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


Question:

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° 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                                                            
  STATUS CRTPT                                                                                     
    
The trim should equal the difference between the (negative of the) critical
point heights shown by the status report.