Building technique using sheet carbon or glass on a plywood one off

[Doug Lord]

I'm in the process of building a one off test model for an 18' trimaran. I'm going to build the hulls out of balsa. From years of past experience the hardest part of building a one off model is getting a professional finish within a weight budget and I haven't been very good at it. I've built many, many plugs with excellent finishes when weight was not a concern.
What I'm going to try is to glue very thin carbon/epoxy sheets(about.25mm)- where one side of the carbon is glossy and the other ready to bond- to the balsa. The shape of the hull is conducive to this with no weird bends.
I'm wondering what experienced one off boatbuilders would think of using this technique on a full size hull-say one made with okoume plywood instead of or in addition to glassing the hull for structural reinforcement. The idea would be to eliminate weeks of finishing since if the sheet installation was done right
it would be ready for paint almost immediately or the sheets could have a gel coat finish and the seams blended in.
I'd appreciate any thoughts or references where this technique has been used.

Here is a supplier of the sheets I'm going to use on the model. For fullsize maybe 4' X 8' pannels (or so) could be laid up on window glass to achieve a near perfect finish. http://www.protechcomposites.com/carbon-fiber-products/

Doug, try wiping PVA (Alcohol, not acetate - The green stuff) on melamine board with a rag and wiping it off. The goal is not to remove but to leave a very light film. Don't sand the board first and don't wax. Forget the pane of glass - the melamine is easier to work with.
Lay up on that. How are you going to insure complete contact with your core without excess squeez-out? Vacuum? You are going to need to address the butting of skins. You can do that on the mold by taping something the thickness of the first ply (plastic laminate?). The goal is to make sheets like sheetrock with recessed edges for taping later. If you just butt completely flat panels, then glass over, you lose the benefit of a fair surface. What the guys do here, apparently, is exactly that, then sand what they just joined mostly away (not good). Poly works well this way, I don't know if epoxy will stick too well. Some of the guys lay up poly directly on the malamine with no prep, then pull off the sheet green and turn out many sheets a day.

[michael pierzga]

A small yacht composite furniture builder employs the technique mark speaks of to generate near perfect surfaces on the outside face of components. They presently have a an open, centre console boat flybridge roof , under construction. I will ask how they apply the edge laminate to the sandwich next time I see the guys

[rwatson]

Originally Posted by Doug Lord

....
I'm wondering what experienced one off boatbuilders would think of using this technique on a full size hull-say one made with okoume plywood instead of or in addition to glassing the hull for structural reinforcement. The idea would be to eliminate weeks of finishing since if the sheet installation was done right
it would be ready for paint almost immediately or the sheets could have a gel coat finish and the seams blended in.
I'd appreciate any thoughts or references where this technique has been used.

http://www.protechcomposites.com/carbon-fiber-products/

me too - I plan to build a hull with the "shiny" side laid up on a melamime board. Derek Kelsall has been doign it for years.

[Doug Lord]

Thanks, guys-the edge seam is a problem on full size for sure. On the model, I would just use a full length sheet. I'm not sure how the bonding process will go: if I use a vacuum I'm concerned that it might press slight waves into the sheet-if I don't there could be slight pockets.
About 1971, I worked for Thomson trawlers and they were searching for ways to build "quick and fair" superstructures. I came up with using a sheet of window glass laid very carefully down on a wooden frame. The glass was waxed and sprayed with a fine mist of PVA which flowed out perfectly and then sprayed with gelcoat. The panel was laid up with a balsa core and pulled. Worked good and such seams as there were were easy to fix. Those were fairly thick panels and used only for superstructure.
Using the thin stuff that I want to presents a whole 'nother set of problems. There is not enough thickness to the sheet to allow a good seam fix w/o destroying the fairness. Mark, your idea is a good one-thanks for the suggestion! At a sheet thickness of just two layers of 5.7 oz carbon it still seems like it would create a "bump" on the bonding side of the sheet-or am I missing something? Maybe ensure the tape "recess" was just the thickness of one layer of 5.7 then the bonding side would be flat-I'm slowly getting it.....

PS- On production model boats I built it was essential that the hull and deck came out with no wax residue so I used Part All and PVA exclusively. I had a hell of a time getting the Part All in a 5' mold without too much spray related "fisheye"(not exactly) distortions. So I tried pouring the PVA into the mold with the mold vertical and a very clean "catching pan" underneath which worked real well but is pretty much limited to models. A good sprayer with good equipment could have probably have misted the stuff on like they did in the old days at Thomson.

[gonzo]

I think that panels that thin will be very lumpy after you glue them. Even if you vacuum them down, they are so flimsy that small differences in the amount of glue will show through.

[Doug Lord]

Originally Posted by gonzo

I think that panels that thin will be very lumpy after you glue them. Even if you vacuum them down, they are so flimsy that small differences in the amount of glue will show through.

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You may be right especially if the glue shrinks the tiniest little bit....

[rwatson]

I dont think trying to build very thin sheets is a good idea. As Gonzo said, they will be thin and flimsy.

You have to get a thickness sufficient for them to be self supporting, and hopefully self fairing. If not, you will create serious problems with deformation when you layup the other layers inside. Curing fg distorts, unless the supporting area is solid enough.

Thats why Marks suggestion about 'raw' edges makes so much sense - if your panels are too thin to use edge 'masking', they will be too thin to do anything else with.

[Doug Lord]

Originally Posted by rwatson

I dont think trying to build very thin sheets is a good idea. As Gonzo said, they will be thin and flimsy.

You have to get a thickness sufficient for them to be self supporting, and hopefully self fairing. If not, you will create serious problems with deformation when you layup the other layers inside. Curing fg distorts, unless the supporting area is solid enough.

Thats why Marks suggestion about 'raw' edges makes so much sense - if your panels are too thin to use edge 'masking', they will be too thin to do anything else with.

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Thanks!