Weight trim is a form of trim and describes the compensation of the lean angle of a sailboat to optimize speed. The force generated by the wind, which acts on the sails , is divided into propulsion and lateral force. The transverse force acting on the sail perpendicular to the direction of movement causes a sailing boat to tilt. However, the more inclined a sailboat is, the smaller the surface of the wind in the sail and the lower the propulsion. In the extreme, at a 90 degree heel , the boat overturns and the sail is in the water (without generating propulsion).
Therefore, on smaller sailing boats and in stronger winds, it is in principle best if the boat is sailed completely upright. This is especially true for gliding dinghies such as Korsar or Flying Dutchman . With dinghies, weight trim is achieved in particular by using the trapeze . In light winds, however, the heavier sails do not stand optimally; a slight heel is preferable here so that the sails fall into a better shape due to their own weight.
However, there are also types of boats, such as sport catamarans , which only reach the optimum speed when they are tilted to a certain extent. Small catamarans reach maximum speed when a hull is lifted out of the water. In large trimarans, even the middle hull should partially lift out of the water. This significantly reduces the water resistance of the boat.
On smaller boats in particular, the crew plays a central role in weight trimming, because the boats are very sensitive to even minor weight shifts. Even on yachts of average size, the crew weight is so noticeable that sailors, especially on regattas, sit specifically on the beltline on the windward side in order to use their weight to straighten the boat. Ballast tanks are also used in modern racing yachts to trim weight.