LDD 30502
SHIP RESISTANCE AND
PROPULSION
TOPIC 1: Total Hull Resistance
TOPIC 1: Total Hull Resistance
All floating bodies (ship and boat hulls) experience resistance when
moving through water.
The resistance of a ship at a given speed can be defined as a force
required to tow the ship at that speed in water, assuming no
interference from the towing ship. The power necessary to overcome
this resistance is called the tow rope or effective power.
The ship actually moves at the same time through two fluids, water
and air, with widely different density. While the lower part of the
hull is moving through water, the upper part is moving through air.
Summary:
Water resistance (submerged part of a hull)
Air resistance
(upper part of hull &
superstructure)
This total resistance is consists of a number of different components,
which is caused by a variety of factors and which interact one with
other in a complicated way.
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Total Hull Resistance
Total Resistance
Frictional Resitance
Residuary Resistance
Wave- making
Wave
Resistance
Eddy Resistance
Air Resistance
RT = RF + RR
R F : Frictional
Fi i
l Resistance
R i
R R : Residuary Resistance
Components of Total Resistance
Frictional Resistance
Exists due to the viscosity of water (seawater/ fresh water).
( due to friction of the water against the surface of the ship)
There is a mass of fluid being dragged along with a ship.
Energy necessary to drag the mass of fluid is the work done by the
ship against the frictional resistance.
resistance
Frictional resistance represents almost;
70-90% of the ships total resistance for low-speed ships (bulk
carriers
i andd tankers),
t k )
and sometimes less than 40% for high-speed ships (cruise liners and
passenger ships).
Th
These
values
l
may become
b
higher
hi h in
i time
ti due
d to
t the
th increased
i
d
roughness of the ship surface
in general can be summarized, which were dependant on
viscosity and density of the liquid, ships velocity and wetted surface
area
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Wave-Making Resistance
Resistance caused by waves generated by the motion of the ship
Waves possess energy. Thus a ship making waves means a loss of its
energy.
Wave-making resistance is important to surface ships,
ships especially those
of high speeds, but may be negligible to submarines.
Wave-making resistance is affected by beam to length ratio,
displacement shape of hull,
displacement,
hull Froude number (ship length & speed)
Bow divergent wave
Stern divergent wave
Typical Wave Pattern
Eddy Resistance
Eddy resistance refers to the loss caused by flow separation which
creates eddies, particularly at the aft end of the ship.
The residual resistance normally represents 8-25% of the total
resistance (low-speed
(low speed ships),
ships) and up to 40-60%
40 60% (high
(high-speed)
speed)
Incidentally, shallow waters can also have great influence on the
residual resistance, as the displaced water under the ship will have greater
diffi lt in
difficulty
i moving
i aftwards.
ft d
large eddy
small eddy
Full ship
Sl d ship
Slender
hi
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Air Resistance
Resistance caused by the flow of air over the ship with no
wind present
Air resistance is affected byy projected
p j
area,, shape
p of the shipp
above the water line, wind velocity and direction
Ships super structures are never faired and as such offer a
range of sharp corners and projections for air resistance to act
against.
Typically 4 ~ 8 % of the total resistance
RA = 0.90 air V 2 Aair
where
air is the density of the air, and
Aairi is the cross
cross-sectional
sectional area of the vessel above the
water
Other Type of Resistances
Appendage Resistance
Frictional resistance caused by the underwater appendages
such as Rudders; Shaft Brackets; Struts; Stabilisers; Bilge keels
224% of the total resistance in naval ship.
ship
Steering Resistance
Resistance caused by the rudder motion.
Small in warships but troublesome in sail boats
Added Resistance
Resistance due to sea waves which will cause the ship
motions (pitching,
(pitching rolling,
rolling heaving,
heaving yawing).
yawing)
Increased Resistance in Shallow Water
Resistance caused by shallow water effect
Flow
Fl velocities
l iti under
d the
th hull
h ll increases
i
in
i shallow
h ll water.
t
Increases frictional resistance
The waves created in shallow water take more energy from
the
h ship
hi than
h they
h d
do iin d
deep water ffor the
h same speed.
d
Increases wave making resistance
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