Dynamic trim

Dynamic trim is the change in a ship’s running trim caused by sinkage and trim moments that develop with forward speed, distinct from the static trim set by the loading. As speed rises the pressure field around the hull lowers the mean level (sinkage) and shifts the longitudinal pressure balance, usually trimming a displacement ship by the stern. It is governed by the depth Froude number and grows sharply in shallow water as squat. For fast and planing craft the running attitude departs strongly from the at-rest waterline, changing wetted length and resistance. It is measured in towing tanks and predicted by CFD, and it is why squat allowances are applied in restricted channels.