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How do I predict Ship Squat?
Tables and curves (MS Excel)
What are the factors governing Ship Squat?
The main factor is ship speed Vk. Squat varies approximately with the speed squared. As an example, if we double the speed we quadruple the squat. Put another way, it can be shown that halving the ship’s speed will quarter the squat. In this context, speed Vk is the ship’s speed relative to the water so the effect of current/tide speed with or against the ship must be taken into account.
Another important factor is the block coefficient Cb. Squat varies directly with Cb. Oil Tankers will therefore have comparatively more squat than Passenger Liners.
The Blockage Factor 'S' is another factor to consider. This is the immersed cross-section of the ship’s midship section divided by the cross-section of water within the canal or river. If the ship is in open water the width of influence of water can be calculated. This ranges from about 8.25b for Supertankers, to about 9.50b for General Cargo ships, to about 11.25 ship-breadths for Container Ships.
The presence of another ship in a narrow river will also affect squat, so much so, that squats can double in value as they pass or cross the other vessel.
Formulae have been developed that will be satisfactory for estimating maximum ships squats for vessels operating in confined channels and in open water conditions.
These formulae are have been derived from analysing about 600 results. some measured on ships and some on ship-models. Some of the empirical formulae developed are as follows:
Let b = Breadth of ship.
Let H = Depth of water.
Let Cb = Block co-efficient.
Let B = Breadth of river or canal.
Let T = Ship’s even keel static draft.
Let Vk = Ship speed relative to the water in knots.
Let CSA = Cross Sectional Area.
Let S = Blockage factor = CSA of ship / CSA of river or canal.
K1, K2 and K3 are squat coefficients.
If ship is in open water conditions, then the formula for B becomes:
B = (7.04/Cb^0.85) ship breadths. This is known as the “width of influence”.
Blockage factor = S = (b x T) / (B x H).
Maximum Squat = (Cb x S^0.81 x Vk^2.08) / K1 metres for open water and confined channels.
Two short-cut formulae relative to the previous equation are:
Maximum squat = (Cb x Vk^2) / K2 metres for open water conditions only, with H / T 1.1 to 1.4.
squat = (Cb x Vk^2) / K3 metres
for confined channels,