For learners in a keelboat, 7 to 10 knots is ideal and 17 knots or more is for experienced crews with a reef in. Above about 22 knots, inexperienced crews should stay in harbour, and in a gale (34 knots or more) everyone should stay in port.
There is no single figure, because it depends on the boat, the crew and the sea, but these are sensible guides for a cruising yacht of about 8 to 10 metres:
- Under 4 knots: not enough to sail well.
- 4 to 10 knots: gentle sailing, and the best range for learning.
- 11 to 16 knots: good sailing for a competent crew, with a reef ready as it builds.
- 17 to 21 knots: first reef, and a crew with some experience.
- 22 to 27 knots: second reef. If you are inexperienced, stay in harbour or stay ashore.
- 28 to 33 knots: deep reefed and best avoided.
- 34 knots and over: a gale. Stay in port.
Small dinghies have lower limits, because they rely on the crew's weight to stay upright. Many beginners find 12 to 15 knots a lot in a dinghy.
Look at the gusts, not just the average. A forecast of 15 knots gusting 25 deserves the same respect as a steady 25, because the gusts are what knock a boat over and break gear. Gusts are usually strongest near land.
The sea state matters too. Wind blowing against a tidal stream, or over a long stretch of open water, builds steep waves that make a moderate breeze hard work. A sheltered harbour in 18 knots can be easier than an exposed channel in 14.
Finally, think about the whole trip. The wind usually feels lighter sailing downwind, so a pleasant run out can mean a hard beat home. Check whether the wind is forecast to increase during the day, and decide before you leave. If you are unsure whether it is too windy, stay ashore or wait for the wind to ease.
More in the book: Chapter 10, When the wind gets up