Nacra 5.5Uni - F17 or F18 square top work?

i own 2 f18 sails that i have used on my mystere 5.5 (30' mast)
a gen 1 and a gen 2 capricorn sail

they worked well, were very powerful upwind but lacked size for downwind sans spin
(the sail is designed to be used w a spin downwind)

i like them both very much but don't use them anymore unless my main is in the shop getting repaired

Are you saying the 'stock' Mystery 5.5 sail has more sail area than the F18, square top sail? I would think any square top would project more sail area than the equivalent pin top? I know my F16 square top main has a LOT more sail area up high were you want it, than my Prindle 18 pin top. In fact, I run the Prindle pin head in big wind, just because it has so much LESS sail area up high.

nope, the f18 has more sail area

but i dont sail stock sails
i use cut down tornado sails or other custom sails i have had made.

and the stock mystere sails has lots of roach and was designed for upwind and downwind sailing

the f-18 has no roach and is designed for upwind sailing and spin assist downwind

Perhaps I'm ill-informed, but I thought the amount of roach was defined by drawing a line from the end of the boom to the top of the mast and looking at the ratio of sail area that is beyond that line. If that's the case, I'd say the square-tops are very

roachy

let me re-phrase that
the mystere sail has a lot more roach than an f18 sail

PS i am not sure but i would think that roach is measured from the clew and head of the sail, not the boom / mast, but could be wrong

Yup - I think you're probably right. I had made an assumption in my mind that the sail is attached at the end of the boom and top of mast, but you know what they say about assumptions <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />

i was under the impression that all the area aft of a line between a point on the head,usually where you attach the halyard, and the clew is considered roach. So a square top has most of it's roach at the top and a H16 sail or Shark sail has the roach distributed lower down the leech.

I modded an existing square top to fit my Nacra 5.2.
But I selected it so that the boom length was almost perfect to start with so I could simply remove a horizontal section and go sailing asap.
Worked well. Check the Texel rating sheets (floating out on the internet somewhere) for measurements, they will give you some idea of what kind of sails may be suitable.
Take note that the sails are either cut for floppy masts or masts with diamonds. Be sure to get a sail cut for a mast with diamonds.