Seamanship
Mean draught and trim
Trim, hog or sag and the mean of means from the six draught readings, the starting point of a draft survey and the dock water allowance.
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How it works
Port and starboard are averaged at each mark. Trim = aft − forward (by the stern when the aft draught is deeper). Deflection = mean of forward and aft − midships: the midships draught deeper than the mean means the ship sags, shallower that she hogs.
The mean of means (quarter mean), (F + 6M + A) / 8, allows for the deflection. Draught marks are rarely at the perpendiculars: forward draught at FP = mark − trim × distance / length between marks, aft draught at AP = mark + trim × distance / length between marks. A full draft survey then needs the ship's hydrostatic tables and the dock water density.
For training and cross-checking. Not a substitute for approved navigation equipment, nautical publications and the ship’s own procedures. Calculations run in your browser and nothing you type is stored; a PDF you download is made on our server and not kept.
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