Originally Posted by BryceGTX
Anyone who thinks otherwise deserves my attention. However, you can easily prove me wrong by producing the RM diagrams for a First 40 and a Catalina 40. It would be interesting to see who is absurd.
Still not interested in discussing this with you but just not to confuse people:
To access a sailboat stiffness through a stability curve the one that should be used is a GZ curve (length of Arm) not a RM curve (righting moment) simply because a RM is obtained multiplying the displacement of the boat for all values of a GZ curve at different angles of heel. The values to be considered will be the ones till the MAX GZ with special incidence till 45º.
Two boats can have similar RM values till 45º and therefore have similar RM curves on that segment but one can weight almost the double. In that case the lighter boat would be much more stiff. It will be the case when we compare for instance a 40 class racer (or a First 40) with a Catalina 40:
A Catalina 40 weights about the double so for identical values on the GZ curve the RM at those points will be the double on the Catalina 40 regarding the racer, but the Class 40 racer will have a GZ curve 60% better, so even if the RM is smaller its Stiffness is much bigger because It needs a lot less sail to go at the same speed.
For the ones that these curves means little: The RM is the force that is making force at a given angle of hell to put a boat vertical and that is opposed by the wind force on the sails. If a boat weights half it will not only be needed less force to right itself up because it weights less but most of all, as it has a much smaller wet surface, it will need a lot less sail area to sail at the same speed of the heavier boat so the force made on the sails will be a lot smaller and it will be needed a much lesser RM to oppose that force.
That's why it is not the sail area that counts for a boat performance but the SA/Displ ratio. The same thing happens with stiffness that has not to do with RM directly but with the sail area the boat needs to sail at a given speed and the available RM. Of course, comparing boats with the same length.
Anyway, as I have said already, anybody that has sailed a race sailboat, or even a performance cruiser will know that the stiffness is bigger than in a cruising boat. For knowing that is not necessary to know a lot about theory, is evident to all with sailing experience in different type of boats.