Heel and Trim

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Heel and Trim

Heel and trim describe two different aspects of a ship’s position. Heel is the sideways inclination about the longitudinal axis; trim describes the ship’s longitudinal attitude based on the difference between its forward and aft drafts. Both conditions can occur at the same time. The axes provide a simple point of reference: heel concerns port and starboard, while trim compares the bow and stern. In technical descriptions, stating the direction and the measurement reference helps distinguish the two clearly.

Heel: across the ship’s centerline

When a ship heels, it inclines to port or starboard. A sideways shift in weight changes the position of the center of gravity and can cause a persistent list. External forces can also produce an inclination. English-language stability literature often uses “list” for an inclination caused by weight distribution and “heel” for one caused by external forces. In German, Krängung is often used as a general term for sideways inclination.

A simple example illustrates the direction: when weights are moved sideways on a ship, the weight distribution across the centerline changes, and the resulting position is described as a port or starboard heel. The direction indicates the side toward which the ship is inclined. A technical assessment also considers the loading condition and the forces acting on the ship. This makes it possible to treat describing the position and analyzing its causes as separate steps.

Trim: longitudinally between bow and stern

Trim is determined by comparing the drafts forward and aft. A common convention calculates aft draft minus forward draft. With a forward draft of 5.00 m and an aft draft of 5.75 m, the result is therefore 0.75 m by the stern. The calculation is: 5.75 m − 5.00 m = 0.75 m. A positive or negative value is unambiguous when the sign convention being used is stated; the direction can also be given in words as trim by the bow or by the stern.

The difference in draft describes the ship’s longitudinal attitude by comparing the two measurement points. If a subsequent measurement records 5.20 m forward and 5.60 m aft, the difference using the same convention is 0.40 m by the stern. Both examples show how the difference can be calculated from the individual values. Loading and ballast distribution affect the longitudinal attitude; the vessel’s operating condition and hull shape are also relevant to a technical assessment. The longitudinal attitude changes the flow around the hull. Resistance and effective wetted surface area can vary with trim, so the appropriate setting depends on the design and operating conditions.

Distinguishing position from motion

Heel and trim describe position, whereas roll and pitch describe motion. Rolling is the repeated rotational motion about the longitudinal axis. Pitching is motion about the transverse axis, in which the bow and stern alternately rise and fall. A ship can therefore have both a sideways inclination and a particular trim while also moving in a seaway. Terms for position describe orientation; terms for motion describe recurring dynamics. This distinction makes it easier to organize observations and measurements according to the relevant ship’s axis.