Keel Shapes and Types
What’s the deal with all these keels?
No matter the shape, all keels have one thing in common – their purpose is to keep a boat from slipping sideways in the water. The modern sailboat keel owes its origins to the leeboard. As the name implies it was a board hung over the leeward (downwind) side of a boat.
Later, this board was moved to the center of the boat and became a dagger board. While the dagger board is a thin piece of wood where extra weight or ballast was needed on larger boats, it morphed into the swing keel.
Swing keels have the advantage of shallow draft with the board up and deep draft and low center of gravity with the board down.
When it was recognized that draft was not an issue, the deep fin keel (made of steel or lead) came about. Sailors ultimately want the performance of a fin keel, but a shallower draft.
Thus, the bulb and wing keels have gained popularity. The bulb keel puts a generous mass at the bottom of a shorter fin while the wing version molds that bottom mass into a hydrodynamic wing – literally pulling down on the boat as it moves through the water.
Not forgotten are full keel sailboats that have no external fin and keep their ballast internal to their hull’s shape.