I have been looking into building a carbon raft frame for a while now. I'm a prosthetist (i make prosthetic limbs) and I build lots of custom carbon stuff. I'm not an engineer, so I probably don't have that nerdy engineering knowledge that you do but I'll tell you what I've come up with so far.
So I have been looking at ways to do this and I have been shopping around for pre fab'd carbon tubing. I think thats probably the way to go, bc it takes some serious equipment to make good tubing. But I have also been thinking of ways to build a frame using the regular carbon mat fabrics and carbon sleeves that I have at my disposal. One thing to keep in mind is that carbon is not that strong unless it is in a 3d shape or unless it is laminated around a foam core. This creates an I-beam like structure for strength. The core could also be a hollow, wood or whatever, think about how skis are made for example. The other option is just a F*** ton of carbon layers but thats outrageously expensive.
Things I have been trying to figure out is how to connect different pieces of the frame or how to connect frame accessories. Set screws can not be used on carbon. In fact I wouldn't trust connecting carbon to carbon unless metal pieces (threaded plates, or metal bushings,ext) were laminated into the carbon. If you look at any carbon parts for cars/motorcycles or in my case prosthetics, the articulation parts or connection pieces are metal lined. For example, there use to be a carbon fiber knee frame that was popular but they now make it out of aluminum bc over time the motion on the articulation points hollowed out the holes. What Im trying to say is that if you drill a 1/4" hole in carbon and put a pin or bolt through that hole and then put it on a raft that will flex and move, that hole is going to measure 3/8+" pretty quick. So I would think that to do this all the parts have to be either laminated together or use metal fittings that are laminated into each part.
I'm not sure how to attach things like oar towers and foot bars to the frame because its very difficult to get strength and adjustability with carbon. In fact, I think it would be extremely hard to build a carbon frame that had any adjustment at all and I certainly have not thought of a way to make a frame that would break down either.
Another downside of carbon or composites is that it wears easily. Things like dry boxes would wear their way into the carbon over time. Also sand between the frame and the raft would easily sand away the carbon. Things like boots, rocks and ammo cans would scratch and scuff the carbon pretty easily. So you could not really expect the frame to have a nice carbon weave look for very long. If you have something made of carbon finer around just lightly sand on it and you can easily see how quickly black dust starts going everywhere.
So far I do not think that the weight to cost ratio would be worth it, however I still want to give it a shot someday just for the challenge of making something cool.
Anyway, Would you be using prepreg or wet lay ups? I certainly don't have the equipment to heat cure prepreg pieces as big as a raft frame. Are you thinking of a tubular frame or something more box like?
I'd like to hear more about this project if you continue on it. Also if you need any help let me know, just remember that Im just a dumb limb maker and not an engineer