A keelboat sails well heeled up to about 15 to 20 degrees. Beyond that she spills wind, becomes hard to steer and slows down, so reduce power by easing the mainsail, flattening the sails or reefing. Small dinghies are sailed nearly upright.
A sailing boat heels because the side force from the sails acts high up on the mast while the keel resists it far below the water. On a keelboat, the weight of the keel swings out to windward as she heels and pulls her back upright, more strongly the further she leans. At some angle the two balance and she sails along steadily, heeled over.
Some heel is normal, and on a keelboat around 10 to 20 degrees upwind is comfortable and efficient. More heel feels exciting but is slower. Past about 20 degrees on a typical cruising yacht:
- The sails spill wind and lose drive.
- The rudder loses its grip, and the boat gets heavy weather helm, pulling hard to windward.
- The keel works less well, so she makes more leeway, sliding sideways.
- Life on board becomes hard work, and it is easier for someone to fall.
To reduce heel, start with the crew sitting on the windward side. Then take power out of the sails: ease the mainsheet or drop the traveller, flatten the sails with the outhaul, cunningham and backstay, and let the kicker off a little so the top of the mainsail twists open. If you are doing this all the time, reef.
Dinghies are different. They have no heavy keel, and the crew's weight is what holds them up, so they are sailed as close to upright as possible. Letting a dinghy heel much is the usual first step towards a capsize.
Heel also varies with the point of sail. Close-hauled is when a boat heels most; on a broad reach or a run she sails much more upright, and because the apparent wind is lighter downwind, the wind can feel lighter than it really is.
More in the book: Chapter 3, How sailing works