Originally Posted by BryceGTX
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?
I don't suggest, I say it is absurd to say that a Catalina 40 is more stiff than a First 40. That would be evident for someone that sailed both types of boats even if he does not understand a thing about GZ or RM stability curves.
The stiffness of a boat is not measured by the smaller or bigger heel angle that is needed for a sailboat to sail at a given speed. Anybody knows that narrower boats will need more heel angle to sail than beamier boats but that does not mean that the beamier boat would be more stiff. It depends on the type of keel and draft (regarding maximizing ballast) and the B/D ratio of both boats. Also it depends on drag and upwind, on wave drag. The beamier boat will need more sail than the narrower boat for the same speed.
Stifness in a sailboat equals boat power. A boat with more sail area will not be necessarily faster. Power in this case it is a relation between the sail area the boat can fly and the drag it has to overcome. Of course heavier boats and beamier boats produce a lot more drag and therefore need a lot more sail area to go at the same speed
The Catalina 40 (assuming you are talking about the 400MKII) has a beam of 4.11m, 9299 kgs and 75.06 m2 of sail area.
The First 40 has a beam of 3.98, 7536 kgs of weight and 98 m2 of sail area.
The First 40, because is lighter and has less beam (less wet surface) needs less sail are to go at the same speed compared with the Catalina and obviously can have more sail up so evidently is a more stiff boat.
This all subject is ridiculous, I am not posting for you (I don't care what you think) but because it may be of some help to other members regarding understanding better the concept of stiffness on a sailboat.
Generally mass production cruisers as the Catalina are less stiff then performance cruisers and performance cruisers less stiff then race boats. That is about the same to say that cruisers are less powerful than performance cruisers and this one less than racers.
The First 40 is a particularly stiff boat among performance cruisers. One of the situations were you need a stiffer boat is to go upwind with waves and bad weather. The First 40 is the most used performance cruiser on the Sydney- Hobart race, a race that is well known by upwind sailing in bad conditions and they are the most popular because they are the ones that performed better, having already won the race in compensated time and finishing in real time among much bigger boats.
Originally Posted by BryceGTX
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.
I am not discussing this with you and who says that I cannot provide the curves? It is just not worth it.
In what concerns comparing boats the GZ and RM curves are used to compare different things and it all depends on what you are comparing. To compare the energy that is needed to capsize a boat you use a RM curve, to see if a boat is well designed and have an idea of its performance, namely stiffness, you use a GZ curve.
When you design a boat you only use GZ curves. what you want is the lighter boat for a given robustness and one with the better possible GZ curve that, in what regards the part that has to do with sailing, means the steepest you can get and with the bigger possible values till 35º of heel and that means normally a big Max GZ. The weight of the boat is a consequence of the weight needed to get the desired strength and the ballast needed to obtain that curve.
RM curves are also more important in what regards static stability and GZ curves in what regards dynamic stability (the AVS point is the same).
It is because dynamic stability is more important for a boat stability than static stability that there is not a direct proportionality regarding boat seaworthiness and the amount of energy that is needed to capsize a boat and that's the reason why boats with a big GZ curve can be more seaworthy than boats that theoretically need more energy to be capsized ( represented by the area under the positive part of the RM curve).
That is obviously the case with a 40 class racer (regarding much heavier boats with the same size) and that's why I have talked about it, just because it is a more extreme example and therefore it is easier to understand even if the principle is true to all boats. But, off course, that would also be the case of a First 40, a very seaworthy and stiff boat, in my opinion more than any main market mass production cruiser.
By the way, the Blue Jacket 40 is going to be also a considerably more stiff boat than the Catalina 400.