Slam

The impact of the forward underside of the hull striking the water.

This usually occurs when a vessel pitches in steep waves and then lands heavily after the bow lifts. It is uncomfortable for crew and can place strain on the hull, fittings and equipment. Speed, course relative to the waves and hull shape all affect how often it happens. Reducing speed, changing angle to the sea, or adjusting trim can make the motion easier and reduce repeated impacts.