Draft and trim: get the sign right
Understand mean draft, bow and stern trim, and reference-length gradients.
Start with two observations
With forward draft FWD and aft draft AFT, the arithmetic mean is (FWD+AFT)/2. Readings of 7.8 m and 8.5 m give 8.15 m. This two-point average is not necessarily the measured midship draft or the corrected draft needed for hydrostatic calculations.
The sign convention used here
EngiCalcLab defines Trim = AFT − FWD. The example gives +0.7 m, or trim by stern. A deeper bow produces a negative result, or trim by bow. Other documents may use the opposite definition; record the convention alongside the number.
Gradient and angle
Divide the draft difference by a reference length consistent with the observation positions. A 0.7 m difference over 100 m gives 0.007, or 7 mm/m. The angle is atan(0.007), approximately 0.401°. For small angles, difference/length approximates the angle in radians, not degrees.
Before applying it to a vessel
Draft-mark offsets, port/starboard observations, hogging, sagging, density and trim corrections are excluded from a simple two-point calculation. Displacement and stability decisions require approved vessel-specific data. The diagram illustrates direction, not an analyzed vessel condition.