Layup on polystyrene

Hi, newbie here. I'd like to build a wing sail using expanded polystyrene core with epoxy / fiberglass skin. This would be for an autonomic sailboat that would sail offshore for extended periods (like the Microtransat challenge).

My question is about what layup would you recommend for the wing skin.

The main element of the wing would be 5.5' tall, 1' chord and max thickness ~2 inches, stays running to the top of the wing. The total weight of the boat will be around 60 lbs.

The main challenge is to survive knockdowns, and while it would be very hard to calculate loads on the wing during a capsize and getting rolled by a wave, I thought I would build it like a surfboard, which are (at least some of them) constructed in the same way, and have to endure waves and a person standing on top.

Looking at various websites I found that some build boards using EPS core and 1 layer of 4 oz glass on the bottom and 2 layers of 4 oz on the top, while others recommend several layers of 6 oz.

Based on info I gathered I'm thinking of building the wing by first cutting the EPS core in half along the wing across the widest part of the foil, and make a stringer with a layer of 4 oz glass glued between the the 2 halves. With the skin, this would create an "I" beam structure. The skin would be 1 layer of 6 oz glass and 1 layer of 4 oz glass. I could maybe even use unidirectional carbon tape running along the wing on the two sides of the stringer to strengthen the I-beam. (I don't want to build the whole thing from CF, would be too expensive.)


So 1) What would be your suggestions for the layup for a wing like that. 2) Is it stronger to use several layers of lighter cloth than 1 layer of a heavier cloth with the same combined weight? (Like 2 layers of 4 oz vs 1 layer of 8 oz.) 3) Does it make sense to use unidirectional carbon tape as reinforcement along the widest part of the wing (embedded in the fiberglass layup)?

Survival is more important than performance, but of course the lighter the wing the better.

Thanks in advance!

 

If this is a single element, cutting two halves and routing out a space for a carbon tube(just forward of the quarter chord point-about 15-19% of the chord aft of the leading edge)then gluing it together and glassing with one layer of 5.7oz carbon cloth might be the best combo of lightness and strength. I'd try to use 1lb per cu.ft. foam but make sure the cut is clean or sanded very carefully to eliminate pockets for resin to add weight and use epoxy only.
60lb is not light for a model with a 5' mast-any details of the boat?
I'm not sure a wing is the best solution here especially a single element at this size.
I'm interested in the autonomous concept and wish you good luck!

 

I would recommend this book:

http://www.amazon.com/Moldless-Composite-Sandwich-Aircraft-Construction/dp/B000BUJP5A

This is written by Burt Rutan, the designer of SpaceShipOne. In it he writes about making aircraft with methods like what you're describing. I think your sail is probably more similar to an aircraft wing than a boat hull.

SPOKEN BY A NON_EXPERT!

 

Thanks Doug! The wing will be a two element wing, I just focused on the main element in my question, since that will be the one that holds the whole thing together. The 2nd element will be a one-piece flap.

I've been following all those threads about wing sails here and on SA, so I know there are more sophisticated ways to build a wing. However, here the main goal is to keep everything together and performance is secondary.

There are several reasons for using the wing on this boat. One is that control systems will be overly simplified to reduce power consumption and possible problems. Thus, it is accepted that sail trim will not be perfect, and I think a regular sail would not survive months of punishment. The other reason is that the plan is to make this boat a catamaran that will self-right itself. For this purpose, there will be a keel (that's why the weight) and I also need the flotation created by the wing when it's underwater.

Anyway, it is just a plan right now, and one crucial thing is to figure out how heavy the wing will be, because if it turns out to be too heavy, then the whole self-righting catamaran concept might fail.

I thought about using a tube in the wing, but then I thought I could place the same amount of material in the skin, where it's more effective. I realize I could save weight by using carbon everywhere, but if the weight difference is not that big, I would rather use glass instead. I want to keep it simple. I don't have experience with vacuum bagging, and I don't want to spend too much on the boat as there is a good chance it will be lost eventually.

So do you think the layup I described above would hold things together - is it too much / too little?

 

Thanks, I'll check that book, but one big difference I see between this wing and an aircraft wing is that an aircraft is not be dipped into the ocean regularly.

 

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You can hand lay one layer of 5.7 carbon or your preferred laminate but a gloss finish would be hard to achieve with anything that thin without a lot of weight gain. I would not try to do the layup you suggested because doing it in styrofoam could be difficult and weighty. But it would probably work. I'd prefer the carbon tube because it gives you something at the bottom and top to attach to and is really simple to install. Based on my model experience and your requirements a 3/4"-1"OD carbon tube with a 1/16"(.0625") wall should work.
There are also places that sell very thin carbon already laid up with a glossy side and a peel ply side-you might be able to find glass done that way to simplify more. Or lay up your own sheet on a piece of window glass or smooth plastic(melamine was suggested in a different thread but I have no experience using that for a flat sheet mold).
Good luck!

 

You might want to look into the design work that Dr. Drela has done with sailplanes.
I have build some vac bagged foam core wings that are much as you are suggesting per the 'I beam' inset in the core.

For the wings I built a piece of uni-directional CF was inlet into the foams surface.
No splitting of the foil was required.
I was using Hy-load 60 foam which was CNC hot wire cut to something like .0005 accuracy (interesting and convenient process).
The wing skins I was doing are lain up on a sheet of thin nylon, then placed over the foam cores and into the vac bag.
A slick process where the layup is wet out in all its component parts on the nylon & resin weight can be very carefully controlled. The nylon carriers leave a leave a very smooth surface when pulled from the core.

The wings I was build were quite small & used 1.5 oz Kevlar. I couldn't begin to quantify the skin thickness for your sail as this should come from a a bit of engineering.

I think Dr. Drela is a member here?

 

Yes I was wondering about creating a smooth finish. Would it make sense to use some very light cloth as the last layer (while reducing weight in other layers)?

How would laminating on a piece of glass help? I'll need to get it off the glass to put it on the core. I don't think it will remain smooth in the process. Maybe I misunderstood something?

 

Thanks a lot. Could you explain how you did the CF inlet into the foam? That might be a better way to do it.

 

Do you mean extruded polystryrene (XPS) instead of expanded polystyrene (EPS) for the core? XPS is Dow Styrofoam blue board, Owens Corning Pinkboard, or Pactiv GreenGuard

 

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I've used window glass as a mold in the past. It needs to be waxed and the sprayed or poured with PVA. Pouring works best on something the size of the sheet for your wing. The idea is to lay up a sheet of glass or carbon on the window glass using peel ply on top of the last lamination so the resulting surface can be bonded to the styrofoam. Laying up on glass produces a cosmetically nearly perfect outside surface. I'm going to back off a bit with this caution: I've used this technique on large panels and it works well. I have not experimented with thin coreless sheets so I'm not sure how well it will work especially for someone with little experience. I'm getting ready to build a model trimaran where I will purchase the thin sheets rather than lay them up and I will experiment with bonding them to balsa. When that's done I can speak more authoritatively about the technique.
If the technique works as well as I think it will it will produce a cosmetically great finish with zero sanding and fairing required-if it doesn't work so well with a relatively thin sheet it will be a mess. I think its best if you don't do this first time around-don't take the chance of creating a bigger problem than the one you're trying to solve.
If you have a hobby shop specializing in model airplanes nearby you could possibly find someone that would cover your styro wing with aircraft plastic which would be strong and light. It requires that the foil shape cut is clean and smooth and should be waterproof BUT I DON"T KNOW THAT FOR SURE. You could test a small sample......
Good luck!

 

No, I'm planning to use 1 lbs. cu.ft white EPS. I can order blanks online if I'm lazy or could wire cut it myself. I don't expect much structural strength from the foam just something to provide the shape for the skin. I thought that having a stringer would be enough to carry the compression loads.

 

OK, I understand now. I will keep the construction simple. I have some experience working with epoxy+fiberglass, but mostly fixing boats and building small RC boats. I don't have equipment for vacuum bagging, and I don't want to spend a fortune on this boat. It's just for testing a very simple concept for autonomous sailboats.

 

I am looking up a tutorial for the technique I am referring to.
I will post as soon as I find something or walk you through it.

It is a very simple process & I just used a cheap plastic automotive vacuum pump for doing the bags.
The results are great and you can do a very complex layup with fussy fabrics- light weight, and glassy smooth finish.

For starters call Laszlo Horvath at Compu-Foam/Core:

http://www.compufoamcore.com/
He is the nicest guy in the world with a lot of experience cutting foam.
His prices are very reasonable and he will cut beautiful cores for you to a very accurate foil.

The core and skin layup work together to provide a structure which will resist creasing/bending/twisting when loaded.
I would look into a higher density foam for the core and light weight fabrics such as kevlar or carbon for the skins.
In other words the weight you save in the skin with higher strength fabrics is invested in a higher density core to provide a stronger structure for less weight all up.
A start would be specifying the foil parameters and boat specifics to get a design window that can be fleshed out if a Engineer/NA happens to wander by.

Don't be fooled by the foams used in surf board construction- the load case for a thinner foil may be far higher.

 

I see you guys won't let me get away with doing someting cheap and simple!

Believe me, I would love to play with CF, kevlar and vacuum bagging, but my wife will kill me if I spend a fortune on a boat that I toss into the ocean and never see again...

Anyway, I'll look into using some simple vacuum pump, maybe that's worth it as it will help to make a nice surface (if I don't mess it up too badly).

Thanks for that and also for the link for the core. I'll give him a call, and based on the name it seems I can even speak Hungarian with him.

I understand the interaction between core and skin. I thought about using EPS, because I found another website - http://www.flyingfoam.com/ where I could get the core for a very reasonable price from 1lbs EPS. I thought that by using a stringer I could compensate for the lighter and softer foam.

As for the loads, here are some simple calculations I did. I'm not an engineer, so forgive me for not using the right terminology. Based on the max righting moment of the boat, I expect the max bending force on the wing under sail (assuming wing is supported on two ends and force applied in the middle) will be ~130 N and max compression on the wing is ~280N. The max thickness of the foil is 2 inches and the wing is 5.5 ft tall.

However, the problem is calculating loads when the boat gets rolled by a big wave and the whole thing is yanked around. Since it just seems too difficult to model, I thought I would throw out all the science through the window, and just make something that would be strong enough to hold a person standing on the thing. I told you I'm not an engineer!

So basically I just need some empirical input like if you use this many layers of that, it will be plenty strong. Like I said the emphasis is on surviving the journey and not on making the boat faster by half a knot.

Thanks for all the advice!