Crossbeam Stiffness, Catenary Loading & Luff Curve Matching

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.

 

Beam of your proa is quite big , so if you see that ama is wobling increasing stiffness is necessary.Generally speaking crossbeam of multihull should have maximum stiffness available. There are some exeptions eg some trimarans.
I can't tell much about your sails .

 

A true proa dilemma that I'm very familiar with ;-)
My old P52 proa was a consummate 'ghoster' with a simple polytarp clubbed staysail. But as the wind picked up and stresses increased; the staysail gave a bit, stretched, luff sagged and camber increased when you really wanted it to flatten out. A vicious feedback cycle that ends with a broken mast.
A curious dilemma. Proa's are easily driven low stress craft, yet rig is high stress.
My current approach is a cambered panel staysail based on the premise that the short span between battens allow the sail panels to maintain an efficient aero shape as wind increases. An idea from Robert Biegler in an AYRS publication.
Current craft QB is schooner rigged with such a rig and shows promise but needs more rig trimming time once water warms up again.

 

I collected data on the deflection versus load using a bathroom scale and a house jack. Using the formula for the deflection of a beam that is loaded on its end, and accounting for its cross section and length, I fit for the slope of the stress strain curve and got an effective modulus of 14 GPa for the beam (it is made mostly of doug fir and some uni fiberglass so this number seems plausible). If that is right, it seems that with a modulus of around 200 GPa for carbon, there is indeed room to makes the beams stiffer even if I keep the cross section and length the same. I wouldn't make them from 100% carbon but would probably make them in laminated cedar (essentially as a form) and then put uni carbon on the outside caps. I'd like to calculate how thick does the carbon need to be to improve stiffness by a factor of 10 over my current beams.

 

As I understood you gonna switch crab claw to more like leaf crab claw sail ?

 

Simplest way to stiffen crossbeam is to add c.f. to top and bottom of existing beam either unidirectional tape from someone like Soller Composites or pultruded rod Marske Aircraft https://marskeaircraft.com/ .
BTW your mainsail is similar in concept to Rob Z's Sidekick which seems to work quite well

 

No, first Proa ACDC started life with a Bolger/AYRS bidirectional sail which was very good part of the time and terrible the rest RIP. I switched to a crab claw which was pretty good all of the time with the following caveats in my experience; 1. downwind sucks 2. reefing likewise.

Since then I've built (and modified) several proas. P52, Bionic Broomstick, Nomad, Trivial Obsession and now QB (Questing Beast). ACDC was an entry in a $50 sailboat race, all the rest has been in quest for a better trailerable camp/cruiser for events like the EC and TX200.
Hydro has been pretty well sorted out, aero still under development ;-).

 

If you make new crossbeams , easiest way is make cross section higher . 1" will make big difference . Is connection between main hull and crossbeams 100% rigid ?
Adding any uni fiber on the outside caps is way to go . How much carbon for existing cross section dimmension ? I don't know . Another question is quality of CFC and adhesion power between carbon layer and cedar wood .

 

Not a rig I have any experience with ( ) but when dealing with unknowns like this, one approach is;

1 - take some bottom-up photos of the sails when fully loaded, so you can assess the draft percentage and shape and the forestay/mainsail stay sag.

2- try to find some roughly comparable craft that appear to be well sorted and find out their draft percentage, point of maximum draft, twist, etc. This can often be obtained by spending some time running around the web. Maybe you could try to search for pics of other ply multis (Kohler designs etc) and try to assess sail depth from there.

3- Once you know your current sail draft and stay sag, you can work out the maths of how much draft can be reduced by reducing stay sag.

4- A rule of thumb in some beach cat classes is that the lee stay should go slightly loose when fully powered up.

5 - There are no "benefits by getting the mast's aerodynamic interference away from the leading edge of the main sail". The idea that the mast is bad for the mainsail is an ancient and incorrect idea, possibly sparked or popularised by Marchaj's utterly unrealistic tests that used ludicrously over-sized masts.

As has been said here on this forum by people like Tom Speer, Boeing and America's Cup wing section designer, sailmaker and CFD expert Mikko Brummer and (I think) MIT professor and aerodynamics legend Mark Drela, modern studies show that masts actually provide a major driving effect, not drag. This, of course, is what happens in reality when we see that devices intended to get rid of the mast's alleged drag fail to show the results predicted by old and incorrect theory.

 

@Ovakus: nice boat
I've no experience with Proas - so the following should be read as a purely geometrical consideration.

1. There are two ways to put tension on the stay - firstly, a strong pre-tension of the entire rig - secondly, to transfer the weight of the ama to the mast top as efficiently as possible. For the former, stiffer crossbeams and water stays would actually be helpful, but this is a rather inelegant solution. The other way would be to position the mast more upright - this would significantly improve the force application angle and leverage ratios.

2. Your mainsail and headsail compete for stay tension - as soon as one gets pressure, it loosens the tension of the other, and vice versa. I would test whether it is better with the headsail alone - and if so, attach the mainsail to the mast instead of a stay. Then there is more tension left for the headsail.

3. The easiest way would be to send your girl to the Ama. (I'm kidding )

 

Rob Z's Mainsail is similar but his is a balanced main with the stay running up the sail (kinda like a balanced rudder) while I'm just hanked on at the luff (like a old barn door rudder). His is a bit more elegant and because he doesn't have cross beams but instead a whole bridgedeck, I bet his structure is just much stiffer. Furthermore he has sails hanked onto both the leeward and windward side of his mast. Loading either sail applies tension to the other. While (as mentioned by ropf) my setup annoyingly has one sail stealing tension from the other.

 

The stiff cross beams would allow better preload and I agree that a more upright mast would help. But I'm worried that the competition between the main and the jibs is problematic and inherent in my setup. You can tell how much they interact when tuning the rig. If the cross beams were stiff enough and the rig was preloaded enough, I think both sails could maintain tension and reasonable luff curves even with this geometry but the interaction between the sails feels rather unfortunate and inelegant. A normal main sail that runs up a mast track would solve this (I think) but it would then require a rotating mast and its complexities. I realize that a rotating mast is not really all that complex but I was happy to avoid it (partly because I had access to a nonrotating mast from a C&C24).

 

The top of your mast is stayed in three directions, fore, aft and to windward. the problem is the base of the windward shroud is a bit more flexible than it should be. If you move the base of the mast to leeward you risk collapse in even a slight backwind. No reason not to attach the sail to mast with lacing or hoops if you want. I'm still searching for a rig that is more in tune with the 'easy does it' mantra of a proa. Having a well stayed mast snapped in two by a 48 s.f. polytarp staysail has colored my outlook.

 

Skip, I do like the fact that the current rig doesn't worry me too much if I go aback. I even think I could tack it in light wind (especially if under main alone) if I needed to. I've gotten my shunts to be much more efficient than when I started with the addition of roller furlers for the jibs and lines to raise and lower the rudders from the cockpit but still, shunting takes time compared to a tack.

 

Or with a luff sleeve, like Wharrams Tiki Wingsails, without the gaff ...