A ship that speeds up in shallow water sinks deeper into it. This is called squat, and in narrow, shallow channels it can eat up all the water under the keel. The water that flows past a moving hull has to speed up, especially where there is little room under and beside the ship. Faster flowing water has lower pressure, so the water level around the ship drops, and the ship sinks with it. Full-bodied ships such as tankers, bulk carriers and large container ships usually squat most at the bow. A rule of thumb by Professor C.B. Barrass: in a confined channel, squat in metres is about the block coefficient times the speed in knots squared, divided by 50. In open water you divide by 100. For a 366 m container ship with a block coefficient of 0.65 and a draught of 14.5 m in a channel 16 m deep, that gives 1.9 m of squat at 12 knots, more than the 1.5 m under her keel. At 6 knots it is under half a metre. Real squat also depends on the width of the channel, the width of the ship and the ratio of water depth to draught, and pilots and port authorities use more detailed methods and measurements. This video is an illustration, not for navigation.
Source: Squat formula: C.B. Barrass, Ship Squat (various editions). Visualisation by The Moving Sea
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