How a sailboat makes progress against the prevailing wind
There is still a great deal of mystery attached to the process whereby a sailboat makes progress against the prevailing wind, but it’s possible to state it in simple general terms. A sail would never generate “lift” if it were flat—its curved, asymmetrical shape causes the wind passing over it to be deflected, and that deflection in turn causes small differences in the pressures on either side of the sail, with high pressure on the windward side and lower pressure on the leeward side.
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| Idealized depiction of airflow over properly trimmed sails |
The net result is a force acting to leeward at right angles to every part of the sail’s surface. In the case of a mainsail trimmed for beating, most of that force is directed sideways, of course, but a small portion is directed forward. The sideways force is counteracted by the boat’s keel or center-board, and the small remaining force is what drives the boat forward. Because of the obvious comparison with the way an airplane wing works, that force is referred to as “lift,” even though the drive is horizontal, not vertical.Naturally, no sail can drive a boat directly into the wind. Modern sailboats mostly beat to wind-ward at an angle of about 45 degrees to the real wind direction, although highly developed racing craft can head up almost 10 degrees more.Lift occurs only when there is a stream of wind passing across the sail from fore to aft, which is normally the case from a beat to a broad reach. As you turn your boat farther downwind from a broad reach, the cross-flow ceases and the sail becomes a stalled airfoil, blowing before the wind in a manner that is intuitively understood.When sailing dead downwind with stalled sails, you can’t go any faster than the wind. But a boat on a broad reach may exceed the true wind speed because its forward motion creates a faster “apparent” wind against the sails.See also
Air Pressure on Sails; Wind and Altitude; Yacht Design.
