So if we are talking about 5knts+ (no laminar boundary layer) you want a fuller sail downstairs and a flatter sail upstairs. I would begin loosening a bit of outhaul (which gives you the lower part) and maybe try with a little more of rotation (which combined with sheet tension gives you the upper part). If the diamond wires are almost slack, maybe you want to try a little bit more of prebend by applying diamond tension, so the mast follows the luff curve better (I think a little of prebend is necessary in all winds). Cheers, Claus <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />
that's a good link. Actually my problem is not how the best trim would look like in theory, but how can I close my leech in reality. For strong wind my sail looks like I wish. In light winds I couldn't close my leech as much as I wished (with tight main sheet and open downhaul). Maybe my wish of a closed leech is not realistic at all, but with your suggestions it could be that the battens in the top are too stiff or I still have too much pre-bend or the sail cut is not ok. Hopefully I will have better wind next week end, so I can figure it out and make some more pictures. So far thanks for your comments.
I spent 1 minute with accumeasure (download on the Uk Halsey site), result attached. No significant twist in that mainsail, but you really should flip the boat over when taking photos like this, so you can get some force pulling on the leech.
Lots of gotchas to look out for in that analyzis, especially the top one where I dont know the angle of the batten! I would say the sail is a bit deep for my taste all over, but the twist profile is where I would expect it to be with no kind of force pulling on the leech.
In this photo downhaul and mainsheet are cranked on as hard as possible, much more then sailing setting, still cannot pull the depth out of the lower part of the sail. We decided the mast wasn't bending enough below hound fitting to suit the luff curve so Gordon (former owner in pictured) recut the luff curve in that area. The sail is now competitive and has won races against other Mozzies, last by a long way before recut.
Darryn Mozzie 1782
Last edited by Darryn; 06/16/0805:41 PM.
Re: Mast theory
[Re: Darryn]
#145266 06/16/0807:41 PM06/16/0807:41 PM
as I said, I don't how to use this numbers. But I have found a pictures of Gilos Blade Sail with open downhaul and it looks pretty much like my sail, at least in my eyes. I agree with the excessive camber, but with the downhaul I can reduce it quite efficient. No pics in the moment, sorry. I will continue with on the water testing, because I can understand what tell-tales tell to me.
Quote
I think your problem is not the leech but the sail shape in the lower part, i.e. chamber too much aft and too flat close to the boom
Claus,
I tried various outhaul tension (on the water) and I am pretty sure that neither the sail is to flat there nor is the camber to much aft. Both more camber and more forward position of camber let the bottom stall earlier.
The larger the squaretop is the more leech twist you want "see".
I write "see" as in this case it is different from actually having more leech twist. Basically the twist angle is the same but the large head shows more falling away to leeward at its tip.
Wouter
Wouter Hijink Formula 16 NED 243 (one-off; homebuild) The Netherlands
I agree with you for upwind courses. But if I am on a reach, drag doesn't matter, only lift until you can't hold the heeling moment any more. And you get the maximum lift out of a sail or wing, if you bring all parts of it close to stall at the same time. And exactly for this condition I want (or I hope at least) to set up the sail as well. I can always open my leech with the downhaul for the upwind leg. Maybe it is not possible to stall the top with a square head sail + wing mast and without jib (which protects the lower and middle part of th emain from stalling) <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" />, but before I make early assumptions I will listen if you guys can stall the top of a cat rigged square headed and wing masted boat.
The mozzie mast is quiet a different animal to the F16 mast.!
Marcus
Hi Marcus, that is the photo before the recut. Looking for the ability to flatten the sail completely, a small pocket down low is Ok but the depth extending all the way up to the hound fitting with all adjustments maxed means that when downhaul and mainsheet are eased to normal sailing positions you cannot point. Fast downwind but low VMG upwind. I'm sure you know this already with your sailmaking skills, I'm putting this on here for those who might not.
Mozzie pear shape mast and pin head sail is very different to F16 wing mast and square top which this topic is refocusing on, a cue for a timely exit I think <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />