On Spinnaker boats:
Get a straight traveller rail. It is less in the way and the curved rail doesn't offer any serious advantages. Mainsail is different to jib because it has a boom and so you can angle the mainsheet back a bit without stuffing up you sheeting angle that the sail experiences. With the spinnaker the boom never goes out more than a foot or so and the offset caused by a straight rail is still neglectable small and will be taken up by the angle-back of the mainsheet that just will point a little bit more straight up.
On sloop boats without spi, you can go either way. Often you sail with twist in the mainsail because of the jib or you're upwinding and centre everything.
Curved tracks are really helpful on cat rigged boats that don't have a spinnaker. Here it is almost critical in getting the right sail shape on deep downwind legs. HOWEVER there is a trick to make a straight track work more or less well enough.
This trick is used on (sorry) F16's. Mostly because no curved main tracks will ever be found on these boats as they are normally sailed with a spinnaker or doublehanded and than the curved track is seriously in the way of the skipper without offering any benefit. But some of the F16 sailor like to just pop the main and go sailing on a later afternoon or evening, just like an A-cat. In this setup they have to correct for the disadvantages of the straight rail. On F16's (and other modern (spi) boats like A-cats and F18's) the mainsheet is directly fitted to a loop that runs past the boom and straight to the clew of the mainsail. This is the cheapest, low weight AND low friction option for spi boats and works very well. When going for A-cat mode (only mainsail) they just loop the clew loop over the boom and hook the mainsheet to SECOND loop and loop that over the boom after the clew loop.
When going upwind the two loops will meet and be pulled against eachother. The force is then transmitted from loop to loop via the wall of the boom, with no bending of the boom. So it will work exactly the same the single loop option only adjusting it will take more force. When sailing deep downwind the clew loop will move forward and the mainsheet loop will move back, they need to be helped sometimes, but often move quite well on their own for same strange reason. The two loops will move apart where the clew loop will follow the curve of the sail and the mainsheet loop will follow the straight traveller when the car is ran out. By now sheeting in you have optimal sailshape and the boom will bend a little but not too much as the mainsheet tension is significantly less on the downwind legs.
Even boomed mainsails that don't use loops can use this modification. Just fit your mainsheet to a loop (webbing) or a loop in some high tension line instead on its normal saddle.
Personally I'm gooing to try to add an extra control line (with cleat) to the mainsheet loop (when I use it) to pull it backward on A-cat setup downwind legs. This way I can get maximum mast rotation as well. But again this is sub optimal to a curved track but in this particular mode of sailing but as I'm doing this in recreational sailing and club races only I don't mind much.
In real racing and serious spi sailing I much prefer the straight rail. Both from a performance point of few as a construction point of view.
Wouter
