Why is trim not always the difference between fwd and aft drafts?

Question QA029 :
I am running some damage stability calculations and in doing so I have realized that the resulting trim value determined by the GHS command is not equal to the differences between forward and aft drafts (basically when it involves a large trim).

Answer :
The trim will equal Draft(AP) - Draft(FP) only when the heel angle is zero. At other heel angles the trim is less by a factor of the cosine of the heel angle. In your case,
6.516 x cos(7.20) = 6.465
The reason for this is that trim is the longitudinal inclination of the waterplane while draft is always in the ship's centerplane. (At 90 degrees heel, draft is undefined while trim remains well defined.) This is why we use "depth" (which is perpendicular to the waterplane) rather than draft, especially when the heel angles are large.
You can verify the trim as follows:
CRTPT (1) "Baseline at FP", 0, 0, 0
CRTPT (2) "Baseline at AP", 270, 0, 0
The trim should equal the difference between the (negative of the) critical point heights shown by the status report.

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