = 1c + 2c / 2
Draft mean for means
4
Draft corr. for deformation
5 (at draft 4)
Displacement at sg 1.025
Trim bow / stern
Trim in cm
LCF
LBP
TPC (t/cm)
MTCT Diff
Density
6
Trim correction 1
7
Trim correction 2
8
Correction for Density
Consumable weights
HFO
GO
LUB
Fresh water
Ballast
Constant/Others
9
Total known weights
10 = 5 + 6 + 7 - 8 Corrected displacement
11
Light ship
12 = 10 – 9 – 11
Cargo weight total
Initial
Final
Cargo loaded/discharged =
Our contact details:
RaetsMarine
Contact: Jeroen Ebbeling/Robbert Beekman
Fascinatio Boulevard 622
2909 VA Capelle A/D IJssel, The Netherlands
Telephone: .. +31-10-24 25 000
Faxnumber: . +31-10-24 25 088
E-mail: j.ebbeling@raetsmarine.com
E-mail: r.beekman@raetsmarine.com
Draft Survey Calculation Sheet, 2010.1
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2. RaetsMarine Insurance B.V.
Draft surveys are made in order to determine the quantity of cargo loaded, carried and
discharged. This is done by measuring the vessels draft and calculating its
displacement prior to loading a cargo and after loading of this cargo, taking variables
such as weight lightship, ballast water, fuel and stores into account.
In order to obtain a reasonable accuracy in this draft survey calculation corrections to
the draft readings and it initial corresponding stability values and parameters must be
made.
General definitions;
Displacement of a vessel is the actual total weight of a vessel. It is expressed in metric tons, and is
calculated by multiplying the volume of the hull below the waterline (the volume of water it is displacing) by
the density of the water. The density will depend on whether the vessel is in fresh or salt water, or is in the
tropics, where the temperature of the water is warmer and hence less dense.
Deadweight of a vessel (often abbreviated as DWT for deadweight tons) is the displacement at any loaded
condition minus the lightship weight. It includes the crew, passengers, cargo, fuel, water, and stores. Like
displacement, it is often expressed in long tons or in metric tons.
Lightship measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board. This
weight will be mentioned in the stability booklets.
Gross tonnage or Gross Register Tonnage is the total internal volume of a vessel, with some exemptions
for non-productive spaces such as crew quarters. There are therefore different gross tonnage measurements
(Suez/Panama).
Tonnage measurements are now governed since 1994 by an IMO Convention (International Convention on
Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In
accordance with the Convention, the correct term to use now is GT, which is a function of the moulded
volume of all enclosed spaces of the ship.
Net Weight or Net Register Tonnage is the volume of cargo the vessel can carry; in other words its the
Gross Tonnage minus the volume of spaces that do not hold any cargo (e.g. engine room, bridge, crew
spaces, depending which country is making calculations (Suez/Panama)).
During a draft survey the vessels draft is measured. The draft is measured at 6 points, 2 x bow (P/S), 2 x
midships (P/S) and 2 x at the stern (P/S). The draft readings are averaged out in the mean draft. Via
calculations this draft reading leads us to the vessels present apparent displacement at the time of our
survey via on board, ship’s specific, hydrostatic tables and stability curves.
The water density at the time of the draft survey is also determined by means of a density meter. A very
common instrument for the direct measurement of the density of a liquid is the hydrometer, a floater. The
density of the water in which the vessel is surveyed directly influences the draft/dept of the vessel.
Considering that the hydrostatic tables and stability curves are pre-calculated for displacements of a vessel
in water with a standard density of 1.025 t/m³ (salt water), corrections to the determined draft must be made
in order to be able to find the correct displacement of the vessel. Make sure at the time of measuring the
density the density meter is free floating.
Displacement correction In order to find the correct displacement we will correct the displacement straight
away, instead of correcting the drafts at first. We will use the hydrostatic tables with the mean draft as entry
value. These calculations are called the trim corrections. Likewise we will also make a correction for water
density but again a tonnage correction and not a draft correction.
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3. RaetsMarine Insurance B.V.
First trim correction,
It is necessary to correct the fore & aft drafts to the true draft at the perpendiculars.
This can be achieved either by calculation or draft correction tables if available. The main calculations must
be accomplished with the drafts at the fore & aft perpendiculars and at the amidships point of the hull.
Note that draft marks are not always placed on the perpendiculars, so the true draft of the vessel does not
always correspond with the drafts found during the survey. The hydrostatic tables are mostly based on the
length between perpendiculars and not to the length between the draft marks on the hull.
1st Trim Correction
=
TRIM(cm) x TPC x LCF
-------------------------------LBP
Please note that the LCF is the distance between the midship point and the position of the LCF.
This correction is either added or subtracted to the displacement depending whether the LCF is forward or
aft of the mid point, or the trim is by the head or stern. Always draw a sketch to ensure you apply this trim
correction the correct way.
Second trim correction
This is a little more difficult to grasp. It is because there is in fact a second movement of the LCF caused
because of the irregular hull shape of the vessel in trimmed condition. The position of the LCF in the ship’s
hydrostatic tables are given for a ship on even keel only, but if the ship is trimmed the waterline has
changed, causing a second movement to the LCF. The second trim correction is always positive and fairly
small, for example with a loaded Panamax at a one metre trim the correction should only be approximately
12 tonnes, with a two metre trim approximately 50 tonnes.
This second trim correction is calculated by the following formula:
2nd Trim Correction:
Trim²(m) x ΔMTCT x 50
-------------------------------------------LBP
In this formula the ΔMTCT is the difference of MTCT values over a range of draft 50 cm above and 50 cm
below the corrected mean draft.
This second trim correction is always added to the displacement.
The second trim correction, sometimes called the Nemoto correction, is intended to correct for the movement
of the LCF with the change of trim. It was deduced by Mr Nemoto after observing a bulk carrier being built.
The theory on which the second trim correction is based only holds true for small changes of trim so it is not
strictly true for large trims. Please note that the second trim correction is rather small for small trims, and
rarely exceeding 30 tonnes for large vessels. It is often ignored but for completeness sake the second trim
correction should always be calculated.
Density correction If the vessel is in a dock water density then she is sitting differently in the water than she
would be if in sea water so we are reading a different displacement from the hydrostatic tables which are
derived for salt water only. The following formula is simply the dock water formula converted to a tonnage
figure, not a millimetre value.
(1.025 – dock density) x Displacement
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4. RaetsMarine Insurance B.V.
Density Correction:
--------------------------------------------------1.025
And of course this is subtracted from the displacement if the dock water density is below 1.025 t/m³
It is worth mentioning at this point that although we always consider the density of salt water to be 1.025 t/m³,
if you were to take a reading in Mediterranean Sea ports you would probably find it to be around the 1.030
t/m³ to 1.033 t/m³ mark!
Please note that the ship’s standard hydrometer is not an accurate instrument when it comes to using it to
determine tonnage for a draft survey. It makes no allowance for the temperature of the water which will
change the density as temperature changes.
Corrected displacement When the true displacement of the vessel is known it only requires the subtraction
of all the known weights on board and the subtraction of the ships light displacement. What remains is either
the vessels ‘constant’ if in an empty condition or the cargo weight if in a loaded condition.
The reason for two draft surveys is at first to find the vessels constant and then include this in the known
weights when calculating the actual cargo on board after loading.
When taking a draft survey upon discharge the cargo weight plus constant are found in the initial survey and
the constant in the final survey, subtract the constant from the initial figure and you have the weight of cargo
discharged.
TRIM(cm) x LCF x TPC
-------------------------------LBP
1st Trim Correction:
=
x
x
-----------------------------LBP
=
tonnes
2
Trim (m) x MTCT Diff x 50
----------------------------------LBP
2nd Trim Correction:
=
x
x 50
----------------------------------- =
LBP
tonnes (always add)
(1.025 – dock density) x Displacement
--------------------------------------------------1.025
Density Correction:
=
Draft Survey Calculation Sheet, 2010.1
x
-------------------1.025
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=
tonnes
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5. RaetsMarine Insurance B.V.
Many commercial ships have a symbol called a Load mark painted on each side of the ship. This symbol,
also called an International load line, or Plimsoll mark, marks the level to which the ship can be safely
loaded. As cargo is brought on board, the ship floats lower and the symbol descends farther into the water.
Before these symbols were made compulsory, many ships were lost due to overloading. Sometimes they
were deliberately overloaded in the hope of collecting insurance money. Ships carrying emigrants from
Europe to America were also lost. The British social reformer and politician Samuel Plimsoll advocated
improved safety standards, particularly at sea, and the mark bears his name in his honor.
The letters on the Load line marks have the following meanings:
•
TF - Tropical Fresh Water
•
F - Fresh Water
•
T - Tropical Seawater
•
S - Summer Seawater
•
W - Winter Seawater
•
WNA - Winter North Atlantic
Letters may also appear to the sides of the mark indicating the classification society that has surveyed the
vessel's load line. The initials used include AB for the American Bureau of Shipping, LR for Lloyd's Register,
and NV for Det Norske Veritas.
These season and zone marks are used to ensure adequate reserve buoyancy for the intended area of
operation. Ships encounter rougher conditions in winter as opposed to summer, and in the North Atlantic as
opposed to tropical waters, for example. A copy of a zone-chart must be on board in order to assure that the
vessel is not overloaded.
Fresh water marks make allowance for the fact that the ship will float deeper in fresh water.
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Draft Survey Calculation Sheet, 2010.1
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Draft Survey Calculation Sheet, 2010.1
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