Basically I thought I'd cut the foot of the sail parrallel to the existing foot, as low as possible, but so that I'd have enough of an off cut to fill out the rear of the foot. Looking at it now I think I could go a bit lower than in the second picture.

That's nothing, you should see when I use it to tattoo people's foreheads... oh wait you have

I don't know the technical reason, but I believe they need to be different because a landyacht sail is designed to always be vertical, whereas a windsurfer sail is designed to be tilted windward and act almost like a wing.

Hopefully someone who knows what they are talking about will be along soon.

Some people have just bent their boom to match the shape of the windsurfer sail.

There is the American model land yacht that has the wishbone boom set up. That is the one that has the pilot sitting in front of the mast. Looks rad and a bit scary re safety. There is footage on Youtube of it alleging 70 mph.

I don't see why a sail such as you have Hillsy, ie with a fairly low boom position and not too much roach at the top, would not work well as is.

The shape and aspect ratio of that sail is fairly close to that of the blokart sail I laid over it.

The wishbone boom arrangement does complicate the sheeting set up for a landyacht but the problem is not insurmountable.

If it is not required to recut the sail to meet sail area specs for a class yacht, I think this sail would work quite efficiently.

Paul seems to have the most experience at recycling wind surfer sails. Can you help us out with this one Paul???

firstly what is the sail size? whats it going to go on? It looks like a cammed sail. If so the sail as is will require massive downhaul/outhaul tension at all times, and you will have a sail that is far too full for your landyacht. in order to flatten the sail once you are going you will need to rake the mast back like a sailboarder does. I dont know how you will do that.. the sail can be flattened by getting rid of the cams, pulling off the pocket, recutting the luff and the pocket( it will be curved too and refitting it. you will need to add batten pocket stops to keep the battens back from the mast.
Will the boom length be longer the your cl 5 sail? I suspect so.
when you decide on the angle and length of you new boom position ,measure it from the Back of the foot ,ie you are going to save the roach of the sail and sacrifice the luff area to get the shape right. Remember how I said to look for sails with cloth on the luff , not monofilm!. now you know why.
I havent yet seen a succesful use of a wishbone on a performance landyacht but the mini landyacht from Bunbury that had a wishbone looks to be working fine.