I'm about to get a new Fireball mainsail in kevlar.  When these came out they tended to be crosscut, however now there are more options available.  You can still buy a fully crosscut sail, however you can now also get a mainly crosscut sail with the area under the bottom batten radial from the clew, and also a fully radial design (eg North Sails).

Over time the Fireball kevlar main seems to develop a problem with a crease developing from the clew up through the inboard ends of the batten pockets and I understand that the semi radial or fully radial design can help stop this. 

Any sailmakers care to share some pearls of wisdom on the merits of the designs?

Thanks!!

I am with Stefan - on a sail as small as a Fireball with cloth that stable I doubt it makes any difference.

Choose what all the top guys use ...

Which is all true but there are scale effects. Radial construction originated for yacht spinnakers, where 40-50 foot yachts use the same cloth as dinghies. Fashion, as much as anything else, drove the construction down to dinghies, where the forces involved are tiny by comparison. Dinghies use the same cloth not because of the stretch forces involved but because anything lighter gets punctured too easily.

Same for kevlar mains. A 35 footer might use the same cloth as a dinghy, because something the weight of a crisp packet might handle the stretch forces alright on a dinghy but it isn't robust enough. Radial construction will make a difference to a 35 footer, but on a dinghy, I'm not too convinced.

It probably depends on the cloth - you're best off trusting the sailmaker to to have made these kind of decisions to get the fastest sail. If you suspect its only cross cut to save money, ask why it's cross cut - with most sailmakers I suspect the answer would probably take at least half an hour - I doubt few if any would just say "because it is"

On the subject of the batten pocket crease, given that it never occurred with dacron sails which are like elastic compared to Kevlar cloth, there may be other powers at work. I've heard rumours its something to do with the mast bending off sideways too much below the spreaders.....

i agree with stefan and rick on this.

in high load, big sails the of axis loads can be a major issue. in dinghys it is marginal.

best bet in my opinion is to split the difference.

-use a cloth with a diagonal warp as well as the major fibres, for stability.

-then align the back half of the sail with the leach and the front half with the luff.

What are they made of? I admit it is a few decades since I bought a kite. (We are talking actual kites here, not spinnakers.)

The main issue with most kevlar cloths is that the mylar laminate is too thin, it has almost no resistance to creasing and breaks down very quickly.

Even if the sails are handled with kid gloves their lastability is unacceptable for an item that costs £600+.

The far more expensive formula 1 engine is being used by professional drivers and is not paid for out of the driver's pocket.

I'm not even sure that Kevlar laminate performs any better than the older - albeit slightly heavier - polyester laminate we were using 5 years ago, but the big differance is you could get a full competetive season out of a polyester scrim sail. Some people are changing Kevlar sails after about 3 months use. (To be fair the very top sailors probably did with polyester then too).

There are many laminate alternatives to kevlar, but for some reason people seem unwilling to try them. It strikes me that as far as practicality goes the kevlar cloths on offer are unsuitable for dinghies becasue it is impossible to prevent the creasing that casues so much harm. The polyester scrim cloths seemed to have more mylar and resisted the cracking much better.

Interestingly back on topic, many classes that have tried radial cut mains have now gone back to crosscuts or hybrids.

Norths make a very odd looking radial jib for the Enterprise.

Dacron racing sails lasting longer than kevlar is a myth. True, the absolute "life" of the sail, (the time until it falls apart) is much longer for a dacron sail, but after a season's use a dacron sail is well past its best. You can still make it look nice for photographs by doing tricks like pulling the cunningham on, but the performance is rubbish compared to a season old kevlar sail.

The Kevlar main on my Fireball (incidentally with a radial bottom section and cross cut upper) has been used for 2 Europeans, 2 Nationals and a Worlds, loads of windy opens and quite a bit of club racing, and hasn't really changed in shape very much even though the cloth hardly looks new any more! I doubt you'd get that much out of a dacron sail.

You do have to look after them, and not do stupid things like leaving them flogging on the jetty or routinely drying them out in direct sunlight, but I'd never go and buy dacron sails if there's an option to get something better.