Sailing to Windward


How can a sail boat sail upwind and what limits how close to the wind direction it can go?


The short answer on sailing to windward is that the sail is shaped a bit like an airplane wing, sort of bellied out to the side which is away from the wind direction, and this airfoil shape creates a force in the direction of the bulge in the sail. In the case of an airplane wing where the bulge is upward we call the force lift. In the case of a sail where the bulge is horizontal we might call the force pull. To keep this shape the sail must not be pointing straight into the wind. If we let the sail point into the wind the excess sailcloth just flaps back and forth, not creating the airfoil shape.

windsurfer What happens to create the pull is that the sail splits the air stream passing over it into one part that travels along the curve of the sail, around the bulge, and another part which takes a shortcut straight across from the leading edge to the trailing edge of the sail. Since the curved path is longer than the flat one and the air must move faster along the downwind side of the sail. When air is moving fast, more of its energy is tied up in its speed and less in its pressure. The difference in pressure between the outside of the bulge in the sail and the inside shows up as this pull mentioned earlier.

So now we have a pull centered on the sail, pretty much horizontal and perpendicular to the boom (that stick along the bottom of the sail). For this pulling force to make the boat move forward, the boom must be at an angle to the centerline of the boat so that part of the force is directed forward. The other part of the force will be in a direction to tend to pull the boat sideways through the water. The fin on the bottom of the boat offers a lot of resistance to sideways motion and very little to forward motion so the sideways force does not have much effect on the boat's motion.

Now we get to the second part of the question. The sail must be kept at an angle to the wind to keep its airfoil shape. Then the boat must be kept at an angle to the sail so that some forward force is available. Adding these two angles together will give you the total angle the boat must be off the wind in order to move forward.

Those of you who are knowledgeable sailors will recognize that I have left out a lot of stuff about relative wind, righting moment, circulation, turbulence etc. For those of you who want to know about these things, there is a sail theory site which might be of some interest.