However, although this may be possible on the shore by reefing or changing sails, it will be too difficult and time consuming during a race. More sophisticated methods of de-powering the sail must be investigated as part of a complete design. 4.2 Basic Sail Performance Characteristics Figure 4.1 shows the lift and drag characteristics of a single sail. If a hull is superimposed onto this diagram, with an apparent wind angle to the boat b and to the sail a , the best sail setting position can be identified. A corresponds to a minimum aerodynamic drag angle.
B corresponds to a maximum forward drive force.
Altering the sheeting angle to move along the polar curve from A to B causes an increase in side force as well as an increasing drive force. Extra dagger board induced drag is created as side force increases. Initially sail drive exceeds the increase in drag and boat speed increases. In moderate weather where the craft is under powered this optimum is the proper choice of sail setting. In reaching conditions it is nearer B, and B itself approaches the point of maximum sail lift coefficient.