Bank effect

Bank effect refers to the tendency of the stern of a ship to swing toward the near bank when operating in a river or constricted waterway.

The asymmetric flow around a ship induced by the vicinity of banks causes pressure differences (Bernoulli's principle) between port and starboard sides. As a result, a lateral force will act on the ship, mostly directed towards the closest bank, as well as a yawing moment pushing her bow towards the centre of the waterway. The squat effect increases due to the decreased blockage.

This can be seen on this footage video bank effects taken during model tests in a towing tank at Flanders Hydraulics Research

This phenomenon depends on many parameters, such as bank shape, water depth, ship-bank distance, ship properties, ship speed and propeller action. A reliable estimation of bank effects is important for determining the limiting conditions in which a ship can safely navigate a waterway.

This phenomenon has several different names, including bank suction, bank cushion, stern suction, and ship-bank interaction.