A few words about keels...
Most North American sailors are familiar with single-keeled boats, twin-keeled, or more properly, bilge keeled boats have enjoyed a lot of popularity in latitudes with large tidal variations. In Britain, for example, Westerly Marine produced a number of boats with bilge keels which were extremely popular. In fact, many twin-keelers out-sold their fin-keeled sisters. A case in point would be the 31' Berwick which sold 309 while the otherwise identical single-keeled Longbow sold 265.
It is difficult in the United States to find much written about twin-keelers. Most of what is in print is not flattering. In his book, The Coastal Cruiser, Tony Gibbs includes the Westerly Centaur in his design portfolio. He notes that its design looks odd to American eyes, but that it provides adequate deck area, good head room, and is tough and seaworthy. After stating that she is well canvassed for a moderate displacement yacht, he concludes: "What these ratios cannot take into account, of course, is the twin-keel underbody, which in my opinion will reduce her normal sailing speed inordinately." My response, after sucking in my gut and buying a twin-keeler is: "Did you ever sail one, Tony?"
I am not a racer. I enjoy the sport of sailing from the standpoint of a cruiser, so everything from this point on carries a particular bias, but one that many may wish to consider. It is also true that I have had limited experience in terms of the number of boats I have sailed, so I am going to give a subjective comparison between my two most recent vessels, a Ranger 30 and Drinian, a 31' twin-keeler.
Though a foot longer overall, Drinian is 1'5" shorter on the waterline. The Ranger had a hull speed of about 6.8 knots and the twin-keeler, 6.6 knots. This would be based on a standard formula for all keel boats. If the twin keels "inordinately" reduce hull speed, don't tell Drinian! In a fresh breeze, both boats could achieve their theoretical hull speed. In trailing seas, both would surf and Drinian has gone over 7 knots. Both boats were well-balanced in terms of their wheel steering. The major difference is in heeling. Strider (the Ranger) would roll over on her side on a screaming reach. Drinian wants to sail upright. When heeling becomes excessive, she slows down. When reefed, she quickly comes back to hull speed. In other words, my experience has been that a major difference is the angle of heel, and for some, that may be perceived as speed. I must admit there is a certain thrill when you have to brace one foot against the cockpit coaming while the crew moves to the high side, but I'll get over it! There are many cruising advantages that come with the sacrifice. Here are a two:
Fewer things roll around below decks
Less adventurous crew members feel more comfortable
There are also advantages from a performance point of view. Consider these:
In short, when compared to similar fin-keeled cruising designs, twin-keelers tend to sail flatter and are less tender. They also are capable of great magic. I'll show you a picture that I borrowed from a Griffon Owners site.
Try drying out in a fin-keeled boat! This is the magic. Shallow waters in a tidal basin become a dry dock for bottom cleaning and repairs. No need for a cradle or jack-stands at haul-out time!
Admitedly, I have a prejudice that is growing stronger the more I sail with Drinian. It is a bias shared on both sides of the equator. Twin-keeled Bruce Roberts designs are popular in NW Australia where tidal ranges are as much as 30'. I guess we are all skeptical of those events and elements beyond our own experience, but keep an open mind to the potential advantages inherent in twin-keeled cruising boats!