II BASIC PRINCIPLES OF SAILING BOATS

Methods of sailing vary according to the manner in which boats are rigged, but the essential principles of sailing are the same for all craft. The simplest and most easily understood point of sailing is called in nautical terms sailing before the wind. The term running before the wind is also used. As the term indicates, the boat follows the same course that the wind is blowing. As shown in the first diagram in Figure 1, the sail or sails are set at approximately a 90° angle to the longitudinal axis of the boat, with power derived from the push of the wind on the sails’ back surfaces.
 

In sailing off the wind, as shown in the middle diagram, the wind reaches the craft from the side, or beam, and the sails are set at approximately 45° from the axis of the craft. In this sailing position, the wind exerts a pulling rather than a pushing action on the sails, which act as airfoils, like the wings of an airplane. The general principle of wind action is that the wind flows at a greater rate of speed along the forward surface of the sail, creating an area of lower pressure ahead of the sail. The actual force exerted by the wind is at right angles to the sail, as indicated by the dotted line a. This force would tend to drive the boat at an oblique angle if the hull of the boat were perfectly flat. Every sailboat, however, is equipped with a fixed keel or a retractable centerboard, which acts as a flat longitudinal plane to prevent the boat from moving sideways through the water. The effect of this plane is shown by the dotted line b, and the actual course of the boat, the result of both the force of the wind and the resisting force of the keel, is the dotted line c, representing forward motion.