Thanks for the quick and helpful replies.

Wouter - it seems that my "big" main is actually the "small" main you mention. There seem to have been three Stealth mainsails. The smallest was apparently an original option when the boat was first launched - I've been told that I'm too heavy to use it unless dogs are getting blown off chains (although not everyone who has said that was quite as polite!)

Dermot - we do use PY for club racing. Those who understand these things have worked out a conversion factor by dividing the PY by the ISAF for a Hurricane and for a Hobie 16 and then averaging the result (I think - I'm a words person not a numbers one!). The end result is you multiply the ISAF handicap by 687.768 to get a PY. And I'll still come last.......

George