⛵ HAPPY SAILING/ How Sailing Works — Lift, the Keel & Sailing Upwind
NO-GO ZONE WIND track sail force DRIVE side force keel grip
What's happening
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The forces on the boat
Total sail force
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Forward drive
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Side force
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Making way
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Leeway (slip)
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The keel
Keel down
Turn it off to see the boat slip sideways.
Steer relative to the wind
Angle to the wind45°
0° into windclose-hauled ≈45°across 90°
Wind strength14 kts
lightfresh
Why the sail pulls — it's a wing
WIND low pressure — pulls ↑ higher pressure — pushes ↑ LIFT
Wind bends around the curved sail. It speeds up on the outside (low pressure, a suction) and slows on the inside (higher pressure). The two together pull the sail sideways — the same lift that holds up a plane.
How a boat sails upwind

1 · The sail is a wing

A sail is not a bag that catches wind and gets pushed. Set right, it is a wing stood on end. Wind flowing across its curve speeds up on the outside and slows on the inside, and that pressure difference pulls the sail — mostly sideways. This pull is the sail force (the red arrow).

2 · Split the force in two

That one red pull does two different jobs. Split it along the boat and across it:

• Drive — the part pointing where the bow points. This is what moves you.
• Side force — the part pushing sideways. This just tips the boat and shoves it sideways.

Close to the wind the pull is mostly side force and only a little drive. That sliver of drive is still enough — if something stops the sideways part.

3 · The keel does the trick

Under the boat is the keel — a deep fin. Water is hard to push sideways through, so the keel refuses the side force (the blue arrow) while letting the boat slide forward easily. Kill the big sideways part and the small forward part is all that's left: the boat goes forward.

Flip the keel off in the panel and watch: the same wind now just shoves the boat sideways. The keel is what turns wind into a course.

4 · So you can sail toward the wind — but not straight at it

You can point to about 45° off the wind and the wing still works. Any closer and the wind hits the sail head-on, it just flaps, and there is no lift at all — the no-go zone. To reach something dead upwind you zig-zag across the no-go zone in a series of 45° legs. Turning the bow through the wind from one side to the other is a tack.

Try this

• Start at 0° — the sail flaps, no force, no drive.
• Bear away to 45° — the sail fills, drive appears, you sail upwind.
• Swing out to 90° (wind across the boat) — drive is at its biggest.
• At any angle, toggle the keel off and watch the green track arrow swing sideways.