Mast bend, Mast curves and sail materials etc

So following chat in other threads, what makes the best performance sail and mast combo for a wind range of say 8-20 knots on a singlehanded racing sailboat?

Edited by yellowwelly - 04 Mar 14 at 5:04pm

Originally posted by JimC Good design

Ok my tuppence worth.

I am not a sailmaker, but I am a chartered engineer and I have variously used, modified and developed cross cut sails, radial sails, mylar sails, dacron sails, aramid sails, as well as carbon masts both stayed and unstayed.

Firstly, Jim C is spot on on a couple of points - good design is 90% of the problem with sails. Also, performance is only  a secondary concern here, but ease of handling / adjustment and responsiveness are also important, and they are functions of performance.

Finn sailors all have different masts - this is why their sails are tailored - it is not a facet of ODL04 or whatever the fabric in use is.

Also - there is dacron and dacron.

When you need dacron is if you have to accommodate a large range of mast bend, where the bias mobility means that the sail will not distort like a laminate sail which has far less bias mobility.
However the lord giveth and the lord taketh away, and the downside of that bias stability is a reduction in control over the moulded flying shape of your sail, i.e it will get deeper and become more draft aft (more weather helm) as the breeze increases, which to some extent can be over come by sail controls.

A modern windsurf sail doesn't need dacron despite the mast being pretty bendy (will get onto this later) as the tuning range is pretty small - i.e 20mm on downhaul, 30mm on outhaul) so it isn't actually accommodating a big range of bends under normal sailing load. Well, except when landing a massive jump or wiping out, neither are design cases per se.
By having the goodness of a non woven material, the windsurf sail then can (helped by lots of downhaul tension and well specced battens) enjoy far more shape stability, not causing the rider "back handedness" and be able to be used over a decent wind range without needing the controls tweaked.

There comes a point with sail and mast combos, and lets say for the sake of argument that is when the max bend offset (to which the sailmaker cuts his luff curve) at mid height is about 250mm for a 5m luff length, that a laminate sail can be used across that bend range without distorting. If class rules / cost allows, then this option will result in better profile stability, greater wnd range on one tuning setting, and less weight aloft.

There is a perception that stiffer masts are harder to sail to - this is only the case if using a sail that was intended for a bendier mast. If a sail is cut correctly to fit the mast, then the stiffer mast will actually be more responsive, (higher natural frequency) plus a sail with the same moulded shape will be better to use in the light (less luff round, so not too full) and in windy weather (not so dependant on vang for flatness, so can be used with more twist for a given fullness, so more forgiving) with no impact on meduim weather performance.

Experience in both unstayed and stayed (jib headed) rigs in the last decade or more, especially since the advent of carbon masts, is that a large amount of adjustability remains with a correctly developed stiff mast and laminate sail combo, and that the loss of absolute adjustability is overcome by the fact that you aren't chasing the loss of profile (draft) stability.

To say that dacron is necessary on a modern rig to accommodate a large weight range is  propoganda. The effect of, say, a 25% difference in righting moment (big in the context of crew weight) on the actual bend offset of even an unstayed modern rig is small (it is the vector sum of the generally far larger f/a bend due to sail controls / prebend / mainsheet, and sidebend due to RM), and provided that mast is not too bendy in the first place so that we aren't exceeding the aforementioned 250mm for a 5m luff length, then we are still fine with the laminate cloth.

To quote the example of the merlin - whilst you can obviously tailor your sails, the masts are still skinny and quite bendy, but the vast majority of that bend in the case of most stayed boats) is f/a from vang / cunno  / main, and as such even significant RM changes make relatively little difference to the overall bend of the mast. These sails are universally laminate these days so that the goodness of light weight and profile stability can be taken.

So to summarise, you only need dacron if your mast is bendier in absolute terms than modern norms for both performance and ease of handling, or you are fitting an unfair luff curve.

There are good dacrons out there though, and they aren't cheap! Dacron will have better resistance against UV, and possibly wear / chafing although that may depend on the dacron. The filler in dacron does break down with use / flogging though, meaning that the bias stability becomes less and there comes a point where sail is essentially a dishcloth and no cunningham will remedy it.

Dan

Is this the Y&Y forum? The posts above are well thought out, educational and totally lacking in BS. Thank you guys, it was a pleasure to read your thoughts.

Complicated old business this sail design stuff