Stability Question

[Jules Martin]

I've learned a lot from the generous contributers to this forum; now I hope to learn some more.
I am in the initial stages of building the "La Chatte" catamaran from Glen-L, a design with a certain Birkenstockian elegance, I find. It is an old design, and the hulls look a little close together. The design of this boat would make it very easy to increase the distance between the hulls, and I was wondering to what extent this would increase the stability. I understand that pitchpoling results at too great a beam. The centrelines of the hulls are drawn 5'-8" apart, the LWL is(are) 14'-9", the sail area is 159 sqft, and the CE on the fractional rig is 11' above the design waterline. Would a beam increase significantly increase stability without compromising sailing qualities? Is there an equation to figure this out? Going fast is fun, but I like to be relaxed even when it blows, and if my kids ever come along. Thanks for your help!

[BigCat]

Originally Posted by Jules Martin

I've learned a lot from the generous contributers to this forum; now I hope to learn some more.
I am in the initial stages of building the "La Chatte" catamaran from Glen-L, a design with a certain Birkenstockian elegance, I find. It is an old design, and the hulls look a little close together. The design of this boat would make it very easy to increase the distance between the hulls, and I was wondering to what extent this would increase the stability. I understand that pitchpoling results at too great a beam. The centrelines of the hulls are drawn 5'-8" apart, the LWL is(are) 14'-9", the sail area is 159 sqft, and the CE on the fractional rig is 11' above the design waterline. Would a beam increase significantly increase stability without compromising sailing qualities? Is there an equation to figure this out? Going fast is fun, but I like to be relaxed even when it blows, and if my kids ever come along. Thanks for your help!

You'd have to strengthen the beams, and maybe the structure that distributes the loading from them into the hulls. This would increase weight, which would be a negative. The rig might be more stressed, too, but since cats heel very little, probably not by much. You might get more wave slap on the underside of the bridgedeck- as greater beam needs greater bridgedeck clearance. You don't mention the overall beam, which is usually about 1/2 the length in modern designs. Many think that pushing this beyond, say, 1.85 to one, increases the risk of pitchpoling, as well as bridgedeck slam.

For stability calculations, see the foot note at the bottom of my web page at http://www.dunnanddunnrealtors.com/catamaran.html