Originally Posted by Pat Ross
This is what we use on the Multihull Dynamics website.
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
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"