I just cut down a Hobie 18 sail to fit my windspeed 15 cat. I only had it shortened to match my old sail. The foot is about 10" longer then the old. The problem, if it is a problem, is where my mainsheet hooks into the clew, its a vertical connection from the traveler. When I try to tighten up the sail the foot will bow. Because of this I dont know if Im getting the best performance from the sail. If I thought it tacked hard before, its even harder now. The cat seems to get pushed back easily or it crabs. My cat has no dagger boards so help prevent the crabbing. I have considered doubling up the bottom batten because its a boomless sail.
Does anybody have any suggestions?
Bob M
Yep. Sounds just like myfootistobigitis. Send us a pic to confirm.
Bob
You will probably never get that sail to perform really well. The luff curve doesn't match your mast/mast-bend and it sounds like the foot is to long, as you say.
Is your boat a sloop or a unirig?
Did you notice increased weather helm when you hoisted this sail?
Some photos would be nice. One taken in profile of boat/sail, one from below the sail towards the top and one from behind you when you are out sailing with it. All photos taken with some downhaul and mainsheet applied, and preferably with some wind filling the sail.
Sorry to say, but I would have started all over with a new sail if you want the best out of your boat.
didn't nacra solve a similar problem by putting a travler track on the mainsail clew?
you could install a boom.
Rolf, I will have to read my "sailing for dummies" again to translate all those terms. I am just a newbie. Learning by the seat of my pants. From what I can see there is no curve to my mast. I don't know the difference between a sloop and a unirig. The sail does fill up in the wind. As far as weather helm? Don't know what that is yet. I will try to gets some pictures. Until I can I will read my book again.
Thanks
Rolf, I just studied up on sloop, unirig and weather helm. My cat is a sloop, I have a jib I do experience weather helm when underway. Much more so then with the old ripped up sail. The cat really wants to turn into the wind. Thank for forcingme to learn a little more about the terms. Will it make me a better sailor? 
Absolutely... 
Sorry if I was technical, did not mean to be.
You can reduce the amount of weather helm by raking the mast forward a bit. It depends on how much you can adjust your standing rigging tough. If you overdo it, you can get other problems, but that's the easiest 'quick fix' for weather helm due to a to large mainsail.
With your amount of initative (chopping the head of an old mainsail, not a thing most sailors would think of or implement in real life), I am sure you will be a great sailor. Get Rick's book "Catamaran Racing for the 90's" available in the onlinemarinestore.com section of this site. That book will help you avoid most mistakes you inevitabely will do by the "seat of my pants" approach.
PS: You do know how to, and are able to, right your boat after a capsize? (I always ask this if somebody are new to the game)
Sorry if I was technical, did not mean to be.
You can reduce the amount of weather helm by raking the mast forward a bit. It depends on how much you can adjust your standing rigging tough. If you overdo it, you can get other problems, but that's the easiest 'quick fix' for weather helm due to a to large mainsail.
With your amount of initative (chopping the head of an old mainsail, not a thing most sailors would think of or implement in real life), I am sure you will be a great sailor. Get Rick's book "Catamaran Racing for the 90's" available in the onlinemarinestore.com section of this site. That book will help you avoid most mistakes you inevitabely will do by the "seat of my pants" approach.
PS: You do know how to, and are able to, right your boat after a capsize? (I always ask this if somebody are new to the game)
Rolf, Interesting that you asked the question on righting the boat. I purpously went out and capsized to see if I could right it. I ended up turtling it and had to get help. I now can handle it myself. 
[With your amount of initative (chopping the head of an old mainsail,]
I didn't actually take the head off the sail. The best match up was to remove about 70 cm from the foot. This kept the sail pretty close to the old sail. The leech curve was also close. The biggest difference was the foot was alittle longer then the old, maybe 25 cm longer. I am going to change the rake and see if that changes the performance. I will keep you posted on the results.