The latest and greatest seems to be zero luff curve on mains. What are the advantages?
I have looked a little on the internet and found some 505 class articles. Let me guess a bit.
More precise control of mainsail shape. Less bending of the mast in puffs. If these are true, why are they true? What else?
I am saying less mast bend in puffs because I figure the philosophy is to try to lock a straight shape into the mast. And because the 505 article in Sailing World in 1981 mentioned that.
To me, less mast bend in puffs means a more difficult to control boat as you will have less dynamics helping the boat steer itself on a high fast line upwind. But maybe that is part of it - more speed but more difficult to tune and steer. (?)
I could see where it would take less backstay tension to flatten the main, if one didn’t have to move the center of the sail several inches/feet (little vs. big boats) forward to accomplish the depower.
On our cats, allowing the mast to rotate, also allows it to bend sideways (now facing forward) which in turn flattens the main. We don’t use backstays, so the rotation of mast coupled with “soft” diamonds is our way of depowering. An interesting mechanical result then happens. As the mast turns, the middle of the mast moves forward flattening the main sail in the middle, and at the same time the upper part of the mast moves aft, which in turn tightens the jib forstay (if so equipped). All of this is based on diamond tension and the ability of the mast to rotate.
Back to the 505 - it would “seem” (???) that a straight luff would, as you note, have a very narrow band of performance - Sails that aren’t too full - but also ones that don’t take much to depower. Probably very sensitive to angle of attack as well.
I’ve not heard of this trend in full size sailing. Have you considered asking some of the full size lofts? If it really is the “hot” trend, then the class experts at North, Quantum or any of the other big lofts should be able to explain what the advantage is.
Speaking from experience, I have sailed quite a few classes where there was a lot of luff curve built into the sails including the Laser and Lark. Generally, in moderate breeze and above, you used a lot of mainsheet tension to bend the mast (neither of those classes has a backstay). The sails were cut so that you would still have some twist even with, for example, the mainsheet “two-blocked” on the Laser. But once you are carrying that much sheet tension just to bend the mast, it leaves you very little ability to control the twist. If you ease the mainsheet to insuce more twist, it will only serve to unbend the mast. so perhaps they have decided that it is better to maintain tuning control by not using mast bend to flatten out excessive luff curve.
Another thing to consider is that generally a mast that is flexible fore/aft is also going to be flexible laterally. Lateral bend can cause the leach to open up and the sail to untwist up top. So it may be a matter that these classes have gone with stiffer masts to control lateral bend and are now finding that in order to maintain the right leach tension they are using less sheet tension and therefore less mast bend and therefore they need less luff curve. So it could be an effect rather than a cause…
And finally don’t forget mast breakage. A lot of classes went with noodle rigs and loose diamonds back in the 1980s and the result was a lot of broken masts. I remeber when most C scow sailors carried at least one extra mast with them to a regatta - the top guys would carry 2 spares. The same was true in amny other classes. Perhaps they have decided that not only is that expensive, but it also hurts your final standings if you have a bunch of DNFs for broken masts. So they have stiffened their masts and tightened the diamonds back up and now the masts aren’t bending as much so they need less luff curve…
But like I say, the best answer to this will probably come from the lofts.
I’d tend to agree with Will – smaller or zero luff curve a result of stiffer masts more likely. But what makes mainsail luff curve desirable in the first place? I think there are two aspects.
One is that you can push draft into the mid-sail (straighten the mast some by slackening the backstay or whatever) or pull it out (bend the mast to match the curve). Difficult to do if there is no curve in the first place.
The other is that, as the boom swings onto the reach, draft is necessarily pushed into the mid-sail by the extent of the luff curve, thereby automatically increasing mid-sail twist. You want this in theory, since ideally sail twist should increase for reaching. If you are only racing on windward-leeward courses, though, this isn’t that important.
That you can twist the head off if you need to can be done with straight or curved luff.
This is good stuff! as I am making my first set of sails. All the sets I have from sailmakers have straight luffs, which is what I have been somewhat copying. My masts (ketch) have slight curve due to my set up (stays) so I was ready to cut a curve into the sails. Which is what I’ll do after reading first 4 posts.
Thanks, yar