Originally Posted by David M
In reference to the first post..
lifting out of the water
for a catamaran is the point of maximum righting moment.
The premise of the OP in the SA thread is wrong...you still have righting moment after the vessel has reached maximum GZ. Monohulls must heel more before reaching their maximum GZ.
I beg to differ. The premise is correct. At some point very near to the hull lifting completely out of the water the max RM of a typical catamaran will be attained. Cats tend to reach max RM at relatively low heel angles. With cats it is important to note that the heeling moment from wind
will tend to decrease less quickly than the RM decreases beyond the point of max RM. So, typically a cat sailed to its max rm will capsize
if the crew does not take some action. This seems like sailing on the edge to me and is quite different than the keel
yacht experience at max RM.
Some numbers can be found in the Deakin
paper. They are discussing both tris and cats. Cats generally reach RM at lower heel angles than tris so this applies even more to them.
The wind heeling moment reduces with heel angle at a lower rate than the righting
moment of a typical multihull. Their range of stability normally is about 60 to 80
degrees, and the wind heeling moment remains significant at 90 degrees. The wind
heeling moment is therefore greater than the righting moment at large angles of heel,
even at low wind speeds, and this fact may have implications for multihulls heeled to
large angles by wave action. In this respect multihulls differ from ballasted monohulls
which, when at 90 degrees, have positive righting moments but negligible wind heeling