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Old 27-08-2008, 18:07   #16
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Me? No, I'm the big hairy guy right next to the cute blond (from Pittsburg) on the right.

Damn, they cut me off again.

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I though one of them WAS you Joli!
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Old 27-08-2008, 18:20   #17
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In Chris White's book The Cruising Multihull he measures from hull centerline to hull centerline. He goes on to state that a cruising cat with balanced longitudinal and transverse stability would have a hull centerline-to-centerline beam of about 40 percent of the waterline length. This would seem to give more consistent and realistic numbers than using overall beam.

The length to beam ratio of the individual hulls, sometimes called the fineness coefficient is the length of the hull waterline divided by the waterline beam. White states that 8/1 is about the minimum, 12/1 produces a very efficient hull, and 16/1 denotes a high performance hull.
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Old 28-08-2008, 12:07   #18
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Steve,

This is what I thought, and as you mention this. I remember getting the information from Chris's book. Hell I sleep right next to the book.......lololololol.... Looks like I am right at 13/1, but I will try to remember to measure tonight. Of course I said I would measure last night too, and I was in the bilge last night, and tonight.....

Joli,

I am beginning to think you like European beer, and pretty women:cubalibre
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Old 28-08-2008, 19:38   #19
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Originally Posted by Steve Rust View Post
In Chris White's book The Cruising Multihull he measures from hull centerline to hull centerline. He goes on to state that a cruising cat with balanced longitudinal and transverse stability would have a hull centerline-to-centerline beam of about 40 percent of the waterline length. This would seem to give more consistent and realistic numbers than using overall beam.

The length to beam ratio of the individual hulls, sometimes called the fineness coefficient is the length of the hull waterline divided by the waterline beam. White states that 8/1 is about the minimum, 12/1 produces a very efficient hull, and 16/1 denotes a high performance hull.

This is what we use on the Multihull Dynamics website.

Pat
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Old 29-08-2008, 09:57   #20
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Kanter's article on hull beam

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This is what we use on the Multihull Dynamics website.

Pat
Waterline length is divided by waterline beam to get the hull length / hull beam.

Read this to understand the importance of that statistic: Southwinds - January 1999

When calculating stability, you measure from the center line of one hull to the center line of the other, or for some versions, the centerline of one hull to the boat's center.

From my website: "The theoretical stability of a catamaran is given in the formula, displacement in pounds x the distance between the centerline of the two hulls in feet x .5 = righting moment in foot pounds. To use my BigCat 65 design as an example, 45,000 x 28 x .5 =630,000 foot pounds (that is, a lever of an amount of feet times an amount of force in pounds.) If considering a capsize from wind force, the theoretical heeling force is the wind pressure x the sail area x the CE (the height from the waterline to the center of your sail area.) Wind pressure can be calculated from Martin's formula, which is windspeed in miles per hour squared x .004= pounds per square foot. So, the heeling (capsize) force for BigCat 65 that generates a theoretical force sufficient to capsize it is: 9.21 pounds x 2400 sq. ft. of sail area x 41 feet (center of sail area above waterline) =630,000 foot pounds. This wind pressure is found at 48 miles per hour, which = 41.7 knots. (One knot equals 1.15155 miles per hour, so divide the mph by 1.15155 to get the knot equivalent to the result of Martin's formula.) So, in flat water, theory predicts the BigCat 65 will capsize at 41.7 knots in calm water if the sails are up and unreefed. This theory applies rather poorly to a biplane rig, which the BigCat 65 design has, because you can't get the full effect of the wind abeam if both sails are up, but it works pretty well for a typical catamaran with a single mast. Obviously, this is all very theoretical, because a capsize is unlikely to occur in calm water, sails are rarely strapped hard amidships in high winds, etc., but it does give one a starting point for considering the forces at work. It does help explain how charter catamarans have capsized by coming out from the lee of islands in strong trade winds with the full mainsail sheeted amidships while under motor, when you consider that gusts are often equal to half again the average wind speed, and that winds will often increase as they funnel through channels between islands, or through gaps in cliffs to windward"
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