PS Advisor May 15, 2002 Issue

PS Advisor: 05/15/02

Deep vs. Shoal Keels
I have been considering purchase of an Ericson 32-200. Two options are available: shoal or competition keel. The competition configuration has a ballast of 4,200 pounds and the shoal configuration has 3,800 pounds. Displacement in both cases is listed as 9,800 pounds.

I have no experience with shoal keel configurations. I would like to gain some insight as to handling characteristics (differences between the two), advantages/disadvantages and the like. Can you point me to some information? Also, I would have believed that regardless of the keel, ballast for a certain displacement and sail configuration should be the same. I thought ballast was part of the equation for stability, righting arm, etc.

I have read your evaluation of the Ericson 32/32-200. It was quite informative.

-Paul Souval
Via e-mail

There are a number of good books on yacht design that spell out the basic considerations of deep versus shoal keel. You might try Killing and Hunter's Yacht Design Explained, or Ted Brewer Explains Sailboat Design.

The deep keel has two distinct advantages. First, it places ballast lower, which gives more righting arm, which keeps the boat more level and therefore makes it faster. Second, the deeper the keel, the longer the foil (and leading and trailing edges) and therefore the more lift generated; this means the boat will point higher. When it comes to windward performance, there's no substitute for a deep keel.

Deep keels, however, are obviously inconvenient when sailing in shallow coastal waters. Over the last hundred years yacht designers have worked hard to develop shoal keels that don't sacrifice quite so much performance.

An early solution was the keel/centerboard configuration, in which ballast is contained partly in the hull and partly in the stub keel. The centerboard is weighted, not so much to provide additional ballast but so it will stay down at speed. The purpose of the centerboard is to extend the keel foil to generate more lift. It works, but not with great efficiency. The shape of the board is by necessity thinner than optimal for the best lift. And because the ballast is located fairly high (giving the boat a high center of gravity, CG), it will be more tender than a deeper-keel boat of the same proportions. One way around this is to give the boat more beam, and make its bottom flatter, thereby increasing "form stability." Boats with a lot of form stability are stiff and don't heel as much initially, but are more likely to pound when sailing to windward and are at greater risk of becoming just as stable upside-down should they capsize.

Some small boats have stub keels and no centerboards. This results in poor windward performance and low stability. Not a very good solution.

Australia II's victory over Freedom in the 1983 America's Cup revealed Ben Lexcen's genius on so-called wing keels. Wings are a development of end-plate effect, which sailors learned from studies in aerodynamics.

Somewhat earlier, Henry Scheel achieved such an effect with his specially shaped bulb keels. The Scheel keel, attached to the right boat, proved a good compromise between shoal draft and lift, without the weed-snagging characteristics of some wing keels.

An efficient wing keel probably provides more lift than a keel/centerboard combination, and more righting moment because the wings are part of a bulb that locates ballast fairly low. But lift and righting moment are still not as good as a good deep fin.

It's somewhat unusual for the same boat to have more ballast in the deep keel than the shoal. Designers usually give the shoal keel more weight to compensate for its lesser depth. Ballast does indeed play a major role in stability calculations, but nothing says it must be the same for deep and shoal keel versions of the same boat.

Shoal keels do sacrifice lift and boatspeed; how much is sacrificed depends, of course, on the boat. For the casual or cruising sailor, it's not a big deal. For a performance sailor, it's everything. But when we think of what shoal keels sacrifice, we're reminded of a friend whose boat lies at a dock that has three feet of water. As he says, it's a shoal keel or no keel.

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