Originally Posted by PCP
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.
You are looking at this way too simplistically. The GZ curve can only be used to compare two boats if they have the same mass. Clearly the Catalina 40 weighs 3000 pounds more so the use of a GZ curve is useless.
On the other hand, since you can't provide the RM curves for the two boats, its not likely you can provide the GZ curves. So your discussion is not particularly useful.
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.
Who cares about a class 40 sailboat... we are comparing two very specific boats the Catalina 40 and the First 40 (2012). Class 40 wasn't in the discussion..
Clearly a Catalina 40 is not double the weight of the First 40.. so not sure where you are going with this discussion about Class 40.
You keep saying the First 40 is more stiff than the Catalina 40. Clearly no numbers support your opinion. And you provide no RM diagrams to support your opinion. Even boat designers do not agree with you.
On the other hand.. my Catalina 40 runs at half the heel angle as a First 40 at 8 knots. And you suggest the First is more stiff?