When a ship has trim, however, neither the draft
amidships nor the average of the forward and after
drafts will give a true mean draft. For most types of
ships, the curves of form may be used without
correction for trim, PROVIDED the trim is less than
about 1 percent of the length of the ship. When the trim
is greater, however, the readings obtained from the
curves of form must be corrected for trim.
Longitudinal stability is the tendency of a ship to
resist a change in trim. The longitudinal metacentric
height multiplied by the displacement is taken as a
measure of INITIAL longitudinal stability when trim
is very small. (It is important to note that the
longitudinal metacenter (M1) is NOT the same as the
transverse metacenter.) A more accurate measure of
the ship’s ability to resist a change of trim is made in
terms of the moment required to produce a change in
trim of a definite amount. The MOMENT TO
CHANGE TRIM 1 INCH (MTI) is used as the standard
measure of resistance to longitudinal inclination.