Building a (low-tech) carbon fiber spar.

[LP]

What is low-tech, you ask? Basically, no pre-preg, no vacuum, no autoclave and no mandrel. Pretty much a backyard build with hand lay-up and wet out. And what? No mandrel? Well, maybe not entirely true, but we’ll get back to that.

I’m on a quest to build a carbon fiber rig for my 24’ split-rig, trailer-sailer that I am finishing the build on. It currently has two masts and wishbone booms made of Douglas fir that weigh in at about 210 lbs total for both. 123 lbs for the main and 84 lbs for the mizzen. I’ve been searching the web for information carbon spars and have found limited information and several dead and missing links to once available information. There are several resources for using woven carbon fiber sleeves, but strand orientation in these sleeves is not really suitable in most spar type applications. My greatest, most suitable resource has been Eric Sponberg’s paper on freestanding carbon fiber masts and other related information conveyed on his website.

https://www.ericwsponberg.com/articles/

More specifically,

https://www.ericwsponberg.com/wp-content/uploads/design-engineering-masts.pdf

His paper and website is a treasure trove of information regarding carbon fiber spars and freestanding masts. There were three tidbits off information that I found extremely useful in designing my layup. 1) Minimum wall thickness needs to be at least 3% of section size, 2) 24 oz. of hoop direction S or E glass should be utilized on the inner and outer surfaces and can be counted towards the overall wall thickness 3) The thinnest section (wall thickness) should not be any less than 50% of the thickest section. Now, I’ve left out a lot of information and this isn’t a thread on spar design. I’m only trying to give some background for my build choices.

Let’s talk about my no mandrel comment. Anybody that works with composites knows that you need a mold of some sort. This can be a mandrel, a mold, the part you want to copy or even a flat surface depending on the shape you want to create. When it spars and masts, the mandrel is a great way to go if you plan to build multiple units. It does however, have its drawbacks: cost and shape limitations to name two. Another option that was offer on the Sponberg website was a wooden core of thin plywood and other elements that would become an integral part of the mast, but this has a weigh penalty that made the carbon spar comparable in weigh to other materials. Another undesirable outcome.

After much thought, I finally had a light go on and I am hoping it is the alternative method I’m looking for. If you will recall, one of the three tidbits of information I found highly useful was the 24 oz. of hoop direction fiberglass used on the inner and outer surfaces of the spar. So......what if we build a mold (or half mold) of our desired spar and made a “splash” of each half of the mold and rejoined the splashes? So here we go. I’m calling it the split, splash system. Picture to clarify my statements.


Here we have my boomkin, wrapped in packing tape, being used as a mold. Only three sides are are wrappedcovered with glass as I will pull this splash off, turn the boomkin over and splash the direction.



Here we have the opposing splashes along with their boomkin mold. I had some 27 oz. triax laying around that worked really nice in making these splashes.



Here the two halves are being refitted after trimming off the rough edges and establishing about a 1 1/2” overlapping joint. I chose to have both legs of one splash lay up inside both legs of the other to provide a mutual compression against each other. I laid out a hole pattern with 3” or 4” spacing using a smallish size drill bit to make the pilot holes. The holes in the outer splash were later drilled to 1/4” and 1/4” pan head sheet metal screws were used to provide clamping force when the two halves were epoxied together. I used a medium viscosity resin to wet out both mating surfaces without thickening the resin.



An inside shot of my spar. Probably before glue up.

It’s little labor intensive, but the results so far have been quite acceptable. So, no mandrel and the core form for my spar will be an integral and require element of the completed spar.


More to come.

 

[LP]

To continue:

Once the two halves were assembled and cured, it was given a rough sanding to knock down much of the texture from the triax and to also feather overlapped edge. A slightly thickened, but brushable coat of epoxy is applied to help fair the fiber glass surface further and the first layer of carbon uni is applied. My spar is about 3” x 3.5” and my tapes 3.75”. This is a nice mix as the tapes will overlap slightly when laid centered on the flat surfaces. The spar tapers to a 2” x 2” section at the tip so the overlap section grows as you work down the spar. To keep the lay-up as balanced as possible, successive layers were then centered over the center of the overlapped section. Basically, tapes were laid on the flats, then on the corners, then on the flats, etc, etc. It seemed a simple way to manage tapes and keep track of what’s been laid.

In the actual process, I laid tapes on three flat sides to leave a dry side to keep the spar rested on, then the two top side corners were applied and finally, top flat side. Once that kicked, I flipped the spar did the remaining side, the two topside corners and then the three exposed flats to balance it all up and let things kick. In the future, I plan to make the fiber glass core slightly longer than necessary so that the raw ends can be trimmed away cleanly afterwards for a crisper finish to the ends of the spar. Additionally, I would insert extensions in the core so the the entire spar can be supported cleanly and clear of the carbon lay-up.

Other lay-up notes:

The uni that I was using 0.006” thick. It was held together with lines of hot melt on about 3/8” spacing. These glue line were fairly small, but still affected the smoothness of the lay-up. This is my first carbon use so I don’t know if there are other options in this regard. Ultimately, I was laying the faces with the glue line face-to-face in an effort to minimize this inconsistency.

I had the choice of using a 0.014 material thickness, but at $26 a pound vs. $16 a pound for the thinner material. I’ll spend the extra dollars next time to save effort by reducing layer count.

Picture time.

This picture is after the first flip and shows the slightly thickened epoxy covering the fiberglass. Interestingly, this lead to a nice discovery that really helped with the wet out of the carbon. Carbon doesn’t change in appearance like fiberglass when it is wet out with epoxy. After finishing with the thickened mix, I had some left in the pot and just built my next round of epoxy on top of the remnants. This made a milky mixture that really became visible when spread on the carbon. It was such a boon to see definitively where I’ve spread the mixture that I put a pinch or two of micro balloons in every pot afterwards. It didn’t really confirm that the carbon was fully wetted, but I could tell for sure that I had adequate epoxy in place to wet the carbon fully. Note the blue cake spatula in the photo. I would use that to squeegee the tapes, starting down the middle and working to the edges work air bubbles out of from under the tapes.



Total lay-up was 6-7 layers of carbon. With the taper of the spar, one layer at the base of the spar turned into almost three layers at the tip. So, while there are 28 tapes at the base, there are only 12 tapes that run full length to the tip. Once the full length tapes are laid-up, progress speeds up as each successive layer gets shorter and shorter. I would pad quantities so that each layer was a multiple of 4 strips. This way the system of laying on the flat (0 degrees) and the corners (the 45’s) could be continued through successive layers. Another process that worked well for me was to pre-cut all strips and roll them into taped bundles with their lengths marked on the bundle tapes. As I worked through the bundles, I would place the notated tapes on my tools shelf for a continuous cue as to my progress through the lay-up process.

The spar with the final fiberglass wet-out.



Filled and faired.



Installed. Yep, the Mizzen sheet is fouled. Still needs paint.



What can I say?



Please leave comments. I’m still figuring it out. My next adventure will be building new wishbone booms.

 

[gonzo]

What is the weight and cost? I am interested in comparing them to a standard aluminum section.

 

[LP]

Gonzo,

I used about 50 sq. ft. of the 0.006” uni at $0.45 a sq. ft. for about $25 in carbon. The 25 sq ft at 0.014” thickness is $1.69 per sq ft for about $45. Estimating $10 in glass and epoxy. So, $35 (+/-) or $55 (+/-), depending on material choice. Between $4 and $6 per foot in my case for this spar. I’ll have to get back to you on the weight in a couple of days. Surprisingly, I haven’t actually weighted it yet.

Price was whole roll, bulk price. Smaller quantities will run more. I bought an 8 or 10 lb roll. I have big plans.

 

[LP]

I just weighed my spar. It weighs it at 11.5 lbs. It’s 9’ long. If I had designed it specifically for this location, it’s sectional would area would be a bit smaller, and thus, maybe slightly lighter. Is it was, I was doing down and dirty and simply splashed the existing spar.