Following a winning trend I suspect.Having never had to subject myself to the IOR rule, why did type formed IOR racers tend to tall skinny mains and huge overlapping genoas (in the earlier days of IOR)?
The first terms in calculating RSAM and RSAF were to determine the areas of the main and foresail. But RSAM had a .35 multiplier while RSAF had the .5 multiplier. Since you use .5bh to find the area of a triangle, they simplified it by using .35 rather than .7 x .5 for the main and .5 rather than 1 x .5 for the foresail. The second part of the first term for RSAF (1.0+1.1*(LP-JC)/LP) was to account for the area of the overlap and typically came out as 1.33 IIRCSome kind soul scanned each page of Mk III, up to 1985. Read it at your peril. This is really simplified so bear with me if you have designed to the IOR Rule! I've only been exposed to it slightly. I may be cutting important corners
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I think the first formula SAM = 0.35 E P + 0.2 E (P - 2E) does underestimate real mainsail area considerably.
For example I have a E = 10 and P = 30.
SAM = 0.35 (10 x 30) + 0.2 x 10 (30 - 2 (10)) = 125 ft2.
A plain triangle of E x P / 2 = 150 ft2
A real main, with roach will be maybe 12% more or 168 ft2 (~74%). The 12% might be a more modern figure where we are using longer battens. So probably 12m is correct = about 70% of actual area is rated.
(I am ignoring the Reduced Girth Factor etc correction assuming it wasn't usually taken. Also ignoring Corrected E and P (EC PC) because they probably have to do with headboards and tack/clew offsets etc.
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I = 30, J = 10, LP = 15
A genoa of those proportions would have a real area pretty close to the rated area.
I think where the big difference lies is a genoa is more efficient without a mast in front of it for equal amounts of sail area. So the rule type forms you toward big genoa + small main. So 12 metre is very correct - the main is less penalized.
In later years (Mk III version something or other or Mk IV) we started to see many more 7/8 rigs with bigger mains. That might have been because they finally realized they could make a more flexible main for trimming or that main area was under-rated.
(This probably also led to the skinny IOR masts where you were trying to make the mainsail as little shadowed by the mast as possible.)
A fascinating history lesson.