Simon,
As I see it the main difference regarding sail area is how the masts surface area is calculated.
Current rule:
Mast area = mainsail luff length multiplied by the circumference of the mast. simon adds x1/2
Proposed rule:
Mast area = mast length *maximum circumference of the mast * 0.5
Mast length is longer than max luff length, which means that the area of the mast under the lower measurement mark now is included in total mainsail area?
Yes, so the rule counts 1/2 the area of the mast that does not have sail attached; yes, sails will get a little less in theory; but in order to get a sail to measure is must have less than 15sqm in it anyway.
Edit to respond to Paul's comments; I'd bet if you got your sail measured, the all up area would be somewhere less than 15sqm; I'm guessing mine is about 14.8 including mast.
