Vessel Interaction

Shallow Water

In the context of ship handling shallow water can be defined as depth of less than twice a vessel’s draft. Insufficient space between the hull and bottom in shallow water will prevent normal screw currents resulting in waste of power, sudden sheering to either side, and sluggish rudder response.

In shallow water the bow will tend to ride its own wave while the stern sinks into the depression of its own wake, causing an effect known as squatting. This is especially noticeable as the vessel increases beyond it’s critical speed.

Bank Suction & Cushion

The effect of bank suction is when the stern of a vessel proceeding along a narrow channel is pulled towards the nearest bank. An additional effect is having the bow pushed away from the nearest bank by the effect of the vessel’s wake, this is called bank cushion. This is especially true in shallow water, where the draft of the vessel is almost equal to the depth of water.

A similar effect to bank cushion can be experienced with two ships passing – the bow wave of the opposing vessels will push the bows apart.

Response to Cushion

If, in a narrow channel, your vessel suddenly sheers toward the opposite bank the best maneuver would be to order full ahead engines with hard right rudder. The increase in throttle will increase the effectiveness of your rudder and help to restore steerage. This will aid in the recovery of control to the vessel before the boat is able to get a great deal of way on. The throttle may then be restored to the previous setting.