The size of your gennaker will be determined by maximum luff. Most ready made gennakers are built to general sizes, like small, medium, large etc in clothing. The foot and leech measurements will both be shorter than the luff because the sail must float out and up and you want to keep the foot out of the water. Whoever builds the sail has this restriction factored in.. so they would not construct a sail with an exessively long leech that allows the sail to sag too low.
Your subject boat is a masthead rig, and thus must have it's spinnaker secured up top with a swiveling block mounted on a crane that will place the sail in front of the forestay. I trust you to do more research on this. Search with keywords like: rigging gennaker, how to set up an asymetric spinnaer, cruising spinnaker, etc. There's a nice little piece on
www.thesailwarehouse.com about their spin packages and how to rig them. Don't forget you'll want an "adjustable tackline" and a pair of "tweakers" for the sheets. Again, these are good search keywords.
When I bought my gennaker from North Sails Direct years ago (I believe this discount, offshore arm of that company is no longer operational) the sails were premade, sold off the shelf and came in a range of sizes as mentioned before. You plugged in your luff dimension and chose the pre cut size that was closest, the J isn't really important for this type of free flying sail.... . The advantage of course was the price... very very good. The sail came witha bag and I had also requested a deployment sock, or snuffer, for only $100 more. Also included was a collar of sail materiel reinforced with webbing and sewn in rings that you could use like the "tacker" ... which allows you to install the sail over the roller furling headsail. I didn't have furled headsail at the time, so I rigged another method to attach clew to the forestay and the adjustable tackline. I find that I don't always attach the clew to the forestay unless I'm sailing on a beam or even close reach, otherwise I like the sail to float out as far as possible when broad reaching or running. Before the ATN tacker was marketed sailors used a device called "parrel beads" to attach the tack to the furled headsail. The "beads" are big round wooden or plastic orbs with a hole that you can run a wire or line through. here's a picture:
I got this from Neil Pride website, they sell them for $70. But you can buy the beads and make your own if you choose. just googe "parrel beads" West Marine sells parrel beads 5 for $10 and you can make the lanyard at their workbench in the store. So....you can spend $220 plus for that tacker thing, or just rig the parrel beads for under $20. Or get the sailmaker to throw in the fabric one like mine did.
Finally, (geesh this turned out to be a length response, sorry)
. to learn more about measuring and rigging gennakers. I advise you to visit various sailmaker websites. All of them have instructional videos and articles of interest to make you a more educated sailor. Good Luck.
