How Wind Moves a Sailboat
Sails are wings. The keel resists sideways force. The combination produces forward motion at angles the wind has no business pushing you toward.
Two forces, working together
A sailboat moves because the wind does two different things to the sail depending on what direction you're heading. Upwind and on a reach, the sail works like a wing. Downwind, it works like a parachute. Both are at work most of the time.
Lift: the wing effect
When the wind comes from the side, air flowing across the curved sail moves faster on the leeward side and slower on the windward side. That pressure difference generates lift — the same force that holds an airplane up — pulling the boat forward and slightly sideways. The keel underneath the hull converts that sideways pull into pure forward motion.
Drag: the push effect
Sailing directly downwind, the sail can't generate much lift; instead, the wind simply pushes on the back of the sail like wind on a flag. This is slower than reaching, which is why downwind boats often sail a series of broad reaches and jibes instead of going dead downwind.
Why you can't sail straight upwind
A sail can only generate lift when air flows across it at an angle. Point straight into the wind and the sail flaps uselessly in irons. The closest most boats can sail to the wind is about 45°; to reach an upwind point you tack back and forth through that arc.
Apparent wind
The wind you feel on a moving boat — the apparent wind — is different from the true wind. The boat's own speed adds a headwind component, so apparent wind on a fast-moving boat is stronger and more forward than the true wind. Trim sails to the apparent wind, not the true.
Takeaways
- Upwind, the sail works as a wing generating lift.
- Downwind, the sail catches wind like a parachute.
- You sail to the apparent wind, which is the combination of true wind and your own boat's motion.
Frequently asked questions
- Why is a sailboat faster on a reach than downwind?
- Reaching uses the wing effect of the sail, which generates more force than the parachute effect of dead downwind. A boat can often sail faster than the wind on a reach but never downwind.
- What is being 'in irons'?
- A sailboat stopped head-to-wind with the sails flapping. The fix: push the tiller hard over to one side and let the boat drift backward until the bow swings off the wind.