Carbon Fiber is more likely to make it heavier than lighter.
I didn't want to clog up my boat build thread with a dissertation on carbon fiber, but since I'm going to use it with my build, I want to dispel a myth, and want to clearly state that I'm not "that guy" that thinks Carbon Fiber is a magic substance that makes things lighter. It is, however, a magic substance that makes things stronger, and if you play your layup right, you can end up with an equally strong but lighter end product than fiberglass, or with a stronger but heavier end product. But the lighter layup will be susceptible to complications and you may end up with a weaker product. ugh. Always complications. Read on.
There are so many variables that this could be a book, but I'm going to touch the highlights. First of all, to save my fingers, I'm going to use "CF" for "Carbon Fiber," and "Fiberglass" to encompass dynel, poly, and your basic "white" fiberglass cloths most commonly used for fiberglass layups. If fabric weave is important to the topic, I'll mention it, otherwise assume I am comparing two similar weaves in different materials, i.e. fiberglass, carbon fiber, or Kevlar. *Also, "Kevlar" is used to encompass aramid fabrics similar to those marketed as Kevlar. I'm going to be assuming a hand layup without vacuum bagging in this discussion unless otherwise noted.
So, the reason I'm posting is I get this a lot- "I want my boat/surfboard/car part/etc. to be really light, so I want to use carbon fiber." Let's say the plans call for 6 oz fiberglass cloth, so they buy 6 oz CF. Same thing, right- 6 oz cloth. No- 6 oz of CF is thicker than 6 oz of fiberglass cloth, so what happens is that the cloth absorbs more resin, and therefore the layup ends up HEAVIER with CF. Of course they can't figure out why their boat came out so heavy. This is a common misunderstanding about CF. The RESIN is where most of the weight is. CF being thicker in my example, it holds more resin. Unless you adjust the weight of cloth relative to the strength, you will likely end up with a heavier end result- but it will at least be very strong!
Another example is someone can't find 6 oz fiberglass cloth, so they go with a similar weave in, let's say 8 oz. Only 2 ounces per yard, right? So my boat is 20 square yards- that's only 40 ounces more weight, right? Wrong! The 8 oz cloth is going to absorb more resin than the 6 oz. The weight difference will be much more. I will never forget me asking the guy at the chandlery if he had any 6 oz cloth instead of 9 oz. He said, "What's the big difference?" I said I was building a kayak and wanted it to be light enough to portage myself. He laughed- "Good grief, what's a few more ounces? Make sure you take a big sh-- before you get in it so you're lighter weight." I just laughed. I remembered his comment as I portaged my 19' expedition sea kayak on one shoulder. My point in this paragraph- it's not as much the weight of the cloth- it's the thickness and how much resin it absorbs. The weight is a good indicator of thickness, but with CF, since it's a lighter base material, 6 oz will be thicker than 6oz of fiberglass cloth. ** Weave also affects how much resin the cloth will absorb. That's more complicated than I care to discuss here, except I will discuss spread tow fabrics in a bit.
So- You could reduce your CF cloth weight in my example from 6 to 5 ounce (close to 15%) and get about the same strength, but lighter end result. SAME strength because you have a thinner cloth. The other option is use the same cloth, and get a 15% stronger layup and deal with a heavier end result.
Although stronger overall, CF can be brittle when met with blunt force. (*lots of variables here depending on types of CF- I won't get into that) Also when bent, it will eventually rupture, and it ruptures with great fanfare as opposed to a slow breaking. I have heard a few CF sailboat masts go "kaboom" - one was just sitting on the mooring with almost no wind. Flaw in the layup maybe, or maybe too much tension combined with hard sailing. (This happened immediately after the Rolex International Regatta, so it had just been sailed hard.) It just decided to suddenly give up. It sounded like a gunshot. So, for a real-world application- if you build your boat with CF, and hit a rock, it's going to have resistance to the force, but if it meets its limit, it's more likely to catastrophically fail than to give and leave a big dent. Something to consider.
So, enter Kevlar. Kevlar is light and strong, and it is way better than CF or fiberglass at taking blunt blows and it doesn't fail with great fanfare like CF does. It's also very abrasion resistant. All this combined is why it's used for bullet-proof vests. The latest thing is fabrics woven with strands of carbon and kevlar combined to take advantage of both material's strengths. Note that just like with CF, this cloth is going to wet out heavier than similar weight fiberglass cloth, but will be much stronger. Typically this is found in twill weave and plain weave. With Twill, you have different weaves, but to be simple, a 2x2 twill has fibers going north and south. One will be carbon, the other kevlar. The drawback- or advantage depending on how you look at it, is that all your carbon fibers run one way, and all your kevlar run another. I prefer a "dual twill" with carbon and kevlar both going both north and south. (warp and weft in fabric terms)
So I should use Kevlar because it will prevent abrasion, right? Well, yes- it is less prone to abrasion, but isn't there always a catch? The catch with Kevlar is that when it does abrade, it likes to abrade in a fuzzy mess which aside from looking bad, also can wick water. How's that for complicating things? Also a drawback to Kevlar is it's just doggone hard to cut. They make electric shears that work well, and also sharpening scissors at a greater angle will help.
Okay, so carbon fiber will make my layup heavier? Well- wait- ther'es more. Enter "spread tow" fabrics. The tow is the bundles of yarns that make up the warp and weft- the stuff the fabric is woven out of. Normally it's bundled up in groups, and those bundles go over and under each other, creating gaps where they overlap, and creating stress as well. With spread tow fabric, the tow is kept flat- like a ribbon, and it is layed over and under each other without the gaps that other weaves have, and without the stress points as well. Spread tow fabrics are usually carbon, Kevlar (aramids), or a combination of both. The advantage to spread tow is you get all the strength of CF and/or Kevlar, but you get a very tight and thin cloth that absorbs less resin, and the end result is supposed to be about a 20% lighter end result, but in my opinion that's only if you are using vacuum bagging. I think in real life small shop spreading resin by hand it would be more like 15% weight savings, but you get an even stronger layup. Win/win! Oh wait- it's hella expensive- like more than twice as expensive as plain weave or twill CF. Other than that- win/win!
So here's what I'm going to do- I am using a 5.66 ounce spread tow carbon fiber layer, with a layer of kevlar on top of that. Here is my reasoning. Spread tow fabrics are very thin, so they are strong, but not as abrasion resistant. I'm saving 15% or more weight with the spread tow. I'm gaining a ton of strength over fiberglass. The second layer is going to resist abrasion and punctures. Yes, it will have the side effect of when it does abrade it will be fuzzy, but it is what it is. Another thing about using two thinner layers is you can control your resin a whole lot better than one thick layer. For instance- two 5 oz cloths versus one 9 oz will likely come out a lighter layup because the 9 oz is going to have larger pockets between the weave to hold resin. You can get a stronger layup with 2x5oz than 1x9oz- but that's a general statement. In my case, with a spread tow bottom layer, and a satin weave kevlar top layer, I should have very good resin control, a very strong overlap of fibers, and a durable layup.
Well, if you can believe it, that's the short version of the story. The longer version would make a 200 page book, so the above contains a lot of generalizations.
