If your jig is 'luffing' when you sheet it fairly tight, move the jib block forward some, this pulls more tension vertically, and puts more tension on the leach, and will often stop the 'luffing', motoring, whatever you call it, it's slow.

And as has been said, it is easy to stall, or backwind the main with the HUGE jib of the Nacra NA's. If so, also move the jib block outward some. It hsould have the 4-way jib system on it, making it an easy thing to adjust.

Good luck, let us know.

BTW, from the looks of the one picture, move the mainsheet block clew forward a bunch, and then play with it to get things flowing right.