The Truth About Twins
As early as the 1940s the real and potential advantages of twin-keel sailboats were being acknowledged by certain authorities. In 1951 author and Naval Architect, Douglas Phillips-Birt wrote, "Twin keels may prove to be the most exciting breakthrough in sailing design that has occurred since the Bermudan rig." He went on to list numerous technical advantages of the twin-keel design, from their capacity to be of asymmetrical form because each keel performs its primary function on one tack only to their greater resistance to pitching. In twin keels, Philips-Birt and others were certain they saw the future. What, then, caused interest in twin keels to stall? We asked Naval Architect and twin-keel expert Patrick Bray.
Bray suspects part of the declining interest is due to incorrect terminology. He notes that there are actually two types of keel arrangements that sport multiple fins: twin-keelers, which typically have two heavily ballasted fin keels, and bilge-keelers, which usually have two lighter fins and an additional heavy center stub keel. Many of the early boats described as twin-keelers were actually bilge-keelers. While bilge keels undoubtedly offer many of the advantages of the more modern twin keel arrangement, there are some significant differences.
"Early bilge-keelers with the three keel system suffered from excessive wetted surface," Bray says, "which made the boats slow in light airs. The large surface area caused high drag at low hull-speeds. Still, this system was used as molded hulls already had the center keel in place, and the costs to change the molds were significant. Also, hull design had not graduated to the degree it has today, and architects did not see the hull as a separate body from the appendages. Hulls had deep wine glass sections with a rudder hung off the back of the keel. How else could you fashion a hull?"
Bray says it was not until the advent of the modern fin keel and a better understanding of hydrodynamics that the twin keel could be applied in a way that would use their features to advantage. "Unfortunately, by this time bilge keels had already shown their disadvantages to the world and the confusion over twin keels and bilge keels had left a negative impact on the world. Today our designs use twin keels to advantage in both sail and long range motor yachts."
It's said the first twin-keel sailboat was Lord Riverdale's 25-foot Bluebird of Thorne, built in 1924. She was plenty criticized but in 1939 Riverdale built a bigger Bluebird (50 feet) with an important refinement her keels were not parallel to the centerline, but canted outward instead. This was significant. It meant that the twin keels became more effective as the angle of heel increased just the opposite of the traditional single keel, which becomes progressively less effective.
Following this early breakthrough, twin keels have evolved to the point that proponents presently claim a number of performance advantages over the modern fin keel. Patrick Bray offered the following list:
1. Higher sailing speeds. Because modern twin keels are of high aspect ratio and present less wetted area than a full keel or long fin keel.
2. The lateral plane appendage efficiency increases with angle of heel. Bray also notes that the windward keel is working more horizontally, creating downward lift that increases righting moment and giving more power to carry sail.
3. Stability is equal to that of an orthodox yacht without the need for extreme beam, and the righting moment and range of stability are at least equal to those of a well-designed centerboard yacht of relatively deep fixed draft.
4. The wave pattern of twin keels reshapes to reduce the fore and aft crests. At hull speed a hollow forms amidships, but the keels cause a wave to form in this hollow, canceling out the stern wave and giving a flatter wake. This increases the maximum speed of the hull by as much as 10%.
5. The deep plunging of an ordinary hull is avoided by the stabilizing action of the fins, which are also very effective at dampening out rolling motions.
6. Speed and fuel consumption under power are better. The prop can work in clear water without being shrouded by the keel and rudder. The twin-keel yacht can also be easily controlled in reverse, something seldom true of single-keel yachts.
7. Windward ability equal to that of an ordinary yacht is achieved on a fixed draft approximately comparable to that of a centerboarder without the problems associated with lifting foils. Windward performance in rough water is improved because of the roll and pitch dampening abilities of the keels.
Experts have also noted that twin keels track better. The additional fins and sometimes center skeg provide excellent directional stability, which is particularly advantageous in strong following seas. And finally, the most basic advantage of twin (or bilge) keels is that they are designed to enable many boats to take the ground in the level position. This makes them ideal for areas with extreme tides and explains their continuing appeal in places like the United Kingdom.
There are, of course, plenty of experts who question the virtues of the twin-keel designs. In his book The Coastal Cruiser, author Tony Gibbs refers to twin-keel boat owners' claims of good windward performance as "wishful thinking," and Richard Henderson says in his Cruiser's Compendium, "...despite some theoretical arguments for the superiority of a twin-keeler in windward performance, a boat with a single fin will nearly always reach the windward mark ahead of the twin-keeler during a race."
Admittedly, a twin-keel boat isn't for everyone. They don't offer the flexibility of a centerboard, daggerboard or leeboard boat, and they often (particularly older bilge-keel boats) won't sail to weather as well as a comparable fin-keeled design. But in most cases the criticisms of twin-keel designs stem from performance comparisons with deep fin keels. For the small-boat sailor seeking a seaworthy, shallow draft cruiser, the twin-keeler might be worth a second look. Joshua Colvin
Bray Yacht Design: www.brayyachtdesign.bc.ca
