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Chapter 15 Trim
Trim may be considered as the longitudinal equivalent of list. Trim is also known as `longitudinal stability'. It is in effect transverse stability turned through 90 . Instead of trim being measured in degrees it is measured as the difference between the drafts forward and aft. If difference is zero then the ship is on even keel. If forward draft is greater than aft draft, the vessel is trimming by the bow. If aft draft is greater than the forward draft, the vessel is trimming by the stern.
Consider a ship to be ¯oating at rest in still water and on an even keel as shown in Figure 15.1.
The centre of gravity (G) and the centre of buoyancy (B) will be in the same vertical line and the ship will be displacing her own weight of water. So W b:
Now let a weight `w', already on board, be shifted aft through a distance `d', as shown in Figure 15.1. This causes the centre of gravity of the ship to shift from G to G1, parallel to the shift of the centre of gravity of the weight shifted, so that:
or
GG1 w d W
W GG1 w d
A trimming moment of W GG1 is thereby produced.
But
W GG1 w d ; The trimming moment w d
The ship will now trim until the centres of gravity and buoyancy are again in the same vertical line, as shown in Figure 15.2. When trimmed, the wedge of buoyancy LFL1 emerges and the wedge WFW1 is immersed. Since the ship, when trimmed, must displace the same weight of water as
