Does the rating change for a Nacra that has been modified to carry a square top sail? Looking at the numbers, the Nacra 5.2 and the Isotope carry roughly the same number in the 2-3 Beaufort Range, however the Nacra 5.2 with a square top rig seems considerably faster.

Yes. It's default rating should have the MN or ML modification factor.

If the square head is equal (within 5%) to the original sail area, the rating should be multiplied by the MN adjustment of 0.995. This was, originally, listed as a square head modification factor.

If the sail area is greater than the original sail area, the ML adjustment of 0.980 should be multiplied on the default handicap rating instead.

Any other modifications continue to be multiplied on the rating.

Modification Factors can be found here:
http://www.ussailing.org/racing/off.../current-tables/mh-modification-factors/

Yes, the rating is modified. Adjustments for "additional sail area" are applied and the rating is changed. Buy your rating chairman a couple beers and parlay a rating if you are just competing locally.

It is for local club racing with a mixed fleet. I'm actually not the one with the square top, but we are trying to determine what his number should be. I'm not one for protesting, but he seems to have a considerable advantage. Thanks Jake and Catandahalf for the info. I'll forward this to our scoring person.

Don't be afraid to protest. If he is not being correctly scored, that is a disadvantage to everyone else, and may reduce the "fun" for them. Which isn't good for any fleet

I agree MN3. We are working to figure out the correct number and inform the sailor. I don't think he is intentionally sailing on the incorrect number and our scoring person may have not been aware that the square top was a modification. Does the material affect the number in any way? Is there a difference between dacron and pentex for instance? We want to make sure everyone is sailing the correct number, especially since some of the members have changed platforms for one reason or another.

Material makes no difference in the rating system. Only if it is a non-factory sail and whether or not the sail area includes more than 5% additional surface area.

Thanks, Jake! That's pretty much what I figured.