I know that the Marblehead class rules were changed some few years ago because of some swing rig sails, but that does not tell me how the new rule is applied. The rule calls for the calculation of “excess widths,” then the use of the excess widths to calculate an “excess area” which is added to the triangular area.
My problem is that it is not at all clear whether the equations used are merely cranked, or if negative excess widths are set to zero. The former makes sense in the general field of sailboat design formulae, I think, implying that additional triangular area may be obtained by using a smaller roach than the “roach allowance,” since negative “excess widths” lead to negative excess area. But my impression is that one is supposed to ignore all negative excess widths, effectively setting them to zero, thus precluding any negative excess area, and the full roach allowance might as well be used in the roach. Neither the rules themselves nor the measurement forms indicate which way to go.
Which is it? Any chance the rules might be cleared up?
(I tacked this question onto an earlier thread about Marblehead rules, but received no response. So this is a second, more direct, attempt.)
Mike Biggs
Mike -
not too many on this board deeply into M Class boats. Suggest a visit to AMYA website, and email the secretary for the M Class here in the US. If you are outside the US - email anyway, since this is an International class and rules are rules are rules - regardless of where you live.
Yeah, I already e-mailed the class secretary. He hasn’t replied to my question. He was the one who, in an earlier related e-mail, said the reason for the excess area calculation was some of the swing rigs, in which there were apparently large curvatures along the luffs. In that same e-mail he said, “When excess cross width is present, X, Y, Z, x, y, z, the area is calculated for it.” That’s what makes it look like my original assumption to just turn the crank and calculate the numbers was wrong. Maybe there isn’t anything to do but to e-mail him yet again. I’ve been unable to find any other group on the Internet, but it seems clear that those who sail Marbleheads in big regattas must know whether a negative excess area is “added” to the triangular area, or not.
Mike Biggs
A couple other alternatives - Bob Sterne (Sternes Boatyard) and Rod Carr (Carr Sails) - as sail-makers for the class, would be up on the rules
(I would hope?)
Okay, I asked the Marblehead class secretary again about the excess area calculation, and this time I got a reply. He said that excess area meant excess area, there was no such thing as a negative excess area, and he hoped I understood.
That’s clear enough. I still think most engineers and scientists, reading the Marblehead rules and seeing the equations for calculating excess area, will do just what I did the first time: make the measurements, do the calculations, and if the so-called excess widths turn out negative, leading to a negative excess area, then a larger triangular area is possible. But I do not have any particular need to fight with anyone about it, beyond pointing out that my original, mistaken interpretation was reasonable.
Mike Biggs
Issue 143 of Model Yachting (just out) does a nice job of answering your question, and provides a link with steps to go to ISAF for more info.
Hopefully you are an AMYA member - right? … 
edit: go to the Marblehead Class news posting.