How does bank effect occur?

The hydrodynamic pressure distribution system around the forward-moving ship can be seen as a boundary layer of water that surrounds a ship when it is making headway.

Forward of the pivot point of the ship, a positive pressure area builds up; however, after the pivot point, the flow of water down the ship side creates a low-pressure area.

This area extends out from the ship and does not cause any concern in deep waters or when the ship is clear of any obstruction or traffic along her route.

However when the ship closes in on a bank the pressure on the bow works on a short turning lever forward of the pivot point but the lower pressure or suction area on the other hand works well after the pivot point and is consequently a very strong force.

The suction area further drops in pressure as the water gets squeezed and accelerates as a result of the two forces the stern of the ship is likely to be sucked into the bank.

It can be very difficult to break out of this hold the ship requiring constant corrective rudder action and power sometimes hard over even to control the hedging.