Why Twin Keels?

John S. Letcher, Jr.


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Twin keels are a pair of smaller fins on the bilges in place of a single, central keel. Draft is typically two-thirds that of a single keel. The idea that a sailing yacht might have a pair of smaller ballasted fins mounted on the bilges in place of the usual central ballast keel originated with H.A. Balfour (later Lord Riverdale) in England in 1922. By 1924 he had built himself the 25-foot twin-keel, twin-rudder Bluebird to try out the idea, and by all accounts she was a perfectly successful small cruiser.

The idea eventually caught hold in Europe, particularly in England, where to date at least 60 stock classes with twin keels have been produced in varying numbers. Twin-keelers now make up a substantial fraction of the population of small cruisers in Europe. In the meantime, a number of larger twin-keel yachts have been built, the best known of which is Bluebird of Thorne (504" x 40'0" x 12'1' x 5'1') sailed by Riverdale three times across the Atlantic, and later to New Zealand.

In the United States, twin keels have enjoyed far less acceptance. I'm aware of only two stock classes that have been produced in this country, and I suppose twin keels make up only a fraction of one percent of our vast fleet of small cruisers. When faced with the need of shoal draft, Americans have voted overwhelmingly for their own invention, the keel-centerboard boat. In considering the advantages and disadvantages of twin keels, I will be comparing them with both the fixed keel and, keel-centerboard types. In relation to the keel-centerboard combination, twin keels have the advantage of simplicity: a simple molding, no moving parts, no holes in the hull and consequently a basically lower cost. Perhaps we can find some logical explanations for the great difference in preferences on the two sides of the Atlantic.

My personal experiences with twin keels have convinced me that the type is worthy of very serious consideration for coastal or offshore cruising. In 1965 I brought my 20-foot fin-keel Island Girl home to California from a three-summer voyage to Hawaii and Alaska. I had met an adventurous girl whom I wanted to marry and take to sea on a similar, farther-ranging voyage. So, on the strength of my experiences in Island Girl, some readings in naval architecture, and my training as an aeronautical engineer, I embarked on the design and construction of 25-foot Aleutka: a slim, light-displacement, double-ended cutter. I planned on twin keels from the very beginning. Yard bills for haulouts had been Island Girl's major maintenance expense; this time, why not use the tide? I had had four occasions to build a cradle for Island Girl; wouldn't it be nice to have a boat that didn't need one?

But I think my main reason for thinking of twin keels may have been a desire for freer contact with the shore. A fin-keel boat is bound to deep water. Any contact with the bottom is so likely to result in a predicament, with the boat lying on her side for half a tide, that she must sail with a large margin of safety most of the time, keeping far from shore, or right in the middle of the channel. In essence, she must keep clear of all kinds of tempting places where the bottom just might be too close to the top.

The only way to land from a keel boat is by dinghy, first finding an anchorage with plenty of water, which might turn out to be rather far from where you want to land. Wouldn't it be fun to have a boat that could poke right up to the shore for a landing, or bump the bottom and sail away, or anchor in water so thin that you might be sitting high and dry in the middle of the night? All this is possible with twin keels, and it is fun. Patty and I have had many occasions to use them in all these ways.

The first big advantage is, of course, shoal draft. Both twin keel and keel-centerboard boats tend to have an upright draft of around 10 to 11 percent of their overall length (a draft of 2 feet, 6 inches to 2 feet, 9 inches for an overall length of 25 feet). This is about one-third less than the typical keel boat draft of 15 to 17 percent of overall length (3 feet, 9 inches to 4 feet, 3 inches for an overall length of 25 feet).

In the tidal estuaries that provide so much of the world's accessible shoreline and popular cruising grounds, the shoal draft cruiser is likely to have twice the water area in which to cruise, twice the number of harbors in which to find shelter, and much greater flexibility in route from one place to another.

Almost any boat except most modern fin-keel racers can be tied alongside a quay or pilings to be dried out at low tide for bottom work, given a sufficient range of tide. However, with a single keel it can be a chancy operation, as the heavy boat balanced on a narrow keel is a very unstable system. If she leans just a little one way or the other as the tide goes out, she puts very heavy strains on the supports and points of contact, threatening to come down with a catastrophic crash. With twin keels, there is no danger at all, so beaching for painting has become a very routine operation for us. As no supports are needed, - we have been able to choose far more pleasant surroundings than the usually dirty, polluted harbors where quays, pilings or gridirons are provided. This capability could be an important safety feature, especially when cruising far from repair yards and assistance, allowing the boat to land safely on any convenient beach in case of underwater damage.

There have been a number of places in our travels where twin keels have enabled us to make a landing where there was no suitable anchorage. I think especially of our months in Glacier Bay, Alaska, on three successive summers. In this extensive system of glacial fiords at the northernmost end of the Inside Passage, even the good anchorages with good depth and bottom are exposed to a constant threat of drifting icebergs. However, at the mouth of nearly every stream and ravine we found small gravel beaches where, with favorable timing of the tides, we could haul out for a secure overnight berth, or for a daytime exploration of the mountains behind. We would simply drag an anchor up the beach or tie onto a boulder or a tree, so the boat could not leave without us when the tide came in.

Shoal draft simplifies moving a boat overland, as a low center of gravity makes for a stable load. Twin keels are better than a centerboard for this because the boat sits securely on a level surface, requiring no cradle. Patty and I moved twice while Aleutka was under construction, each time carrying her along in a rented stake truck. Then we had to move her to the water for launching when she was finished. Coming east from the West Coast, we crossed the isthmus of Mexico (Salina Cruz to Coatzacoalcos) on a flatcar of the Mexican National Railroad, cutting off about 2,000 miles from the distance via Panama. The arrangements for this trip were complex to say the least, and we were thankful that we didn't have to deal with the additional complication of getting a cradle constructed.

One advantage of twin keels that I never thought about much until we reached the shallow waters of the East Coast is the draft being greater heeled than upright. This means when sailing you can often get away with touching the bottom if you're prepared to react quickly. Tacking up a narrow channel is the most likely place for this to occur, when we're tempted to carry a favorable tack too far. The instant you feel her bump or start to slow down, put the helm alee. She straightens up, draws a few inches less, and usually by the time she's paid off and trimmed on the new tack, she will be back in deep enough water. If you ground on the windward side of a channel, just slack the sheets and she usually will drift right off.

Grounding on the lee side of the channel, a different tactic is required, as straightening up will only let you drift farther on. Here I have kept the sails trimmed flat, holding her in position while I recover from the surprise and size up the situation. Then I try to ascertain the most likely direction toward deeper water. I rig my small Danforth high-tensiile anchor with a light, 3/8-inch rode and give a hearty toss in that direction. I tug on the rode to set the anchor, and then take a strain on the rode. Only then do I slack the sheets or drop the sails, when she straightens up, floats free, and tows obediently out to the anchor.

Sometimes it takes a second toss. Once, in Chesapeake Bay, when I got my landmarks mixed up and ran confidently into the wrong cove, it took five or six long tosses to get back into 3 feet of water. It's a pretty lubberly operation, but with twin keels it works. Of course in a keel boat or a centerboard boat with the board up, the keel draws more and digs deeper as the boat straightens up, and unless the tide is rising you're very likely to spend several hours there.

I imagine the biggest reason for the popularity of twin keels in England is the crowding of the anchorages. Yachting became so popular after World War II that all the deep-water anchorages were filled to capacity; there was nowhere to put any more boats. Twin keels made it possible to moor in the marginal waters where the boat might be aground at low tide. You may not have complete freedom to come and go whenever you please, but at least you can own a cruising boat and keep it in the water. So far in the United States, construction of new marinas by dredging has approximately kept up with the demand for mooring space, and most anchorages are not yet so crowded that a few more deep-draft-boats cannot be fitted in.

But this same principle works wherever you cruise with twin keels. Coming into a strange port in the evening you can look around for a vacant mooring and hope that its owner isn't coming back that night. If you don't find one, in a deep-draft boat you may as well go all the way back out beyond the last line of moorings to anchor, because you know all the more convenient spots are occupied. With twin keels we go right on up through the fleet and usually find, between the first row of moorings and the shore, enough water for us to swing comfort ably. And if the tide should go out and leave us aground in the middle of the night, who cares? It will be back in the morning. Thus, almost everywhere we go, we get the best berth in the harbor closest to the landing, close enough to see what's going on onshore, surrounded, by the local fleet instead of outcast on the fringes. And when we want to be alone, we have often been able to find neat little coves all to ourselves, just because there's not enough water in them at low tide for any but a twin-keel boat.

Okay, I guess we have to talk about the disadvantages now. There is only one, really, as far as most sailors are concerned--sailing performance. Whereas a keel-centerboarder can be fully competitive boat-for-boat with a keel boat of similar dimensions, it is likely that the shoal-draft, twin-keel boat will never achieve this parity.

We have several indications of this. No twin-keel boats have ever become, successful racers, even though they are ordinarily rated on the basis of upright draft, and receives substantial amounts of draft credit in their ratings. Whenever a stock boat has been produced in fin and twin-keel models, the fin-keel model has proven faster, especially sailing to windward and in light air. How much faster is hard, to say, since I have not seen any published data, but calculations I have made on the basis of published dimensions indicate a differential of 10 to 20 percent in speed-made good-to-windward below 5 knots true wind, and about half this in 15 knots true wind.

Various writers, as enthusiastic as I am about the advantages, have held out the hope that this defect might be overcome by design refinements--camber, twist, decalage and airfoil sections of the keels. But so far it hasn't happened, and I am just about convinced by my investigations that some sacrifice of speed is unavoidable. It's quite possible, of course, and a worthy challenge for any designer, to design a good twin-keel boat, within the medium.

How important should speed be? No one enjoys being passed up by a boat of comparable size. And it's not as if you had a gaff mainsail sticking up there to make excuses for you. Your excuses are underwater, where no one can see them or is even likely to guess their existence. Can you take it? I think it takes some strength of character to own a twin-keel boat and actually sail it in crowded waters.

On the other hand, consider the long-distance cruise. When Aleutka was finished, in September 1967, we set out immediately down the coast to California, using the prevailing northwesterlies to get south before the autumn storms. The sailing season was over by the time we got to California and we seldom saw another sail on our cruises to the Channel Islands. In February we left for Hawaii, making an easy run in the northeast trades. In May we headed south for Alaska - first reaching a bit west of north in the northeast trades, but before long picking up the prevailing westerlies which carried us right up to the coast. By taking advantage of the prevailing winds we avoided windward sailing; by choosing remote and out-of-season cruising grounds we largely missed the embarrassing comparisons with other boats.

In fact, it happened to be three years and 10,000 miles of sailing before we were passed up by another 25-footer sailing to windward. It was a surprise to see how quickly the other boat gained on us. Right up until that time, you see, we had believed Aleutka sailed to windward just fine; we never knew what we were missing.

So Patty and I concluded long ago that the modest performance penalty of our twin keels is easily compensated for by a long list of solid operational advantages. Cruising can surely be enjoyable in almost any kind of boat, but we've found that it is especially enjoyable in a shoal-draft boat that will stand on her own two feet.

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