Trim

Trim is the difference between a ship's draft forward and its draft aft — in other words, its fore-and-aft balance (longitudinal inclination) in the water. A ship is “on an even keel” when the drafts are equal, “down by the stern” (or trimmed by the stern) when deeper aft, and “down by the head” (by the bow) when deeper forward.

Trim is controlled by the distribution of cargo, ballast and fuel along the ship's length, and it matters for good reason: correct trim affects fuel efficiency (a well-trimmed ship has lower resistance), propeller immersion, steering, seakeeping and under-keel clearance. Optimising trim for each loaded condition and speed is a recognised, low-cost way to save fuel and cut emissions (trim optimisation). Trim (the fore-and-aft balance) is distinguished from list (the sideways tilt), and both are read from the draft marks. Managing trim is a routine but important part of loading and voyage operation, with direct effects on efficiency and safety. It reflects how the ship sits along its length.