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Old 02-15-2013
PCP
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Re: Lets have a chat about multis.

mdi posted a link with interesting information about multihulls, including stability on another thread. I think it is useful to have it here also. The link:

Multihull Dynamics, Inc. - Account Login

Quote:
Originally Posted by PCP View Post
Thanks, interesting stuff.

But in what regards stability the ratios they use are a bit limited. Both take into account the sail area but in extreme situations one does sail in bare poles or with very little sail area so in fact those ratios don't tell about the absolute stability of a boat but about the stability for the max sail area the cat can carry (you can always reef).

Multihull Dynamics, Inc. - News Article

Looking at the formulas they attribute the same importance to three factors: Length, beam and weight. That seems a bit arbitrary to me.

I remember some years ago of some huge racing multihulls (60ft) that were capsized on a huge storm, carrying no sail, just for the windage on the mast. If I remember rightly 70 to 80K winds.

A good way of having an idea of an absolute static stability measure on a multihull would be to know what the intensity of wind would be necessary to capsize the cat only with the wingage over the mast and cabin. I don't understand why that is not used. It is not difficult to computer predict that and surely If I sailed a cat I would know more about that than about a relative stability regarding Stability/sail area. I can always reef or go to bare poles.

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Interesting that the stability ratio used by the designer of Tom cats multihulls is not the same:

"The stability formula gives a result which is the wind speed (in Knots) required to just lift the windward hull (the point at which, in a cruising boat, not a beach cat, you scream "Let go the sheets!", or better still "Let's put a reef in her").

Stability = 13.7 sqrt(Wt * 1/2Bm/SA * HCE)"

Welcome to TomCat Boats

As you can see they are different:

Multihull Dynamics, Inc. - News Article

Anyway on this times of CAD design, with powerful programs I don't understand the need of approximated ratios when it would be easy for the designer to calculate for each boat (taking into consideration all boat's dimensions) this value and most of all the force of wind needed to capsize the boat on bare poles. This could be a king of AVS point for multihulls, not related to heeling but with wind force. If those values were mandatory we could have a pretty good idea of the stability of each cat.

Anyway, as we cam all see, Beam, length and weight are all determinant factors in a multihull stability and that's why I had made some personal reserves to the use of small light cats for offshore work.

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Last edited by PCP; 02-15-2013 at 08:44 AM.
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