The keel does two jobs: its weight stops the boat blowing over, and its shape stops her sliding sideways through the water, so the sideways pull of the sails is turned into forward motion.
The keel is the large fin under the middle of a yacht. It does two separate jobs.
The first is stability. A keel carries a heavy weight of ballast, lead or cast iron, low down; on a cruising yacht it is often a third to a half of her total weight. When the wind heels the boat, the keel swings out to windward and its weight pulls her back upright, more strongly the further she heels. This is what lets a keelboat carry a lot of sail without blowing over, and what brings her back up when a gust passes.
The second is resisting leeway. Most of the force produced by the sails acts sideways rather than forwards. Without something to resist it, the boat would slide across the water. A deep, thin fin is very hard to push sideways but slips forwards easily, so it resists the side force and leaves the forward part, the drive, to move the boat ahead. As the boat moves, the keel meets the water at a small angle and works like a wing, producing its own lift to oppose the sails. Even so, the boat slips sideways by a few degrees. This is called leeway.
Keels come in several forms: the deep fin keel of most modern yachts, often with a bulb of ballast at the bottom; the long keel of many older cruising boats, which is steadier on a course but turns less sharply; and lifting or swing keels on boats that need to go into shallow water. Dinghies have a centreboard or daggerboard instead, which resists leeway but carries no ballast, so the crew’s weight keeps the boat upright.
The keel also sets the boat’s draught, how deep she is in the water, which you need to know whenever you sail in shallow water.
More in the book: Chapter 3, How sailing works