Author Karen Buhler Gale.

Author Karen Buhler Gale.



Short, semi-monthly columns are not conducive to the vast and complicated physics of sailing and sail trim. What begins as reductive talk on foils (air, sails, hydo, keel) soon degenerates into numerical formulas and the obligatory mention of aeronautical engineers. Really, the head can spin. And while everyone seems to know that sails work like airplane wings, the crowd really thins out when you ask how it is that airplane wings work.

Simply put, sails, like airplane wings, are curved foils. When air travels over the convex (curved) portion of the foil it travels faster, having to span as it does a greater distance. This higher velocity results in a lower pressure relative to that on the under surface of the foil (Bernoulli's Principle). Since high pressures try to push into lower pressures in an attempt to equalize, a force is produced. This force is called Lift Lift depends on the shape of the surface, the speed of the flow, and the angle (called the angle of attack) at which the foil meets the airflow.

As with the airplane wing, lift is produced perpendicular to the boom by both reduced pressure on the lee (non-windward) side of the sail, and reaction forces from the deflection of the wind by the angle of attack. Maximizing this effect is the job of sail trim. Novice sailors soon learn the importance of the headsail and mainsail working together to shape the air that flows between them. The slot is the area between the two sails. The headsail helps to accelerate the air across the leeward surface of the main helping it to produce more lift.

Generally, the headsail should be trimmed for course and conditions first, and then the main. An easy rule of thumb for sail trim? The farther off the wind, the farther the sails are out from the boat's centerline. The closer to the wind, trim as close to the centerline as the wind and boat will allow without luffing (fluttering the leading edge-the luff-of the sail).

An effective way to tell if your sails are trimmed properly on any given point of sail except when running (sailing directly downwind) is the use of tell-tales. Tell-tales are small pieces of yarn attached to the sail. If the outside telltale flutters, let the sail out. If the inside tell-tale flutters, sheet the sail in.

For a refresher on the points of sail, check out www.sailingusa.info/sail_trim.htm. For simulated sailing fun, "Take the Helm" at www.nationalgeographic.com/volvooceanrace/interactives/sailing/index.html

Class dismissed. Now go sailing.