Carbon Mast

I have training in making stuff out of carbon and fiberglass, etc. What I'd like to do is make a carbon mast for a sailing skiff. The mast is about 16' long for a lug sail rig.

My initial idea is to shape a core out of blue construction foam as I would a typical wood mast, 4-sided with taper, 8- then 16 sided, then round it. I'd do the same then for laminate use carbon socks, a biax layer, a uni, then a biax. I'd vacuum bag the layers and then use acetone to melt out the foam.

I'd love to hear how others have made a one-off carbon mast.

Cheers

 

Hi Clinton,

I made beams for a small tri by wrapping plastic around a solid shape and vacuumed that. I pulled the former out afterwards with a block & tackle since it was't taper, but wasn't too hard to do.

The thing that creates problems with wrapping round things is when you vacuum them the outer glass implodes on the inner shape, which causes wrincles and inconsistencies. You have to wind very tight to prevent this.

Off late I was thinking more of ways to use compressed air instead of vacuum, higher density and more strength with less weight, but not so easy...
I should mention, I was thinking of applying the compressed air from the inside to expand the material to an outer former.

 

I made one in a similar way. It was 42' with a 4.5 X 9" airfoil section for 32' and the last 10' (above the masthead) tapered. The mast rotated.

The problem I can see with your proposed build-up is that you need a slow curing resin system to be sure that you don't wind up with a potentially fatal flaw from the resin system curing too quickly, because it's going to take many hours for you to complete this layup.

So you will need a elevated temperature curing resin system. Between the heat and the vacuum (you will need at least 25"Hg and 120-180*F), the foam you are proposing to use will not survive with its shape intact and the whole thing will cure mis-shapen and become scrap. There are foams, even expanded polystyrene foams, which will work, just not your proposed blue construction foam. The typical density that all of the foam cutting/molding shops (which exist for the architectural decoration business) is too light and soft also and equally will not work.

I had to find a foam packaging manufacturer that was willing to make a small run of airfoil sections. I had them cut a 2" square section out of the center, then all the pieces could be indexed on a long piece of square 1/4" wall 2" aluminum tubing, which formed a sort of 'backbone' for the layup. You'll need to wrap the assembled foam cores in some kind of plastic material. I used box tape, which worked fine and left a nice smooth finish on the inside of the mast.

I have more than enough carbon braid and uni sleeve left for you to build this mast, which I will gladly sell you for a song. PM me if you're interested.

 

Thanks Jimbo, i may take you up on that after I learn some more.

Here is my latest thought. The mast I want to build tapers from over an inch to between 2-3" around the partner then retapers down to somewhere between 1-2". I can get the exact numbers. But the point is, it seems that the best way to make the mast is to make a wood plug, release agent it really well, and use the wood plug as the mandrel and infuse the carbon around the "mandrel". But, the taper prevents removal of the mandrel. So why not just continue the taper all the way to the butt end of the mast so that I can remove the mandrel afterwards. So the mast is much bigger D at the bottom. Why would that matter. In this case, the foam core/dissolving away with acetone-technique is crazy. Instead the wood plug/mandrel could be reused. Thanks for help, what do you think?

Cheers,
Clint

 

The other option would be to keep your double taper, and make a multi section core. Basically it would be a rod a little thinner than the inside diameter of an opening in the mast. From there you can cut narrow curved slats that would wrap around the rod like barrel staves. Coat everything with release material, and then hold your mold together with a light weight plastic. Then apply your fiber and epoxy as normal.

When done, pop the rod core out, and then pop the staves out. The thing to remember is that you have to design with two stave types, a trapezoidal cross section type, spaced with triangular cross section ones. With out the use of a triangular (with the tip pointing inward), you would get a barrel, which would lock them in place even after the core was pulled. The triangles would let the wider slats fall into the core's hole and then be removed.

 

You can buy a windsurfer mast in the approximate dimensions that you mentioned. You can pick one up for less than the cost of the materials you will need for the CF mast.

 

Clinton,

Been there, done that and learned a lesson. Nothing fundamentally wrong with a carbon mast but your schedule is not very efficient. The socks that are much easier to use than uni carbon tows add almost nothing except cost, weight, time and work to the project. The main reason to use carbon for any project is low tensile elongation and light weight compared to other materials. In bending a spar made with a combination of bias and uni carbon, only the uni contributes anything at all to the stiffness. The bias cloth only contributes hoop strength, which is necessary but easy to obtain with a light sheath of carbon or glass cloth.

You don't say how much of each type you are using but it would appear that at least 2/3 of the material is bias and adding very little to the performance of the mast. A hand layup is also not the best to get a high fiber to resin ratio which means higher than optimum weight. A resin rich carbon layup is mostly a high cost, low performing job that will not give the best results.

For the amateur, an aluminum mast is still very competitive with carbon in weight and far cheaper.

Edited to add that messabout's comment on windsurfer masts is a good one. Keep in mind that a Windsurfer mast is subjected to much lower and less concentrated bending stress than a free standing one with the same sail area. If the mast has standing rigging, the stresses are mostly compression and little bending. Carbon is not so great in compression.

 

This is not entirely true. The braided, bi-directional socks will stretch lengthwise over the mandrel and shrink diametrically when sized appropriately. When stretched, the tows will lay at very low angles (like 15* or so) to the long axis, so they do contribute to the compressive strength of the mast. A&P technology has a little calculator (nomograph) on their website which will show you how each of their carbon sock products behaves over a given stretch, so you can select the right product. Although the uni material is obviously the most efficient in building compressive strength (the only thing that matters in resisting the normal loads in a stayed rig) you still absolutely must have some bi-directional material in the mast or it will wind up fragile WRT 'incidental' loads, like something 'whacking' the mast during service and make the mast susceptible to failure from buckling or just getting a hole knocked into the side. Even though these potential incidental loads come only from the outside of the mast, in order so have a balanced layup, you will need to mirror the layup so it winds up that the first and last layers are bi-directional socks, while the central layers will be uni-socks.

I have not found that uni socks are any harder to handle than bi directional socks.

It looks to me like the socks I have are probably too large in diameter for you needs, BTW

Jimbo

 

Unless the bias angle is very small, the axial uni carbon takes most all of the tensile load. You say 15 degrees, which may be small enough to make a difference. but most amateurs don't stretch the sock that far.

While 15 degrees is a small angle, the fibers in the sock are still woven and wound around the spar. Both of these detract from their ability to absorb tensile and compressive stress. Axial uni fibers are way out front in this regard.

 

Thanks for thoughts. A few things. The rig is unstayed, a balance lug rig. I plan to stretch the socks and plan to figure out how much to stretch the socks. I'll look at the calculator. What I am getting here is that a laminate schedule of the bidirectional on either side of a uni sock is a good choice. I will get some data on the mast. I have another CMT training this weekend and plan to pick some brains there, but this helps too.

I may seem like an amateur, but don't plan to be for long as I will be integrating infused composites into my business. I don't plan to hand lay this up, in fact everything will be VIP. I think windsurfer masts would be OK for small boats, but not the ones I have on my drawing table. One boat is a 16' skiff and the other is a 19' daysailer. The windsurfer masts would be usable for the mizzen masts, but I cannot find any used windsurfer masts.

Besides the laminate schedule, the bigger Q right now is how to make the mast. The wood masts I make taper to a max diameter around the mast partner and then taper back to a smaller diameter at the butt. My BIG question, is what do I wrap the carbon around and how do I get it out. Should I disregard the taper below the mast partner (remember, these are unstayed masts) and do a constant taper. If so, couldn't I just make a wood plug and use it as a mandrel for the carbon?

THX

 

Yeah, you could use wood. You could even use foam of a grade that would stand up to the compression you'll need to compact the layers. There are even grades of EPS that will work too, if you are still committed to the idea of getting a foam core out later. Just not the grades you originally proposed (that I've also tried to make work without success ) . You should consider perfecting a reverse balloon molding method, where a two-piece outer female mold makes the outer shape and finish of the mast, while an inner bag is either fed compressed air or the air between the bag and mold is evacuated for the compressive force. The mold is disassembled to remove the mast and the bag of course collapses, leaving a hollow tube.

Jimbo

 

So here is where I am at: design the mast to be a constant taper from butt to mast head. Make a wood plug (much more pleasant than foam). Infuse the carbon around this plug. I am concerned about how to get the resin to flow from one end of the spar to the other. This seems impossible based on how poorly resin flows through carbon. I could feed the resin in one side of the mast, with a spiral wrap down the length and the vacuum on the opposite side, but then I may have cosmetic issues from where the resin feed and vacuum lines contact the laminate. So that is a challenge.

The air bag inside of a mold trick sounds difficult. I could make a plug quickly, but make a two piece mold off this plug that gets the job done nicely is beyond me at this point.

If infusion is not the best solution maybe doing a wet-bag around the plug is better.

Thoughts very much appreciated. Thanks.

Clint