THE SQUAT & FACTORS GOVERNING SQUAT
To understand squat, first you have to know about Under Keel clearance, Draft & Bernoulli Effect in Physics.
The distance between the keel of the ship & the seabed, at that particular instant in known as Under Keel Clearance (UKC).
In the other words, it is the distance between deepest point of the vessel & the seabed.
The distance between the water line & the ships keel is known as Ship's Draft.
Bernoulli's principle states that for an in viscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.
Squat is the increase in the ship’s draft when the ship is proceeding in shallow water. When the ship is moving in shallow water, the water relative to ship starts to move. This will reduce the water pressure as per Bernoulli’s principle. The decrease in water pressure will pull the ship towards the seabed.
In the other words imagine a ship is afloat on a paste like Margarine. When it moves ahead by one length, it will leave behind a visible hollow space. The volume of this hollow space will be equal to the ship's submerged volume. Same thing will happen when the ship is moving in water, but since water is liquid it will immediately replace this hollow by filling it with water taken from below & sides. As the water is taken from below the ship will be pulled downwards.
Factor’s Governing the Squat
1. Ship’s speed- The speed of the ship plays very important part in squat of the ship, more the speed more the squat & lesser the speed lesser the squat. If you encounter squat immediately stop the ship or reduce the speed, otherwise you may Ground the ship & seriously damage the keel.
2. The Trim of the Ship- If forward & aft drafts are different, the ship is said to have a trim. For Example if the forward draft is 5 metres & aft draft is 6 metres, ship is having a trim of 1 meter by stern. If a ship is already having a trim, the ship will squat by deeper end regardless the shape of the hull.
3. The Hull style- From the experiences & experiments it is found box like ships (full form ships) will squat by head. E.g. Tankers, Bulk Carriers Whereas streamlined ships will squat by stern. E.g. Container ships, Battle ships.
4. The block coefficient- The block coefficient of a vessel is obtained by dividing the underwater volume of displacement of a ship by the volume of a block of the same length and breadth, and of height equal to the draught of the ship. The block coefficient depends upon the “lines” of the ship. Passenger vessels & Container vessels with fine lines have a lower block coefficient than Tankers & Bulk carriers with full lines. The abbreviation for Block Coefficient is generally given as Cb
Higher the block coefficient, higher the squat.
5. The draft to depth ratio- If this ratio is 1, ship has already touched the ground. If it is 2, there is negligible squat.
6. The narrowness of the channel or cannel-In narrow channels side water to the ship is less, so void created by ship will be filled by below water so the ship will squat more.
In shallow water
Squat = (block coefficient x speed x speed)/ 100
(Speed in Knots & squat in metres.)
In shallow narrow channel/canal
Squat = 2(block coefficient x speed x speed)/ 100
(Speed in Knots & squat in metres.)
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