The design of soft wing sails for cruising

David,
Has any progress been made in fixing the capsized batten problem?

 

Let me rephrase that. I'm now certain that if there is a secondary axis of articulation, the amount of articulation must be considerably less than that in the primary axis of articulation, the one that produces the camber. Otherwise, there will be the kind of malfunction that I experienced.

 

After two months ashore and completely away from sailing, travelling in the UK and Kenya, I’m back aboard and trying to get my head around what I need to do to get Tystie back into full sailing trim. You may recall that at the end of last summer, in two separate incidents, I broke six out of nine of the noses, and several hinge boxes, and concluded that that particular design approach was a dead end. The components that I’d made were too compromised to be worth repairing, and I could not see a way to design out the flaws.

I cannot afford to have any more failures. I have to use a design in which, if possible, there are no unknowns, no experimentation.

I could go back to a flat junk sail, removing the doubled part of the sail on the starboard side and adding straight alloy battens. I cannot bring myself to consider such a backward move.

I could go back to a flat junk sail, removing the doubled part of the sail on the starboard side and adding hinged straight battens. Better, and hinged battens have been used for crossing oceans, but I now know how good a wing sail can be.

Or I could stay with the wing sail concept, and find a way of engineering the battens that uses proven design principles.

Attached is the dxf file for the batten concept that I propose to try.
I can re-use the after battens, which gave no trouble. They will be cut shorter, which will decrease the bending load on them. The forward end will be reinforced with several more layers of carbon braid, to add circumferential tensile strength in way of the hinge.
The double wedge hinge component can be cut out of hardwood. Its cross section is similar to that of a timber batten to PJR scantlings, but being made of hardwood, it will be stronger. Its calculated weight is less than that of the CFRP hinge boxes previously used. Its lower faces will be cut at a small angle to ease friction. It will be secured to the battens by a figure 8 Dyneema lashing, to avoid the risk of wedging itself apart, as has happened in the past to unsecured hinges.
The forward batten will be moulded on a generally parallel mandrel 95mm x 38mm in section, with a short taper at the after end to obtain the required angle of articulation. The layup will be triaxial, with one layer of unidirectional 12K carbon sleeve, and one layer of 3” 12K carbon braided sleeve. Again, the short section in way of the hinge will be reinforced. I will make an external mould, to give a good as-moulded finish and avoid lengthy filling, fairing and painting, which I will not have the time and energy for.
At the forward end of the forward batten, I will add a moulded U-shaped jaw to engage the mast. This of course is common practice in several western rigs. As a core, I have bought some ½” closed cell foam of high density with a smooth skinned surface, which should avoid the entrapment of excessive resin that occurred when we tried lighter weight packaging foam. The layup will be two layers of 2 ½” 12K carbon braided sleeve, on a core of two layers of 1/2” foam.
There will be a moulded luff former, with a layup of one layer of 2 ½” 12K carbon braided sleeve on a core of ¼” closed cell foam. At each batten position, a luff former will be bonded to the forward batten, around the mast, and secured to the luff of the sail using a method that will permit the sail to be removed, leaving the batten in place. There is sufficient space for the batten to rise to an angle of 45 degrees without fouling the mast. There will also be another luff former at the mid height of each sail panel, to reduce the amount of inward deflection that occurs in the windward side of the sail.
The downhauls will be tied to the forward batten, just behind the U-shaped jaw. The fall of the halyard will need to lie on the outside of the sail, to reduce friction.

The sail section is loosely based on the airfoil Clark Y. When I analysed this, and a similar section based on Wortmann fx77w153, I found that the Clark Y based section has 8.5% camber at its midline, the Wortmann fx77w153 based section has 10.5% camber at its midline, and interestingly, the symmetrically-nosed Wortmann fx77w153 based section that I’ve used up to now has only 8.5% camber at its midline. Now, since the junk sails I’ve used offshore have had no more than 8% camber, since I’ve found this to be sufficient and since I need to take a “safe” position, I propose to use this Clark Y based section. I suspect that it will perform just as well as the much thicker symmetrically-nosed Wortmann fx77w153 section.

I have managed to make .dat files for these three sections and have entered them into XFLR5 for analysis. I cannot get the symmetrically-nosed section to converge; but the first two, my preferred choices, seem to work out well. The Wortmann section performs a little more strongly; but in keeping with aiming for a “safe” position, I am concerned that there is too much loose sailcloth for comfort. While the XFLR5 analysis indicates that there is stable negative Cp on the leeside between alpha = 8 and alpha = 13, this is only a static analysis. At sea, with a pitching boat, it may be a different story, and the profile might tend to collapse. Thus, I propose to stay with the Clark Y section. On the weather side, the Cp goes from negative to positive (at the aft end of the luff former) between alpha = 8 and alpha = 9. This indicates the angle at which the sailcloth here will start to lift and flutter when sailing too close, and this seems fine to me. The working range of alpha seems to be 9 to 13, with maximum Cl (1.9) at around alpha = 13 and maximum Cl/Cd (100) at around alpha = 9 and this also seems fine to me.

I should be able to get into the workshop in February to start putting all this theory into practice.

 

David, only about 5% is now held in place at the nose with this new design. You think this is enough? There were discussions earlier about the cloth not assuming proper shape if it's too loose, with the leeward "bulge" moving too much aft.

It would seem that you need to be pretty sure that this will work. Have you considered building a scale model first?

I'm mainly asking because I'm in the process of designing a soft wing sail myself for next summer, heavily inspired by your R&D. Mine will only be around 30-35 sqm though, so I would build the U-shaped jaw out of wood or plywood, with aluminum battens. Not sure yet about the hinge part.

For those of you who can't read dxf files, here's the "batten redesign 12" in image format.

 

Try reducing the angle of attack to get convergence. Then change AOA by small steps and recalculate for each small change in AOA.

 

How about extending the luff formers, for example like this (here to ~8%)? Wouldn't it be an improvement? They could of course also be extended further, as far as the foil shape allows.

 

Yes, I think it's enough, though obviously can't say for sure until I've built and sailed it. I deliberately backed away from using UI1720 section, where the amount of loose cloth and angle of articulation were definitely in the "unsafe" category. I even backed off from using fx77w153, which ought to work, but I can't be certain enough. I settled on Clark Y as the basis because it has enough camber to be useful, when using pretty much just the low pressure side, but also because the amount of slackness in the cloth becomes minimal. I'm attaching a screenshot of the pressure distribution at alpha = 8,the minimum usable, showing that there is quite strongly negative pressure over the curved part of the lee side, with the strongest effect very well forward. This indicates to me that so long as I've got that all important first 5% into shape, the rest will follow. The pressure is very low over that first 5%, and remains low over the curved part of the lee side, increasing gradually further aft. With the lower pressure forward, the greater fullness should be forward, as well, I think. The thing that will cause the fullness to drift aft too far is drag, and I haven't much control over that. What I can do if there's a problem is to haul the cloth aft, tightening it up and reducing the camber. That at least would permit me to keep sailing, whereas broken battens don't.

I believe that my bigger problem is with the structure, not with the sailcloth behaviour. So making a scale model doesn't help with that, and what I must do is to test the first-off batten assembly as brutally as I can before it leaves the workshop.

Before I settled on the 5% of chord luff formers, I drew some bigger, wider ones, 8% or more. Yes, you can do it that way, but you're pushing the weather side out further, more towards the shape of the symmetrical noses, resulting in less lift. Also I don't think you've spotted that the luff former has to have a wide enough slot to enter the mast (unless you plan to drop it over the top). That means that the aft ends of the luff former are in air and a bit vulnerable to accidental damage.

I'm glad to hear that you're working on a wing sail as well. Please could you post some drawings, as soon as you have them? I'd like to compare them with mine.

 

Sorry, didn't work for me. I tried at alpha = 1 and nothing happened.

 

Yes, it would seem so.

I need to get better aquainted with XFLR5. I've managed to import and create foils and do basic plots ("XFoil Direct Analysis") but I have no idea how to plot the pressure distribution, any quick tips or pointers on where to look?

True, makes sense.

My idea was to have the extension fastened to the jaw, not to the luff former itself, so there's no problem with getting it around the mast. But yeah, if keeping the windward side as narrow as possible helps overall lift and as long as it's possible to do keep the cloth behaving using only 5% shape then that's probably the better way to go.

Of course, I'll let you know when I've sketched something together.

 

How did you manufacture the carbon battens, as in what type of carbon, resin etc and technique used? The thread has become too long to find where you made them in case you described it there... The strength of the battens can vary wildly depending on the fiber orientation, please tell me you used a large percentage of uni directional?

Is your mast tapered? If not, then incorporate a sleeve in the batten which wraps the mast and resists all other degrees of freedom other than rotation. It would most likely have to be made as a split bearing to allow installation without a crane over the top, and use a very low friction plastic such as uhmwpe etc as a liner.

As much as you may think its a pain in the ***, if considering a entire new system, consider reinstalling the mast itself in the boat on bearings, so the mast rotates rather than the battens. Might solve a lot of issues. If this an option, I can put you in touch with professionals that have made these bearings for masts before.

Lastly, I still believe the discussion on foil sections is largely purely accedemic. Unless you can ensure accurate reproduction of the shape over the entire span that is, and from the pictures I've seen to date, this is far from the case with large variations in fullness between battens...

 

If you're on the "Direct analysis" screen, you have a bar at the top with (L to R) new project, open, save, OpPoint view, Polar View, a box showing which foil is active, a box showing the conditions you entered into your analysis, a box showing what alpha is active. If you click "OpPoint view", you get the view I gave a screenshot of. On the right, halfway down, tick the box "Show pressure". You can also "Show boundary layer", which is useful, but I wish there were an option to show flowlines and stagnation point, as well.

 

Both of ours are.

Wasn't this topic already discussed on pages 6 and 7?

Of course it's not possible to replicate a foil exactly using a soft wing sail, but it doesn't hurt to try to come as close as possible. Plus doing the research in what makes a good foil shape for this purpose gives a better overall understanding of what works and what doesn't and more importantly why.

Got it, thanks a bunch!

About the sails themselves, I've been planning to use canvas 2.7 meters wide* and I've been thinking about ways to make the sail construction as simple as possible, combined with easy assembly of the rig. The easiest way to create the batten pockets would probably be to sew them without any cutting of the canvas, just by folding and sewing (attaching rudimentary picture to illustrate, second drawing). Leech reinforcement would of course be sewn before the pockets by doubling or tripling the canvas. Pockets for the the jaw and luff former could then be made in the exact same way, but by cutting off the pockets afterwards (first drawing) where they're not needed, to make rig assembly possible. The part of the canvas that wraps around the mast would then be attached to the single aft part using velcro, in a way that it can be loosened/tightened a bit in the case where the unattached part of the canvas doesn't behave properly.

Using batten pockets for the front part, the jaw and luff former, introduces a potential problem. The position of these pockets might not be perfectly predicted and thus some lacings should perhaps be added (or replace the front pockets altogether?), to be able to adjust where the canvas is attached to the front. The jaws could for example have vertical holes in them for attaching dyneema lacings. This on the other hand is not an optimal solution since it would make the jaws weaker and sewing in a lot of dyneema or webbing lacings is a lot of work. Any bright ideas on how to tackle this?

Edit: Also thought about having the luff part of the canvas allowed to slide in its pockets somehow, without being locked in place along the "battens" (in this case jaw and luff former), but it seems like a not so great idea as it would most likely mess up the formation of the front 5-20% of the sail. My intuition says that the canvas around the luff should be fastened to the nose, to have equal amounts of loose cloth on both leeward and windward side.

Back to the think tank...

*) The canvas for a 30 sqm sail would be in the ballpark of 10 meters high times 6-something meters wide so it would require three pieces of 2.7 m wide cloth. The third one, the one that attaches to the single part aft, doesn't need pockets at all so it could just be a straight uncomplicated piece of canvas.

 

Using this kind of construction (which I certainly would with your size of sail), I'd insert a wooden wedge-shaped end into your aluminium tubular battens, and then have these enter a square or rectangular section tube, 1/8" wall thickness, with two pivot pins through the thick end of the wooden wedges and near the ends of the square/rectangular tube. As you have CAD, you are probably already aware that my design needs a half-angle of articulation of 20 degrees.

 

That's very wide cloth - what make is it?

Your batten pockets in the after part of the sail will work fine; they are similar to the ones in my current sail, though I worked with horizontal cloths 1.5m wide that form the pocket as part of the seam. You'll need to make the pockets quite wide, with the seam well above where the batten will lie, as it droops down under the influence of gravity.

The midway join: in my previous wing sails, I used Velcro with buckles at the batten positions; in my current sail, I used individual zips with buckles at the batten positions. I don't like either way. It's very difficult to get the Velcro to engage smoothly and without puckers. It's more difficult than it need be to assemble a big sail around the mast. I've come to the conclusion that the Beneteau sailmaker is right, and the join to assemble the sail around the mast shouldn't be at the mid point, but forward, somewhere. The two forward parts should be sewn to the single after part, with quite a wide un-sewn height in way of each batten. My current thought is to use a full length zip (which I've bought already), just athwart the mast. Here, it should be in the separation bubble on the lee side, and in slow moving air in the weather side. It shouldn't be right at the luff, as it is in the Beneteau sail, as all the important aerodynamic action is taking place here.

I plan to have a solid enough forward end to my U-shaped jaws that a pair of screws can pass through spurtooth grommets in the sail, and into the jaws. On the zip side, the screw would be aft of the zip, to carry the horizontal tension and keep the zip unloaded. Since I plan to bond the luff formers to the jaws, around the mast, for extra strength and rigidity, I can't put the luff former into a pocket. I can, though, put a short Velcro-fastened flap onto the sail right at the luff, in between the jaw ends.

However, any extra luff formers, between battens, can be in pockets - much quicker to assemble than lacings, though once assembled, they can stay in place.

 

I don't quite understand what you mean. I'm visualizing something like this, which would allow two axis articulation, but I don't see how/where I would use wedges..?