Blockage factor is caused by the volume of the ship’s hull, occupying a greater part of the cross-sectional area of a waterway. The illustration shows that because the underkeel clearance is small the volume of water under the keel is small and would not have the same buoyancy effect on the hull as noted in deeper water. Bearing in mind that the position of buoyancy is defined as the geometric centre of the underwater volume. If the vessel is heeled by external forces the water plane will increase, the position of ‘B’ would move upwards, but at the same time, also outwards towards the angle of heel. This leaves the low side, at the turn of the bilge, liable to contact with the ground.
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