How does the roll decrement function work on Catamarans?
Q) What is the roll decrement calculation doing with cats? Having run the monohull and catamaran, the roll amplitudes are clearly different but it is difficult to see where the centre of roll is for the cat. A) The only modification we do for cats is work out the metacentric height correctly — i.e. taking into account the separation of the demihulls. You still only put in a monohull on the centreline, but if you say its a cat the transverse waterplane inertia takes into account the separation that you have specified. The problem for cats is that you get a blanketing effect of the weather demihull, and this is not taken into account. For the roll decrement curve, there is no wave excitation, so you are probably not too far off provided you get the damping right — the main reason for putting in the roll decrement simulation was so that you could get a feel for whether you had put in a sensible damping. We are working on a method for estimating the damping based on hull shape.
