By: Bill Gladstone, Director of North U and author of the North U Racing Trim, North U Racing Tactics, and North U Cruising and Seamanship books and discs.
Each sail has three sources of power: angle of attack, depth, and twist.
Proper trim means sailing at full power and with the proper mix of power. For a given wind speed, a deeper more twisted sail shape is preferred in big waves or chop. A flatter, less twisted shape is better in smooth water. Same total power, but a different mix. The deeper, more twisted shape provides better acceleration, a wider steering groove, and more consistent power as your boat pitches and is slowed in waves. The flatter, less twisted shape provides better pointing and less drag for smooth water sailing.