High density foam for battens on soft wing sail?

I was originally going to build foil shaped battens out of laminated marine plywood coated in epoxy.

Now I'm considering making them out of 8lb density 2 part pour foam(us composites). All solid except for the hole for the mast. They would basically be a naca 0018 section with 4' chord and 8.6" wide; 7 battens supporting a total 50ft2 of sail.

These would probably be less work overall than the plywood and weigh half as much.

Yes, no, maybe, it depends?

 

The foam won't have the strength for the compression and bending load. Further, it will turn to dust the first time the sail flogs.

 

Part of the whole point of this sail design is that the loads are evenly distributed across the batten and between the individual battens, like on a junk rig. People with soft wing rigs say their sails don't flog.

Here are the specs if you're interested:
Parallel Compressive Strength: 250 psi
Tensile Strength: 225 psi
Shear Strength: 130 psi
Flexural Strength: 350 psi

"This 8LB density foam is extremely hard and rigid, like that of a soft wood. Your fingernail can penetrate its skin, however it cannot be dented by hand."

For the sake of argument, what if I were to use the 16lb, would you say the same thing?

They would be 2" thick, forgot to add that.

The thinnest part of the batten, excluding the sharp end, would be 5.5 square inches.

 

Are you making battens or frames like an airplane wing?

 

I apologize in advance, I do not know much about sailing.

I do understand materials a bit, as a builder.

The plywood battens you intended to build would have had a certain amount of flexural rigidity. What you have not explained well is how you determined the size of the ply batten vs the size of the foam batten.

Structural pour foam is not sun stable, nor is it very waterproof. Independent tests done by the fellows at boatbuildercentral.com showed very large amounts of ingress; despite the foam being closed cell. This was due, we believe to large amounts of surface area and probably would dry out; so it goes without much saying the battens need painting, but you already know it. To paint the foam; you’d first neat coat it maybe 3 times.

If you have already accomodated the flex; then show us the data.

An okume box beam vs a solid 8 pound foam would be very different over 4’.

I should add, not understanding how a 2” thick piece of foam attaches to the sail..but not a sailor either as I said.

 

How does pour foam compare to XPS? You can wire cut it (or jigsaw) and glass/epoxy the edges.

 

xps has very low compression and shear ratings, in general, this makes it a poor substrate/core for most marine use as glass shears easily away and delam is sure

there are a few variations, but none suitable for this

 

It can be imagined like an airplane wing, with the mast representing the spar, the battens are the ribs, the cloth the skin; though it will not behave the same structurally.

The chord and width were determined by the overall sail dimensions, the thickness by the needs of adhering the sail material to the batten. Both would be the same thickness.

The original plywood design would have been exactly that, a box beam with 1/8 plywood and stringers throughout.

There will be little to no flex given how the loads are distributed. Warping of the material would be the concern. To my knowledge structural foams do not warp.

The battens will be encapsulated by the sailcloth and will see very little UV. They would be coated in epoxy.

I was thinking of using a less dense pour foam with fiberglass skin, but that would have been complicated to build.

 

Well, so, you are mixing apples and oranges and saying it doesn’t matter.

But, the flex differences are high between say an okume 6mm box 2x5 and a piece of 8# foam. If the goal of the battens is just to keep a minimum shape of the sail and not flex; then 4# foam could also work and no mould prep, just laminating them to each other over any intended shape. Any curved part should be overcurved in such case.

You’ll have to paint any epoxy or it’ll chalk out in uv, even under a cloth.

 

Have you done a structural analysis, including a weight calculation? Airplane wings have been developed to the point that there are only minimal increments of improvements left to make. None of those improvements can be acomplished with low cost materials like you refer to. Airplanes are very weight sensitive, so you won't find other technology that will make a rigid sail lighter or stiffer.

 

All the battens will be hanging off the top batten via the sailcloth, the top batten is the only one that won't be evenly supported, as the halyard will be attached to the balance point. the top batten will have to be made much stronger than the others.

Thank you for the tip, painting them will be easy so no problem.

The total weight suspended by the halyard would be about 30lbs(battens, sailcloth, epoxy, etc), assuming I were to use the 8# foam for the battens. The individual battens would weigh about 3lbs, the top one maybe double that. The only really significant load would be the horizontal wind loads, which would attain an absolute maximum of slightly over 100lbs per batten(at which point I would be dead meat on this boat anyways), but realistically will never see more than half that. The load would be transferred to the mast by the hole in the batten that slides over the mast. This is one thing to worry about, because the load will be transferred through a pretty small contact area, and has the potential to crush the foam here. Considering epoxying sleeves into the holes to spread the load, or maybe just make the epoxy coating really thick in the hole. The battens I had originally designed of marine plywood would have weighed about twice as much and would have been way overkill.

I'm not aiming to make it ultralight, just not unnecessarily heavy.

 

I’d say you threw us off a bit with the plywood and sizes. This explains it better.

Is there a shape? If not, forming them with structural foam versus just using a 4 pound foam seems silly.

 

Refer to original post. The battens will be a naca 0018 foil section 2" thick.

 

I reviewed the basic shape.

So, the mould is a 2 part?

And then, how will you fill it? I’m not sure the thin part of the section will fill easily from holes on top. Or rather, I would be a bit nervous about not filling the narrow area well. I suppose incomplete fill could be added to, I’m not sure how well the stuff bonds to its prior bonds is all. Most of the pour foam I’m familiar with is 2 pound for buoyancy.

That said, I am starting to understand the concern and why you asked because I have some doubts.

There are a couple forum contributors that are far more familiar with moulds than me. Perhaps you think the building is ez and you understand repairing pour foam better than me.

@rxcomposite may have ideas about how to build it and/or @ondarvr may also be familiar with moulding with foam

I would be concerned about them losing shape overtime as well either on their length or the thin edge.

And for that reason and the fill concerns, I may be inclined to build them in an open mould and add a backbone. Of course the backbone material then becomes important. I would think 6mm okume could work.

You’d make an open face mould with flanges for a saw guide. Fill it, then saw using the flanges as a guide, then precoat the plywood and bond it to the half with epoxy putty. Then bond the other side. It might be hard to keep the part in the mould for sawing, but maybe with help, could work.

It may also be possible to find someone with a 3 axis cnc to cut these out using 4 pound H80 one inch foam. It would be two sides per and you could experiment using a backbone or just bonding each half to the other under vacuum and pins. Or even just clamping and pins between a couple 2x4s with a 40 mil target bond thickness which can be achieved with putty and a 1/16” vee trowel each side. I don’t know that the cnc would do it perfectly, but 4 pound foam is sort of a dream to sand as long as you don’t oversand. But sanding out 8-10 parts would be less time than building one half of the mould.

 

Also, @rxcomposite would better understand the load analysis for the mast.

But I’ll take a crude shot.

For a 2” mast, the area of the cylinder would be roughly a cylinder of say 1.75”? length, area is pi•r^2 or 3.1415 sqin. So total area of cylinder is 5.5 square inches and half of it subject to compression loads or 2.75 square inches only. I don’t think we can use parallel compressive strength, but maybe half of tensile or say 112 psi. The hole would tolerate about 300 pounds of force. Of course, this doesn’t go into details about edge damage, etc. And I don’t know the forces! There is more though, the forces, could be uneven and cause all the load to end up on one spot on the batten.

I’m not sure if you can buy carbon tube or g10 tube that’d fit over the mast, but I probably would just to make sure the holes don’t deform. I think the problem would be uneven loading on the foam hole. So, say the forces are 400 pounds on just one edge of the foam only version; it’ll crush a bit..then next time; the elongated hole will have one spot doing all the work, etc., etc.