Trim

Trim is the longitudinal inclination of a vessel expressed as the difference between the forward and aft draughts. When the aft draught exceeds the forward draught the vessel is said to be trimmed by the stern; when the forward draught is greater she is trimmed by the head. A vessel on even keel has equal forward and aft draughts. Trim affects propulsive efficiency, steering response, visibility over the bow, and structural stress distribution across the hull. The chief mate uses the loading computer and the ship's trim and stability booklet to plan cargo, ballast, and fuel distributions that achieve the desired trim prior to and during a voyage. Most vessels operate most efficiently with a slight trim by the stern, which also improves rudder effectiveness at low speeds. In bulk carriers and tankers, trim must be managed carefully during loading and discharging to prevent excessive bending moments or shear forces in the hull girder — values that are monitored against class-approved limits. Pilotage authorities in some ports specify maximum trim conditions for transiting shallow channels, and the master must confirm compliance before entering such waterways.