carbon fiber for a dinghy?

19oz twill hand laminated is something over 1/32" thick. I don't think you can get away without 2 layers, or a minimum thickness of 1/16". Even so you will probably need some ribs inside, so the correct answer to what carbon is "the cheapest you can get".

 

So, in fiberglass only, you were at
1.5 oz mat
24 oz roving
1708 = 17 oz + 6.5 oz mat
== 49 oz cloth in a hand layup

So, it soinds like two layers of 19 oz carbon on the bottom and one on the sides with some gunwhale reinforcement would work.

 

Thanks guys. Quite a bit of useful information has popped up now.

That glass layup would be pretty heavy for a cartopper that emphasized weight reduction. If we assume that the weight of the epoxy is 2/3 of the total, we're looking at about 44 pounds without anything else. Doesn't leave much for the hardware, the deck, etc.

If I went with the 19 ounce carbon, which is about $35/yd, I think two layers would be $300 or $400. At the same resin ratio, maybe 34 pounds.

A core starts to look better and better. Maybe end grain balsa, which has compressive strength in the direction of the grain that's considerably more than many woods across the grain. Somewhere between that of douglas fir and oak. I'm thinking with that, I could probably use a layer of much lighter stuff on each side. However, I'm also thinking that working with this stuff might be a real pain. I don't know if a full mold would be required or just a multitude of stringers to support the balsa. I also don't know how much epoxy it soaks up. Strip planking with fairly light fabric on it looks better and better. Especially if paulownia was available. Hmmm, if they sold end grain balsa as planks...

 

8mm pvc foam and 450gm m2 either side. Thats about as light as I would go, 10mm foam would be better. At this size its hard to beat bare ply for weight . For longevity, ease of build and smooth useable surfaces free from stringers and frames etc, foam and glass is better though. Lot of work to build a dinghy. Why not spend a few dollars extra on materials and have something that will never rot. Also no need for flotation compartments as the foam provides the buoyancy. There are a couple of plans for small plywood rowing dinghys on duckworks.
L'il Nip Free Plans https://duckworks.com/lil-nip-plans/ is a small catamaran dinghy. The great thing about small cats is that the largest unsupported span might be less than 12 inches wide. The largest unsupported span in your design will greatly influence the scantlings you require to be successful in your endeavors. If the distance between keel and the turn of the bilge is 2ft then you would need much heavier layup than if it were only a foot. You can aid in the stiffness of these panels with compound curves, similar to the eggshell principle. Also in a cat such as this you have a seat for and aft with which you can easily slide for and aft to trim such a tiny craft, no extra seating to be built. I would change the design by eliminating the structural beams as shown and simply use a transom full width and a bow section at 45 degree angle full width of the boat to provide a front beam. I do this in a much bigger power driven cat with a much greater beam/unsupported span, with no difficulties. Good luck.
Cheers

 

Thanks. That's a very specific layup, so it's possible to figure out how much it weighs. I think about 27 lbs assuming the weight of the layup is 40 percent glass/60 percent epoxy and that the foam is around 5 lbs per cubic foot. Also assuming I didn't mess up the calculations. The shapes I'm thinking about are mostly curvy, so I wouldn't have to worry much about single curvature areas.

I figure the foam would float maybe 80 lbs beyond its own weight. However, that's all over the hull. If the weight of the hull is around 50 lbs, and the rig weighs 10, then, when righting it, the side would probably sink pretty far down and scoop up a lot of water. Particularly if I was righting it by stepping on the dagger or centerboard. I'd want to add more in order to deal with this. If I sat upright in the boat, while swamped, it might keep my head and part of my torso above water, while it sank below the surface. I've had a boat like that. This one would be a little better, but I don't know if it would be enough better. I suppose the foam would be nice if any buoyancy tanks flooded.

I have a reasonably old plywood boat, where the plywood, at least, is in good shape. If the boat outlives my ability to sail it, that's long enough. 50 years isn't required. Unless they hurry up with that rejuvenation pill. But then I'd have time to build another.

A catamaran dinghy might be nice, but I'd probably give it to someone whose needs or wants it would actually meet. I have at least one plan for one of those, because I found it interesting. What I'm contemplating here is something like a character boat. Probably looking much like a Cape Cod cat.

 

If you want super light go Skin on Frame.
Whitehall Skin on Frame Dinghy (available for up to 3 students) — The Australian Wooden Boat School, Wooden Boat Centre Tasmania http://www.woodenboatcentre.com/coursecalendar/2021/5/17/whitehall-skin-on-frame-dinghy-build-your-own-5-places-available-7nteb-z8s3g