Trim

Trim describes the longitudinal, fore-and-aft attitude of a ship in the water, measured as the difference between the draft read at the bow and the draft read at the stern. If the forward draft is greater the ship is trimmed by the head; if the after draft is greater it is trimmed by the stern; and if the two are equal it floats on an even keel. Trim is controlled by how weight (cargo, ballast, fuel and stores) is distributed along the length of the ship, and adjusting it is a routine part of loading and of managing a vessel on passage.

Trim matters for several practical reasons. It affects how the ship handles and steers, since a vessel trimmed heavily by the head can be difficult to steer and prone to yawing, while a moderate trim by the stern usually gives better directional stability and keeps the propeller and rudder well immersed. It also affects fuel efficiency: for a given speed and draft there is often an optimum trim that minimises resistance, and trim optimisation has become a recognised way to reduce fuel consumption and emissions. Trim influences propeller immersion, the risk of slamming forward in a seaway, and clearance under bridges through its effect on air draft.

Trim is closely related to, but distinct from, other loading concepts. It concerns the fore-and-aft balance, whereas list and heel concern athwartships inclination, and stability concerns the ship's tendency to return upright. During cargo operations, especially the rapid loading of bulk carriers, the crew manage trim together with the bending and shear stresses on the hull, using a loading computer to keep the ship within safe limits at every stage. Reading the draft marks forward and aft gives the trim directly, and mariners routinely calculate and adjust it as part of safe and efficient ship operation.