What is a keel, and what does it do?
The keel is the central fore-and-aft structural member along the bottom of the hull—the boat’s backbone. On sailing boats it extends into a fin or full keel that resists sideways slip and carries ballast, and its lowest point is what draft is measured to.
Related terms
How it works
Structurally, the keel is the spine the rest of the hull is built around. On many sailboats it continues downward as an external fin—full, fin, bulb, or wing shaped—while dinghies and some cruisers use a retractable centreboard instead of a fixed keel.
Under sail the keel does two jobs at once. Working as a foil in the water flow, it generates the sideways grip that keeps the boat from skidding to leeward; carrying ballast low, it lowers the centre of gravity so the boat stiffens against heel and returns upright. Both jobs need water moving past the keel, which is why a slow boat slides sideways more.
Backbone, fin, and ballast
The word originally named the first timber laid when building a ship—the backbone every frame attached to. As sailing yachts developed, designers merged that structural spine with the lateral plane and the ballast into a single underwater appendage, which is why one word now covers both a structural member and a hydrodynamic fin.
On the water
- A yacht drawing 2 m measures that draft to the bottom of her keel, not to the hull skin.
- Close-hauled, water flowing past the keel supplies the lift that limits leeway.
- An unballasted dinghy relies on a centreboard for lateral grip and on crew weight for righting.
Common mistake
The keel is not just dead weight. It is also a foil: without water flowing past it there is little sideways grip, so a boat makes the most leeway when she is moving slowly.
Remember it
The KEEL is the backbone below: it keeps the boat on her feet and on her track.