Twin keel

Twin keels or bilge keels are two keels that emerge at an angle from the hull of a sailboat (and some ships), at or near the bilge. The angle allows the boat to have a shallower draft while still allowing for minimum leeway while sailing. The placement of the twin keels also allows the boat to stand upright when out of the water without additional support, as opposed to a single-keeled boat that would fall over if water levels dropped. Twin-keeled boats are typically used in coastal areas that experience extreme changes in tide. When the tide is low, the boat will sit on her keels and remain stable and upright. This configuration is especially useful for sailors in Britain and might in the future be applied in the parts of the Atlantic North America that are extremely tidal such as the Fundy waters that are shared by Maine, New Brunswick, and Nova Scotia.

Twin keels are meant to make sailing safer and easier in shallow or extremely tidal waters while preserving the stability and performance of deep-keel vessels. Yet, twin keels are often shown to provide better theoretical performance on paper, and, in reality, are only slightly slower performing when compared with single keels on identical vessels. They are also said to be safer in downwind conditions as they are more resistant to rolling, and hence should not broach as easily. (Broaching can occur when a boat "surfs" down a wave and its stern gets lifted and pushed by the wave, causing the vessel to turn sharply, and is then rolled over by the wave.)

The first twin-keeled sailing vessel is widely believed to be the Bluebird that was built by Lord Riverdale and launched in 1924. Despite being the first twin-keeled yacht, and being built without much in the way of guiding engineering and science, it was quite radical as it had twin rudders (as racers have today), and a long narrow hull with a pointed, or "canoe," stern. This gave her speed and seaworthiness, and the ability to point closely to the wind. The vessel's other characteristics included self-steering ability, efficiency under power, and structural strength. The most notable benefit presented at the time was care-free grounding that includes the ability to sail away from a grounding incident by righting the vessel.

Riverdale succeeded the Bluebird with two vessels named Bluebird of Thorne. The first was a step backwards in that it re-introduced a normal keel as a place for ballast and to collect bilgewater. It is advanced, nonetheless, as its fins were aft of the center keel, and its rudders extended from the fins.

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Wikipedia