Andrew,

I saw a number of 18 Squares in the late 80's that had curved travellers. These had a center of radius behind the mast, so the clew of the sail would be pulled toward the mast as you let out the traveller. This makes the sail fuller and more powerful. The ends of these travellers were higher than the center, inducing twist as the traveller is let out. These tracks were mounted to the top of the rear beam in the center and then a molded track support attached the ends of the track to the hulls. When properly designed, the mainsheet would be set at the start and then the traveller was the only adjustment made as the boat went upwind or downwind or anywhere inbetween. Nice for single-handed boats, however, the traveller is a bit of an obstacle when moving around the boat.

All the systems I saw were purchased from Harken.

P.S. - All the Nacra 18 Squares started as 11' beam. Why reduce it to 10'? I would have made mine 12' beam if I had to change it.