North Sails One Design

How do sails work?

Article by Paul Bogataj

"Rote knowledge of how to trim a sail is one thing but really knowing how a sail works is quite another. The information contained in this article,
written by one of the top aeronautical engineers in North America, is designed to give you the knowledge you need to make sail trim decisions on
the fly and on the racecourse. After digesting the information in this article you will be able to think your way around the racecourse and make intelligent trim and tuning decision that will improve your boatspeed."

Chris Snow
J/24 National Champion

Sails are wings that use the wind to generate a force to move a boat. The following explanation of how this occurs can help understand how to maximize the performance achieved from sails.

Sails are Flexible Wings
It is useful to recognize what a typical sail is. They are normally built from a flexible material in order to allow the sail to work with the wind on either side to allow tacking. This is a significant restriction that prevents many shapes from being built because they would not be able to support themselves in the wind. This leads to the traditional triangular planform of sails, since the material below has to hang from the material above, which eventually is reduced to a point at the top of the mast. So, the problem becomes how to build and operate a flexible sail in the wind to produce a substantial force component to move the boat.

As the restriction that sails support themselves is diminished (full battens and stiffer materials for example), sails can evolve to be more efficient. Their appearance then becomes more wing-like and less sail-like. Analyzing how a sail works as a wing is useful, not just for modern sails that look more like wings, but also for very traditional sails that, while they look like sails, operate very much like wings.

Velocity and Pressure
Flow accelerates over the top surface of an airfoil, either because it is at an angle to the flow, or because the top has more curvature than the bottom, or both. When a fluid (like air or water) is accelerated, the pressure that it imparts on an adjoining surface decreases. This lower pressure pulling upward on the upper surface of a wing produces lift.

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