(a) Describe three types of resistance affecting a vessel forward motion through the water.
This has two element skin friction and viscous friction. Skin friction is due to the friction of
water against the hull; its value increases with ship’s speed, length, wetted surface area and
surface roughness. On the other hand viscous friction is due to seawater density and
temperature (greater in cold weather). Hence fouling and deteriorating hull surface will
increase skin friction and so reduce the vessel speed.
WAVE MAKING RESISTANCE
Only occurs at the interface between two mediums, as the vessel moves through the water
pressure changes are generated in the water adjacent to the hull, hence an increase in pressure
ahead produces a bow wave whilst a decrease in pressure along the side of the ship causes a
trough. The energy transmitted by these wave devices from the vessel and hence increases its
resistance to forward motion. Waves making resistance is influenced by the ship’s form and
varies directly proportional to speed and inversely as the vessel length.
Although the flow of water close to the hull is stream lined a little further away the flow is
turbulent. The agitated water whirls round in eddies which are absorbing energy from the ship.
Also certain parts of the ship together with the shape of the astern in a poorly designed vessel
with cause further eddying, the smoother the flow around ship the less the eddy making
resistance. When the depth of water is limited eddy-making resistance will increases as the
small under keel clearance will create greater turbulence around the hull.
REDUCING WAVE MAKING RESISTANCE.
The elongated spherical shape service to produce additional wave patterns, which counteracts
and partially cancels out the ships wave pattern thereby saving energy.
REDUCING FORM RESISTANCE:
Here the bulb service to alter the flow of water around the bulb so reducing turbulence / eddy
in this case the bulb is well below the surface and more appropriate for the large tanker or bulk
careers in loaded condition. These vessels have a bluff body due to their relatively large beams
which results in an increase in frictional and form resistance
EDDY MAKING RESISTANCE:
As the vessel moves through the water the bulb alters the flow of water around the vessel
reducing turbulence and eddying. This is more appropriate to the loading tankers and to the
bulk careers which have large bluff bodies due to large beams which increases both frictional
and form resistance
Increases frictional resistance particularly relevant when vessel proceeding at reduce speed
where wave making resistance is much less.