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sdwyer 11-21-2002 04:30 PM

Twin bilge keels?
Are twin bilge keel designed boats safe offshore. I have read they are more seakindly but would like to know more about them. are they safe for ocean crossings?

Jeff_H 11-22-2002 02:03 AM

Twin bilge keels?
In theory twin bilge keels can be designed to be safe for offshore useage. To do so they end up adding substantial weight and wetted surface over an equally optomized single keel with a bulb, and in most cased over even a single keel without one. To one degree or another, depending on the design, bilge keels adversely affects some combination of the ability of the boat to carry stores and gear, the ability of the boat to go upwind, and results in generally less performance than a single foil.

I don''t know where you read that bilge keels are more seakindly, but, as bilge keels are normally designed, that is simply not the case. A single deeper keel would offer greater dampening and a lower center of gravity, both of which would result in a better roll comfort for the single keel.


ndsailor 11-22-2002 03:44 AM

Twin bilge keels?
Anyway, to answer you questions on "TWIN KEEL" boats. Yes, they are safe for ocean passages. As far as roll comfort that Jeff mentions, twin keels produce less roll than a monokeel, to me that would indicate more comfort., and the additional weight makes a less bouncy , albeit slower, ride. Check out this web site for more opinions on twin

Jeff_H 11-22-2002 04:02 AM

Twin bilge keels?
A couple quick points here- Twin keels are a specific type of bilge keel in which both foils are identical. Bilge keels can be either twin or asymetrical. Assymetrical foils offer less drag for the amount of lift.

I don''t know why you beleive that twin keels (or any other kind of bilge keel) offers less rolling than a single keel of equal or even slightly less wetted surface. That simply has not been born out in tank testing. The conjectured reasoning is that the full area of the single keel acts in water that has not been disturbed by the rotation of the boat while the bilge keels act in thier own disturbed water. For an equal area or a smaller deeper area on a single keel, the single keel also have a lower roll moment of interia due to dampening than the bilge keels.


ndsailor 11-23-2002 06:52 AM

Twin bilge keels?
Reasoning, Schmeesoning. Just comparing the ride performance concerning roll behavior on my previous ComPac and Cape Dory compared to my current Westerly Centaur. With the windward keel rising up toward the surface of the water, creating what could be compared to having rail meat lining the rail, and the leeward keel dropping deeper into the water, the boat rolls less. My wife rarely accompanied me sailing in the previous two boats we owned,Too Scary, however she is now comfortably relaxed sailing in all kinds of wind conditions on the Centaur.

Jeff_H 11-23-2002 08:48 AM

Twin bilge keels?
Large scientific sampling and in depth research there, NDSAILOR. I guess I am convinced of the error of my ways. 8^)


geohan 11-23-2002 07:22 PM

Twin bilge keels?
If the moment of inertia of an area is proportional to its distance squared from the axis of rotation, is the change in angle of attack due to the arc swept by the deep keel a factor for a single keel having a lower roll moment of inertia compared to bilge keels? Cheers, George

Jeff_H 11-24-2002 07:34 AM

Twin bilge keels?
If we look at a single keel with equal area to a pair of bilge keels as normally configured the single keel would be deeper and so, as per your desciption of a moment of inertia, its distance squared from the axis of rotation would be significantly greater and so would its dampening moment of inertia. But even comparing equal depth equal area single and bilge keels, the single keel would have a greater dampening moment of inertia because it is acting in undisturbed water.


ndsailor 11-24-2002 09:35 AM

Twin bilge keels?

DelmarRey 11-27-2002 06:54 PM

Twin bilge keels?
I, at this time own a twin keel s/v. It has it''s goods and bads. At anchor she''s very stable and I can get very close to shore. Some nights I''ve ended up on the hard and didn''t know until morning. And in rough weather she handles well. The down falls are performance. They do not point up well and you loose COG to the lee, Badly. Going up a channel one day a fixed keel s/v of the same size tacked three times compaired to my six times although we were traveling at the same speed. Before the wind or a broad reach, she screems.
My $.02 ...........Del

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