Ship Manoeuvring
Squat, shallow water and similar effects
Shallow water
When a ship is moving in shallow water the gap between
the ship’s hull and the bottom is restricted, the streamline flow of water past
the hull is altered and the result is seen as a greatly increased transverse
wave formation at the bows and again at the stern. In fact, the increased size of the stern wave
is a sure indication of the presence of shallow water. The energy expended in the waves formed by
the ship is a loss from the power available to drive her, and therefore in
shallow water her speed is reduced.
Furthermore, the restricted flow of water past the
stern reduces propeller efficiency, which also tends to reduce her speed. Usually, the higher the speed the more pronounced
is the reduction of speed.
These effects may become excessive if the depth of
water is less than one-and-a-half times the draught, particularly if the ship
enters such water at high speed. She may
become directionally unstable and
fail to answer her rudder at all, and the draught aft may increase so greatly
as to cause the propellers to touch bottom.
The effects are likely to be particularly pronounced
in ships where the propeller slipstream does not play directly on to the
rudder. The effects of shallow water on
steering in restricted waters such as canals or rivers are usually worse than
in the open sea, and are more likely to have dangerous results. The only way to regain control is to reduce
speed drastically at once.
When manoeuvring at slow speed
or turning at rest in a confined space in shallow water, the expected effects
from the rudder and the propellers may not appear. Water cannot flow easily from one side of the
ship to the other, so that the sideways force from the propellers may in fact
be opposite to what usually occurs.
Eddies may build up that counteract the propeller forces and the
expected action of the rudder. Stopping
the engines to allow the eddies to subside, and then
starting again with reduced revolutions, is more likely to be successful.
Smelling the ground
The effect of water pressure against the bows from the
presence of shelving water on one side, causing the bows to swing away into
deeper water, is the phenomenon known as smelling
the ground. In a narrow passage or
canal it can produce a dangerous sheer towards the opposite shore or bank, but
it can be beneficial if the water opposite the shoal is deep add
safe. The effect is most marked if the
bottom shelves steeply.
If in shallow waters and with great speed, the effect of squat is
amplified. There is very less water
below the keel and also there is reduced water flow by way of the rudder, thus
the following may be experienced, the requirement is to slow down:
Increased
vibrations
Rudder
sluggish
Vessel
tending to take a sheer
Mud being
churned up aft
Acutely a sharp drop in speed.
Squat
Squat is defined as the reduction of under‑keel
clearance resulting from bodily sinkage and change of trim, which occurs when a
ship moves through the water.
The effect of squat is noticed only in shallow waters
and then it is a potential hazard.
Entering and navigating a canal/ river or a narrow
channel without having taken the squat into account can lead to grounding in
the extreme case or may damage the propeller blades.
Cavitations may take place at the propellers due to
lack of water flowing past the propeller and rudder.
The effect of squat is to sink the ship bodily as well
as to trim her gently by the stern.
It is accentuated by increase in speed – and the only
way to avoid be grounded on some occasions is to reduce the speed, when the
under water clearance allows the ship to pass over the shallow patch.
Shallow water effects are magnified by squat.
The amount by which a ship would squat depends upon
the:
Speed of the ship
The underwater cross section of the ship and
The cross section area of the sea area through which
the ship is transiting
The ratio of the ships underwater cross section to the
cross section of the sea area is called the blockage factor and as this ration
increases the squat increases.
