Page 124 - Introduction to Naval Architecture

SEAKEEPING 111

Slamming is likely when the relative velocity between the hull and
water surface is large and when the bow is re-entering the water with a
significant length of bottom roughly parallel to the sea surface. It is
amplified if the bottom has a low rise of floor. The pressure acting in a
slam can be shown to be proportional to the square of the velocity of
impact and inversely proportional to the square of the tangent of the
deadrise angle.
Wetness
By wetness is meant the shipping of heavy spray or green seas over the
ship. The bow area is the region most likely to be affected and is
assumed in what follows. It may limit a ship's speed and the designer
needs some way of assessing the conditions under which it will occur
and how severe it will be. To some degree wetness is subjective and it
certainly depends upon the wind speed and direction as well as the
wave system. In the past it was often studied by running models in waves
but it is now usually assessed by calculating the relative motion of the
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bow and the local sea surface . The assumption made is that the
probability of deck wetness is the same as that of the relative motion
exceeding the local freeboard. The greater the difference, the wetter
the ship is likely to be.
Increased freeboard, say by increasing sheer forward is one means of
reducing wetness. At sea the master can reduce wetness by reducing
speed and, usually, changing the ship's heading relative to the
predominant waves. Good round down on the deck will help clear
water quickly. A bulwark can be used to increase the effective freeboard
but in that case adequate freeing ports are needed to prevent water
becoming trapped on the deck. The size of freeing ports to be fitted is
laid down in international regulations. The designer would avoid siting
other than very robust equipment in the area where green seas are
likely. Any vents would face aft and water traps provided.
Propeller emergence
The probability of the propeller emerging from the water, as the result
of ship motions, can be assessed in a similar way to wetness. That is, by
calculating the motion of the ship aft relative to the local sea surface. If
the propeller does emerge, even partially, it will be less effective in
driving the ship. It will tend to race arid cause more vibration.
Human performance
It is a common experience that ship motions can cause nausea and then
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sickness - . This discomfort can itself make people less efficient and
make them less willing to work. Motions can make tasks physically more
difficult to accomplish. Thus the movement of weights around the ship,
say when replenishing a warship at sea, is made more difficult. Also tasks