Page 285 - Introduction to Naval Architecture

270 MANOEUVRING

the ship. Anything entering and disturbing the pressure field will
cause a change in the forces on the ship, and suffer forces on itself.
If one ship passes close to another in overtaking it, the ships initially
repel each other. This repulsion force reduces to zero as the bow of
the overtaking ship reaches the other's amidships and an attraction
force builds up. This is at a maximum soon after the ships are abreast
after which it reduces and becomes a repelling force as the two ships
part company. When running abreast the ships experience bow
outward moments. As they approach or break away they suffer a bow
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inward moment . Such forces are very important for ships when they
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are replenishing at sea .
Similar considerations apply when a ship approaches a fixed object.
For a vertical canal bank or jetty the ship experiences a lateral force
and yaw moment. Open structure jetties will have much less effect
than a solid one. In shallow water the reaction is with the sea bed and
the ship experiences a vertical force and trimming moment resulting
in a bodily sinkage and trim by the stern. This can cause a ship to
ground in water which is nominally several feet deeper than the
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draught .
The sinkage is known as squat This phenomenon has become more
important with the increasing size of tankers and bulk carriers. Squat is
present even in deep water due to the different pressure field around
the ship at speed. It is accentuated, as well as being more significant, in
shallow water. In a confined waterway a blockage effect occurs once the
ship's sectional area exceeds a certain percentage of the waterway's
cross section. This is due to the increased speed of the water which is
trying to move past the ship.
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For narrow channels a blockage factor mid a velocity-return factor have
been defined as:

A formula for estimating squat at speed Vin open or confined waters
is:

Cg being the block coefficient.