Sail Panel Layout

Why do some boats have radial sails that radiate only from the clew all the way to the luff (e.g. Laser Radial, 9ers) whilst other radial sails have a panel running up the luff e.g. RS800, RS400? I imagine sails with a luff panel might cost slightly more and stretch less under downhaul load, but I can't imagine the 49er has compromised performance just to save a few pennies. Similarly, in the Solo class the North radial sail has a luff panel but the HD radial sail does not, and they are competing on performance not price. So, what's the thinking behind it?

Thanks Mozzy. It hadn’t occurred to me the reinforcement on the 29er was for that.

The Original reason for radial panels was to follow catenary load lines (look it up I'm not going to explain it) the inclusion of a vertical panel follows the foot to head load line and is preferred if there is going to be big downhaul tensions which there are obviously in windsurfing sails within which this was a popular feature once upon a time.

Now depending on where the sailmakers put their shaping, in the luff like windsurfing or panels as was the case i dinghies I presume still is, then there will be an element of both. I.e. the vert panel will be shaped at the luff and the radial panels will have some seam shaping, but not too great or it'll defeat the previous objective of locking the shape in place via the catenary stress lines.

Yes, got you. My understanding is that the difference with dacron is that it stretches a lot more diagonally than in line with the threads, and more in line with the weft than the warp, inevitable 'cos of how it's made. the filler/coating reduces this (and that is one reason why old sails perform badly). Laminate sailcloth is inherently less stretchy than dacron and can be made with very little stretch in chosen directions (some stretch is necessary or the sail would only set correctly at one precise setting and be, effectively, non-adjustable).