Boat Heading vs Sail Set and Forces on the Sail

The set of your sail depends on your sailing angle to the wind. You’re working to maximize the aerodynamic forces on the sail.

2.1.1 Going Downwind

Contrary to a wing, when heading in downwind directions, the sail is not acting 100% like an airfoil of a wing. If it was to act like an airfoil the sail would have to swing forward of the mast spreaders and side stays. Practically, the farthest we can let out a sail is to a point where the sail is touching the side stays. In this case, you can liken the sail to holding up a big sheet of plywood perpendicular against the wind and having the wind PUSH you downwind.

In a couple of knots of wind, holding a sheet of plywood against the wind is going to bowl you over. Hold its edge to the wind, and you’ll hardly feel anything. So in this instance of going downwind, you’ll want the sail perpendicular to the wind, i.e. no airfoil here!

2.1.2 Going Upwind

Going as best as you can upwind (30-40 degrees off the wind) is the complete opposite. You want to maximize the lifting capability of the airfoil shape just like in the wing-to-sail morphing animation in Module 1.

2.1.3 Plywood vs Wing

Thus at every position between heading at your best upwind angle (30-40 degrees away from the wind) and heading directly downwind, is a gradual transition between an airfoil and a sheet of plywood. Without getting completely mathematical, depending on the boat and sail pattern and things, somewhere about 90-120 degrees off the wind you’re getting pulled by the sail’s leading edge airfoil shape and pushed by the wind against the sail.

To demonstrate the concept of sail set vs. wind angle, we created NED, the sailing instructor. Visit NED on the next page.