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A twin keel is a sailing boat design which incorporates a pair of angled fins, or keels, one on either side of the bilge or lowest part of the hull. The keels are attached to the hull fairly close to the center line and angle outwards, generally ending beyond the gunwale line. Twin keel hulls feature several benefits including relatively shallow draft, reduced leeway motion and, in areas with extreme tide change levels, the ability to stand unsupported on the exposed seabed at low tide. The design does suffer from performance penalties in light wind conditions due to the increase in water contact surface area, which has dampened interest and demand for twin keels in certain countries. Advancements in marine construction and materials have, however, largely done away with this disadvantage.
Conventional yacht and sailboat hulls feature a plunging central keel, or fin, which lends the craft stability against the heeling motion imparted on it by wind pressure in the sails. Twin keel hulls have two separate keels which are located at the lowest point of the hull or bilge. Also known as bilge keels, these two keel members attach to the hull close to the center line and are inclined outwards typically ending at or a short distance beyond the gunwale profile. This keel arrangement was first designed and built in the United Kingdom in the 1920s and proved, over a number of years and design improvements, to be very effective under local conditions.
One of the main benefits of the twin keel hull design, and one particularly appreciated in its country of origin, is the ability to support itself when beached. The extreme tide change levels in the UK often require craft to be moored far out if they are to remain afloat at low tide. The twin keel hull, on the other hand, may be moored closer to shore and will stand firmly, supported by the two keel members, on the exposed sea bed when the tide is out. The design also allows for a shallower draft, or depth of hull, than single keel hulls.
Another major plus with twin keel boats is the reduction of leeway movement. Leeway is a boat's tendency to drift sidewards away from the wind due to the pressure of the sails. Twin keels offer better directional stability with less leeway motion than single keels. A single keel also becomes a less effective stabilizer as the heeling or leaning angle of a boat under sail increases. A twin keel exhibits the opposite reaction and becomes more efficient with an increase in heeling motion.
Twin keel hulls also tend to be faster due to improved hull hydrodynamics under sail. The design creates a flatter wake and is often up to 20% faster than rated hull speeds. Twin keels do tend, however, to lose some performance under lighter wind conditions, especially when compared to shallow single keel designs. Fortunately, advancements in the fields of marine design, construction, and materials have seen great improvements in this regard, thereby leading to renewed interest in the design.