A keelboat is very hard to capsize, because her heavy keel pulls her back upright, harder the further she heels. A strong gust can knock her far over, but she will come back up. Only large breaking waves offshore, or flooding, are likely to roll or sink one.
Under the hull of a keelboat is a fin keel carrying several tonnes of lead or iron. When the wind heels the boat, the keel swings out to windward and its weight pulls her back towards upright, and the further she heels, the harder it pulls. This is why a keelboat feels so stable once she has heeled a little, and why yachts are described as self-righting.
What can happen in harbour and coastal sailing:
- Heeling hard in a gust. An overpowered boat may heel steeply and round up into the wind. It is alarming, but the sails spill wind as she heels and she comes back up. Easing the mainsheet helps at once.
- A knockdown, where the mast goes down near the water. This is uncommon and usually comes from a violent squall or a broach with a spinnaker up. A well-found yacht recovers, but loose gear and people can be thrown about, and water can get in through open hatches.
A full capsize, with the boat upside down, is rare and happens mainly offshore, when a large breaking wave hits a boat side-on. Keeping hatches and companionway boards closed in rough weather keeps water out and helps her come back up quickly.
A keelboat that fills with water will sink, because of her ballast, unless she has built-in buoyancy. That is why the hull fittings, seacocks and bilge pumps matter.
Dinghies are different. With no heavy keel, they rely on the crew's weight to stay upright, and capsizing is a normal part of learning. They float when capsized, and the rule is to stay with the boat. Multihulls are very stable upright but, once capsized, they float upside down and cannot right themselves.
To avoid trouble in a keelboat, reef early, keep a hand on the mainsheet in gusty conditions, and keep hatches closed when it is rough.
More in the book: Chapter 3, How sailing works