- There are as many sailboats keel designs than there are boats. For a keel, the objective is to offer a lateral plan to counteract the wind pressure on the sails by presenting a form below the waterline to help prevent sailing sideways. The keel produces the hydrofoil action, influences handling and steering, and offers a location for ballast. But why so many options? If our concern is towards relatively small sailboats, we can attribute the evolution to going from workboats to cruising boats and onto racing.
- Note: Also on the Blog.
Swing keel; Centerboard.
- From the immemorial time, the keels were long and adapted to the type of boats, commercial activities, conquest
but mostly for building consideration. The possibility to be launched from the beach, deep-sea fishing, and private enterprises, they all show that feature which offers directional stability, ability to hove -to, all included with the logical thinking of the time. They did not get involved, but superficially into the elements of keel action to resist the drift force.
- Colin Archer's Norwegian Rescue boats, showing the underwater profile of a long keel.
- Keels remain fins of low aspect ratio. Over time, influenced by competition and if the vessel is not of a type like the above rescue or a pilot boats, the tendency led to cut down the surface area. The longitudinal section of the keel as seen on Jullinar, she was the first yacht in which for instance, the length of the keel was less than half that of the waterline. Gloriana in the USA shows a markedly modified bow with shallow lines, eliminating completely the forefoot compared to the vertical or near-vertical stem usually associated with boats of that time.
TYD#231. Long keel, steel cruising boat. TYD#966. Notice the aft keel-bite.
- For the next step, one has to look at America's Cup boats to see that, with the introduction of the International Rule after 1900, keels and rudders being part of the sum underwater, their surfaces became smaller. The triangular keel shape over the next sixty years became the norm. The lateral profile with the combination of the hull, keel, and rudder gave the high lift-drag ratio needed for efficiency to go to windward. Adapted to ocean racers like the Olin Stephens designed Dorade, the shape remained unchanged, and differences were only about the aspect ratio in relation to the draft.
Twelve Meter Weatherly. An America's Cup winner.
- The efficiency of a hydrofoil like a keel goes up with its aspect ratio. From 1 to 2 for example, the lift-drag ratio increases by 50%. The importance of the fin is clear, contributing up to 80% of the lateral force. The problem is to reduce the resistance of the main hull which contributes 75% of the drag at heeling angles. With reduced area either in the hull or in the keel and with deep draft keeping the yaw angle low, the speed to windward will increase
- The lateral plane of keelboats with the attached rudder can be further defined by the proportion of the projected area against a rectangle limited by the load waterline and depth. A 6 Meter (black) will typically have a coefficient of .47 against the backdrop. An ocean racer (green) .62 and for comparison, a long keeled centerboard will use 74% of the rectangular surface. With the reduction of the length of the lateral profile, the question of the longitudinal section of the keels remains. The object is to get a high lift and low drag by choosing a shape and thickness appropriate for the condition. It is only after the War that the NACA. wing sections began to see their use and gradually adapted to keels. The typical triangular keel did show some form of streamline, but it is with the introduction of NACA that proper foil became the norm.
- The stalemate lasted a long time. Then gradually the separation between the keel and rudder became prevalent. Period of experimentation with at first skeg and rudder then, with the coming of age of the suspended spade rudder. This essay is about keel, but with the new roles maybe some explanation is required. The advantage of a skeg: to increase the lift coefficient of a given area and to extend the angle at which stalling occurs. The pursuit against wetted surface allowed the combination skeg with rudder to have only 75% of the area of a spade blade.
- Then came the 1983 America's Cup with Australia 2 winning the event. Barely, but certainly helped by her revolutionary wing keel. Under the Twelve Meter rules depth is limited, but by spreading the width of the keel at the bottom when the boat is heeled over, the lower center of gravity plus the increase in draft gave A2 the advantage to win the Oldest Sporting Trophy in the world. It did not take long to see variations of the wing keel on just about every production boats.
(to be continued).