CAKGO AND TRIM 521

at this draught be 30 tons, the sinkage would be 3 inches forward
and 3 inches aft, thus increasing her mean draught 3 inches.

But if the weight (w), 90 tons, were placed 100 feet before the tipping
centre the vessel would trim by the head and the ship's waterline W L
would be changed to W1 L1, showing a wedge of immersion forward

............--............-.....

Fig. 39.
and a wedge of emersion aft as in Figure 39. Conversely, if the weight
were placed aft the ship would trim by the stern. In order to compute
the change of trim we must know:—
(1) The weight required to alter the mean draught 1 inch, that is
the "tons per inch immersion," abbreviated to T.P.I, (get used to these
abbreviations as they save a lot of writing). This quantity varies
slightly with change of draught owing to the altering shape of the
underwater form of the ship.
The T.P.I, can be got from the displacement scale.
(2) The Moment to Alter Trim by 1 Inch, or Inch-Trim-Moment,
abbreviated to I.T.M. This information should be supplied by the
builders as it is determined by calculation or by experiment, and it
also varies with the draught.
The following formula gives an approximate inch-trim-moment.
I.T.M. = **
where T = tons per inch immersion as found from the Displacement Scale
B = moulded breadth of vessel
(3) The weight added to, discharged from, or moved about in the
ship and the longitudinal distance of its centre of gravity from the
tipping centre must also be known; then weight Xdistance =moment.
Knowing the T.P.I, and the I.T.M. the method of computing the
change of trim may perhaps "be best demonstrated by means of an
example.
Example.—Given T.P L 40 tons, I.T.M. 1500 foot-tons, draught
12 ft. 00 ins. forward, 14 ffc. 00 ins. aft. The after peak tank, 180 feet