Wednesday, December 5, 2007


As a ship begins to make way through the water she undergoes a change in mean draft known as sinkage. This change may be equally forward and aft or may be greater at the bow or the stern, the resulting change in trim is called "squat."
When passing through the water the ship displaces an amount of water equal to her own weight. This water must move outward from and around the hull in all directions. The water so displaced moves along and under the hull and returns astern of the ship to fill the space left by the ship as she moves on. The faster the ship is moving the greater the velocity of this flow under and along her hull, and the greater the pressure drop as a result of that increased velocity. Depending upon where the greatest drop in pressure occurs along the length of the hull, this reduced pressure will result in greater sinkage (increase in draft) at the bow or stern, the draft increases to some degree all along the length of the ship.

As the ship enters shallow water the flow of water becomes increas­ingly restricted due to the reduced clearance both under and on one or both sides of the hull. The effect of this restriction or blockage factor is dependent on several things:
1. The speed of the ship through the water.
2. Ratio of the ship's draft to the depth of water.
3. Ratio of the ship's cross sectional area to the cross sectional area ofthe channel. 4. The ship's block coefficient.
5. The ship's displacement, which determines the amount of water that must pass around and under the ship's hull at a given speed.
First look at the effect of ship's speed since this is the factor over which the you have the greatest control. It has been found, based upon observations of both actual ships and models, that squat varies in proportion to the square of the speed. If ship's speed is doubled, squat increases by a factor of four. With large ships and minimal under keel clearances speed and squat must be considered.