As winter closes in here in upstate New York, I'm thinking about modifications for next year for my 28' pacific proa (approx 1700 lbs and 300 sq ft of sail area). It uses a regular Bermuda sail plan similar to the one on CLC's Madness or any other Brown-derived pacific proas like Jzerro. But the rig is a little different -- see below. For this winter I'm interested in improving my upwind performance. To do that I think I might need to recut my sails or design stiffer cross beams (or both).
In light air the boat can ghost along when everyone else is becalmed. But even in light air, upwind performance is only ok. And when the wind picks up to 12-14 knots, when the ama is just skimming the water (ie when the pacific proa is at maximum loading), my sail shape goes to hell or at least that's what seems to be happening. In gusts, rather than taking off like a scalded rabbit -- as a properly designed multihull might do, my boat seems more to load up in the gusts and we surely speed up but not like we should. And I feel like I can't point above 70 degrees AWA. Perhaps most tellingly, fairly large luff scallops appear in the sails.
I'm guessing that my cross beams aren't stiff enough and they just flex too much, allowing much more luff curve than the jib (and the main -- see below) was cut for and the boat is just overpowered in the gusts with crap sail shape.
I can certainly give more details about the cross beams and the rig (See below) but, as an initial matter, I wanted to know if there are any rules of thumb that might help. 1) Is there conventional wisdom about multihull cross beam stiffness? For example, under normal sailing loads (in the 12 knot wind range) how much flex is acceptable? My two beams are about 16 feet overall with 10 feet long extending from cockpit to outrigger and are 4.375 inch high by 3.625 inch wide box beams made of doug fir caps with plywood webs (plus some unidirec glass on tops and bottoms) and they bend by about 4-5% of their length with 200 lbs of load on each. Does that seem too bendy? 2) And as to the sails, is there conventional wisdom on how much luff curve is reasonable? For example, on a 30 foot luff, how much luff curve is normal for a jib? The more luff curve I allow, the lower the loads on the cross beams.
The above questions are somewhat general and have some application to many boats but in my case the problem may be much more acute. As perhaps can understood with the aid of the pictures, my rig is a little weird: both my jibs and my mainsail are hanked onto stays and the mast is canted to leeward. The jibs are normal jibs hanked onto stays. The mainsail is not attached to the mast but rather is hanked onto a stay also. In this way I don't need a rotating mast yet I can easily rotate the main through 180 degrees for shunting. And in addition I hoped that this design feature would gain some benefits by getting the mast's aerodynamic interference away from the leading edge of the main sail. But I'm concerned that this weird rig choice has its downsides.
These rig particulars are relevant for the present discussion because for my boat both the jib and the main act as loaded cantenarys and, because the mast is canted, those cantenary loads aren't just compression loads supported by the mast alone rather they also contribute to the tension in the windward stay out on the windward ama and therefore they add to cross beam flex. In short, I suspect that my somewhat unconventional design is causing issues and the design is quite susceptible to large changes in sail shape due to cross beam flex. This is especially true if I require my luff curve on the main and jibs to be small. I can of course provide more diagrams etc to make all this clear but I wanted to start with the more general questions.
So I'm thinking that I'll need to recut the sails (especially the main: it has luff round right now and I'll need luff hollow instead). But I think that I'll need to stiffen the crossbeams as well by either a) adding unidirectional carbon onto the bottom cap b) adding water stays but the angle is pretty crappy ~ 8-10 degrees or c) rebuild new stiffer beams but unless I really want to change things, I'm kinda stuck with the 4.375x3.625 inch cross section.
But before I make any changes, I'd like to discuss the issues and perhaps calculate and analyze first. Thanks in advance.
In light air the boat can ghost along when everyone else is becalmed. But even in light air, upwind performance is only ok. And when the wind picks up to 12-14 knots, when the ama is just skimming the water (ie when the pacific proa is at maximum loading), my sail shape goes to hell or at least that's what seems to be happening. In gusts, rather than taking off like a scalded rabbit -- as a properly designed multihull might do, my boat seems more to load up in the gusts and we surely speed up but not like we should. And I feel like I can't point above 70 degrees AWA. Perhaps most tellingly, fairly large luff scallops appear in the sails.
I'm guessing that my cross beams aren't stiff enough and they just flex too much, allowing much more luff curve than the jib (and the main -- see below) was cut for and the boat is just overpowered in the gusts with crap sail shape.
I can certainly give more details about the cross beams and the rig (See below) but, as an initial matter, I wanted to know if there are any rules of thumb that might help. 1) Is there conventional wisdom about multihull cross beam stiffness? For example, under normal sailing loads (in the 12 knot wind range) how much flex is acceptable? My two beams are about 16 feet overall with 10 feet long extending from cockpit to outrigger and are 4.375 inch high by 3.625 inch wide box beams made of doug fir caps with plywood webs (plus some unidirec glass on tops and bottoms) and they bend by about 4-5% of their length with 200 lbs of load on each. Does that seem too bendy? 2) And as to the sails, is there conventional wisdom on how much luff curve is reasonable? For example, on a 30 foot luff, how much luff curve is normal for a jib? The more luff curve I allow, the lower the loads on the cross beams.
The above questions are somewhat general and have some application to many boats but in my case the problem may be much more acute. As perhaps can understood with the aid of the pictures, my rig is a little weird: both my jibs and my mainsail are hanked onto stays and the mast is canted to leeward. The jibs are normal jibs hanked onto stays. The mainsail is not attached to the mast but rather is hanked onto a stay also. In this way I don't need a rotating mast yet I can easily rotate the main through 180 degrees for shunting. And in addition I hoped that this design feature would gain some benefits by getting the mast's aerodynamic interference away from the leading edge of the main sail. But I'm concerned that this weird rig choice has its downsides.
These rig particulars are relevant for the present discussion because for my boat both the jib and the main act as loaded cantenarys and, because the mast is canted, those cantenary loads aren't just compression loads supported by the mast alone rather they also contribute to the tension in the windward stay out on the windward ama and therefore they add to cross beam flex. In short, I suspect that my somewhat unconventional design is causing issues and the design is quite susceptible to large changes in sail shape due to cross beam flex. This is especially true if I require my luff curve on the main and jibs to be small. I can of course provide more diagrams etc to make all this clear but I wanted to start with the more general questions.
So I'm thinking that I'll need to recut the sails (especially the main: it has luff round right now and I'll need luff hollow instead). But I think that I'll need to stiffen the crossbeams as well by either a) adding unidirectional carbon onto the bottom cap b) adding water stays but the angle is pretty crappy ~ 8-10 degrees or c) rebuild new stiffer beams but unless I really want to change things, I'm kinda stuck with the 4.375x3.625 inch cross section.
But before I make any changes, I'd like to discuss the issues and perhaps calculate and analyze first. Thanks in advance.
) but when dealing with unknowns like this, one approach is;
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