Sailing Theory
Sailing theory describes how the wind generates the driving force of a sailing boat. Only the interaction of sail, hull, and keel or centreboard makes it possible to sail at an angle into the wind.
True and Apparent Wind
The true wind is the wind felt by a stationary observer. As soon as the boat moves, the boat's wind is added, blowing against the direction of travel. The combination of both gives the apparent wind. This is the wind that acts on the sails and to which they are trimmed.
On close-hauled courses the apparent wind is stronger and comes more from the front than the true wind. On a downwind course it is weaker and comes more from behind.
Forces on the Sail
The sail acts like a vertical wing. On the windward side the air is dammed, and on the leeward side a suction is generated. The pressure difference produces a force that acts perpendicular to the sail: the sail force.
This sail force can be split into two components:
- Driving force in the direction of travel.
- Lateral force perpendicular to the direction of travel.
On close-hauled courses the lateral force is large and the drive small. On a beam reach the driving force is greatest.
Effect of Keel or Centreboard
If only the sail force acted, the boat would mostly drift sideways. The keel or centreboard under water opposes the lateral movement with a large resistance, but the forward movement only with a small one. Only this interaction makes sailing forward — and therefore beating to windward — possible.
Heeling and Righting Moment
The lateral force of the sail makes the boat heel to leeward. On keelboats the deep-set ballast keel produces a righting moment that counteracts the heeling moment. On dinghies this force must be applied by the crew hiking out.
Sail Trim
For the sail to work optimally, it must be at the correct angle to the apparent wind:
- Sheeted in too far: the sail stands too flat, doesn't luff but slows the boat down with too much side force.
- Sheeted out too far: the sail luffs at the leading edge, the boat loses speed.
- Trimmed correctly: the telltales on both sides of the sail stream horizontally aft.
Rule of thumb when luffing or bearing away: trim the headsail first, then the mainsail.