Kaos, I decided to take a picture of my current H18 sail and a picture of the square topped H18 sail and draw up the two for a comparison point. The square topped sail is 154 sq ft to the classic sail's 151 sq ft. The classic sail has an aspect ratio of roughly 4.42, and the square topped sail has an aspect ratio of roughly 4.5. Throwing some numbers together, in theory the square topped sail on an H18 in 30mph apparent wind is going to generate up to about 8 pounds more lift, which is an improvement of about 2%, at the expense of a half a pound more drag - but nearly all of the difference is due to the slightly larger sail area and leech length. If those are kept the same, the square topped sail's better aspect ratio accounts for a 1% reduction in drag on the mainsail and 0.2% increase in lift - less than a pound force for each. So in essence, it seems you're right, a square topped sail really doesn't do much of anything to improve the H18's performance, especially not enough to justify the additional cost imo.
Conversely, the daggerboards have a very low effective aspect ratio of 2.9, while the F18's have daggerboards with effective aspect ratios of about 15.0 (calculated for the Nacra Infusion MkII image I was able to find). Higher aspect ratio daggers would be a much more significant improvement to the H18, providing better lift with considerably less drag (water drag is far and away the larger factor in boat performance) and allowing the boat to point higher upwind and provide the same lift on the daggers without nearly as much drag. But of course that's far more complex of a retrofit, and not nearly as simple as swapping sails.
However, I'm still curious to hear from anyone who may have put on a square topped sail, whether they've noticed any practical/real world improvement. I don't always trust the numbers 100%, and I'm still tempted to believe that the square top provides more of an advantage than what I've approximated... why else would sailmakers have bothered to change the sail designs in the first place?
https://www.grc.nasa.gov/WWW/K-12/airpl ... nwash.html (a short explanation on the importance/effects of aspect ratio)
Is the H18 community very particular about the "Hobie only" rule for sails? I see Hobie's price has gone up considerably since I last checked, and I'd much rather spend that extra money on mainsheet blocks, a sail bag, boat cover, new rigging, club membership, race entry fees, replacement wheels for my cattrax, etc, etc. Still don't see much advantage in buying Hobie, they've already discontinued the boat and seem to be jacking up prices for parts. I'm not technically race legal anyway, since I have an original mast and won't be spending the money to replace or convert it.