How sailing works

True and apparent wind

The true wind is the wind a stationary boat feels. As soon as the boat moves, the wind created by its own speed is added. Together they make the apparent wind, which is what the sails really work with. Upwind it is stronger and comes from further ahead, downwind it is weaker.

Points of sail

Points of sail. Grey: the sector of about 45° either side of the wind in which a sailing boat cannot sail.

In irons (no-go zone): within about 45° either side of the wind a sailing boat cannot sail, the sails flog.
Close-hauled: as close to the wind as possible, sails pulled in. To reach a point upwind you beat, tacking from side to side.
Beam reach: wind from the side, around 90°. Often the fastest and most comfortable course.
Broad reach: wind from behind at an angle, sails well eased.
Running: wind from straight behind. Watch out for an accidental gybe.

Why a boat can sail upwind

Close-hauled, air flows around the sail. On the leeward side it flows faster and creates low pressure. The force pulls forward and sideways. The keel under water stops the boat from sliding sideways. What remains is forward drive and some heel.

Downwind, by contrast, the sail acts like a parachute and the wind simply pushes.

Trimming with telltales

Telltales on the headsail show how the air flows. The aim: both, inside and outside, stream evenly.

Windward telltale flutters: sail too far out or steering too high. Sheet in or bear away.
Leeward telltale flutters: sail too tight or steering too low. Ease the sheet or head up.

Manoeuvres

Head up / bear away: Turn the bow towards the wind or away from it.
Tack: The bow turns through the wind. Calls: "Ready about?" – "Ready!" – "Lee-ho!"
Gybe: The stern turns through the wind. The boom swings across, so it is controlled with the mainsheet.
Shoot up: The boat turns into the wind and loses way. Useful for stopping, for example at a mooring buoy.