Impact of the bow re-entering the water after pitching out, causing localised pressure spikes on the forefoot and forward bottom.
Regulatory detail & full definition
Slamming is the heavy impact that occurs when a ship's bow pitches clear of the water in a seaway and then re-enters it, producing sharp localised pressure spikes on the forefoot and forward bottom plating. Repeated slamming can buckle bottom plating and start fatigue cracks in welds, so it is a structural concern as well as a matter of comfort. The main control is a reduction of speed and/or an alteration of heading in heavy weather — a decision squarely within the Master's overriding authority under the ISM Code — and modern hull-stress monitoring systems provide slamming alarms from measured accelerations and bending stresses to guide it.