1.
NA-352
Marine Engineering
Lecture 6: Shafting and Propellers

2.
SHAFTING SYSTEM
Main Objectives:
• To transmit the power output from main engine to the propeller
• Support the propeller
• Transmit the thrust developed by the propeller to the ship’s hull
• Safely withstand transient operating loads (e.g. high speed maneuvers, quick reversals)
• Be free of harmful vibrations
• Provide reliable operation throughout the operating range

3.
TRANSMISSION SYSTEM No gearbox

4.
SHAFT BEARINGS
Main propulsion shafting is supported by bearings which maintain the shafting in proper alignment
Classification based on location:
1. Line Shaft Bearings: those bearings which are inside the watertight boundary of the hull
2. Outboard Bearings: those bearings which are outside the hull watertight boundary
Classification based on Load Type:
1. Radial Bearings
2. Thrust Bearings
REQUIREMENTS
• Operating range: from 0 to 100 or more rpm in either direction of rotation
• Reliability, as there is no redundancy for bearings and a single bearing failure may halt the propulsion system

5.
SHAFT BEARINGS

6.
SHAFT BEARINGS

7.
THRUST BLOCK

8.
STERNTUBE SEALS

9.
ENGINE-PROPELLER MATCHING
• The engine flange location and the propeller location are essential information in establishing the
shafting arrangement
• The location of the propeller is determined by the propeller diameter, the acceptable clearance
between the propeller and the baseline of the ship, and the acceptable clearances be tween the
propeller and the hull in the plane of the propeller

10.
SHAFT ALIGNMENT
• Shaft alignment is a process where the axis of power transmitting elements is made collinear
• Measurement:
In shaft alignment two measurements are critical: i) Offset ii) Angularity
1. Offset: Offset is defined as vertical and horizontal distance between the two shafts’ rotating axes
2. Angularity: It’s the angle between the two rotating shafts. It is generally measured in terms of
slope
3. Other techniques e.g. reverse dial indicator method, laser alignment etc are also used for shaft
alignment

11.
VIBRATIONS
• When a mechanical system performs oscillations about an equilibrium position, due the action of a
disturbing force, the system is said to be in the state of vibration
• The two most noticeable effects vibration has on the ship are structural fatigue and discomfort of
crew/passengers
• SOURCES of SHIP VIBRATION:
1. Internal Sources:
a) Main & Auxiliary Machines: Engines, Motors, Auxiliary Machinery, etc
b) Unbalanced Shaft: UNBALANCE occured due to disturbed centre of mass, or/and due to
shaft misalignment
2. External Sources: Due to Hydrodynamic loading on Propellers, cavitation, vortices, external
moments

12.
VIBRATION MODES
• Vibration mode is designated by number of nodes in a system
• Node: A point in a vibrating system where the amplitude is zero
• Antinode: A point in a vibrating system where the amplitude is maximum

13.
VIBRATION MODES

14.
VIBRATION TYPES
• There are three basic types of vibration which can occur in a main propulsion shafting system;
these are
1. torsional,
2. Longitudinal/ Axial, and
3. Transverse vibration
• It is essential that a preliminary vibration analysis of the shafting system be made in the early
design stages to avoid complications later

15.
VIBRATION TYPES

16.
VIBRATION DAMPING

17.
VIBRATION CONTROL
How to avoid dangerous vibrations of the ship’s hull?
By Avoiding exciting forces at frequencies close to the natural frequencies of the ship’s hull

18.
VIBRATION TYPES

19.
LATERAL VIBRATION
• This mode of vibration occurs in the direction perpendicular to the axis of rotation of the
shaft
• Due to the bending of the shafts, the centre of gravity of the shaft does not coincide with the
ideal centreline of the shaft, therefore when the shaft rotates, the centrifugal force on the
centre of gravity would cause it to shift further away from the ideal centreline, resulting in a
vibratory motion called whirling of shafts
• Care is to be taken by the designer that the natural frequency of lateral mode of vibration of
the shaft does not coincide with the natural frequency of the engine
• In such a case, resonance would result in the extreme case of whirling, which would lead the
shaft to snap and cause damage or accidents
Medium
speed diesel
engine,
crankshaft
fatigue
fracture
Figure:
Eccentricity
between CG
and CL of
shaft during
whirling.

20.
LATERAL VIBRATION
• Sources:
1. Propeller Fluctuating Loads
2. Diesel Engine Excitation
3. Shaft Alignment Errors
4. Manufacturing Defects in Gearing System

21.
PROP EXCITATION AND ADDED MASS
• There are two types of excitation caused by the
rotation of a propeller, and they are as follows:
a. Alternating Thrust: This results in
longitudinal vibration of the propulsion
system
b. Propeller Cavitation: Propeller cavitation
results in formation of bubbles that form on the
propeller blade
• ADDED MASS: Added mass is the additional
mass that an object appears to have when it
is accelerated relative to a surrounding fluid
• When propeller moves, it carries water along
with it (also known as entrained water)
Cavitation erosion