Hull trim is the adjustment of a boat's loading so as to change its
fore-and-aft attitude in the water. In small boats, it is done by
positioning the crew. In larger boats the weight of a person has less
effect on the hull trim, but it can be adjusted by shifting gear, fuel,
water, or supplies. Different hull trim efforts are required for
different kinds of boats and different conditions. Here are just a few
examples: In a lightweight racing dinghy like a Thistle,
the hull should be kept level, on its designed water line for best
performance in all conditions. In many small boats, weight too far aft
can cause drag by submerging the transom,
especially in light to moderate winds. Weight too far forward can cause
the bow to dig into the waves. In heavy winds, a boat with its bow too
low may capsize by pitching forward over its bow (pitch-pole) or dive
under the waves (submarine). On a run in heavy winds, the forces on the
sails tend to drive a boat's bow down, so the crew weight is moved far
aft.