See attached pic. The boat is also widened to 8.5' and also has a much larger jib and bridal foil like the Nacras. Also the rudders have been raked under the boat. The boat really sails nicely and is very competitive.
That looks a really nice job. Did you fit the stronger forestay chainplates, fitted on newer Darts ? The originals have been known to fail. The foil will help to spread the load anyway.
Headhunter: You may not get an answer to your query on the UK Dart 18 website. There are very few modifications made to Dart 18s in the UK, as any changes would hugely devalue the boat.
The Dart 18 is a very strict one design class and they always say "If it did not come with the boat when new, it is not allowed", but that should not matter where there is no class racing - like in Florida.
I know that the Canadians have quite a few Darts and fitted spinnakers to some a few years ago.
http://www.dartcatamaran.ca/ 
