a) What is meant by squat and explain how does it occur.
This is a term used to define changes in draught and trim which occurs when the depth of water
beneath the vessel is less than one and a half time the draught of the vessel when travelling at a
When there is a limited clearance under the keel the restriction increases the
velocity of water flow which causes a drop in pressure thereby reducing the buoyancy force on
the vessel. This effect is increased still further when vessel is in the confined channel since the
velocity of water flow must increase due to further restriction.
Since the weight of the vessel remains unchanged the ship will have to sink further
thereby increasing her draught in order to restore equilibrium. There is likely to is a change in
trim since the LCB likely to change therefore creating a trimming moment. Where LCF is
greater than LCB there will be a trimming moment at astern, where LCF is less than LCB there
will be a trimming moment by the head and where LCF = LCB there will be no trimming
effect and maximum squat will be of equal value at fwd and aft.
b) List the factors, which effect the magnitude of squat.
1. Speed of the ship.
2. Draught / water ratio.
3. Propeller revolution.
4. Form of bow waves.
5. Length / breadth ratio.
6. Block co-efficient.
7. Change width / beam ratio.
8. Initial trim.
c) Describe the overall effect of shallow water on the maneuvering characteristics of a
1. Speed of the vessel decreases as squat is directly proportional to square of speed.
2. R.P.M. decreases and high R.P.M. increases astern trim.
3. Higher the draught to depth of water ratio greater the squat which results in lesser U.K.C.
4. Vibration may occur.
5. In shallow water squat causes abnormal bow and stem wave to build up there by the type of
bow effects wave making and pressure distribution.
6. Steering is effected because the water displaced by the hull is not so easily replaced by
other water and the propeller and rudder might be working in partially vacuum conditions.
The vessel takes long to answer her helm and response to engine movement become
7. It will be extremely difficult to correct a yaw or sheer with any degree of rapidity.
8. The moving vessels bow wave, stem wave and trough increase in amplitude.
SIGNS OF SQUAT
1. Speed decreases.
2. RPM decreases.
3. Vibration may occur.
4. Steering is affected vessel become sluggish to maneuver.
5. Ship made waves increase in amplitude.
6. Ship wake changes color and becomes muddy.