Building 10 foot fiberglass boat with a foam core

[alexbarker2]

I am building a 10 foot boat out of 2 inch thick pink foam insulation from home depot. I cut cross sections, attached them together, and sanded them smooth. now I have 1 layer of 1708 biaxial cloth with epoxy resin on the bottom, and I am working on the top side. my plan is to use 5 layers of 1708.




The grey back half of the first picture is paint because at that point I didn't know that epoxy doesn't melt foam, and I was trying to seal it up. The black in the second picture is colored caulk that I used to smooth the surface once the boat was in position for fiberglassing. There are some air bubbles in the first layer because of small gaps between sheets of foam. I am going to fix the bigger ones, but I'll probably just leave the small ones alone, since it's just between the first layer and foam.

A couple questions:

What paint should I use / what should I do to paint it? I used epoxy resin.

After I finish my first layer, would it be a bad idea to test it in the lake? I am worried that it might be bad to have unfinished fiberglass get submerged. If I did put it in for a test at that point, it would just be for a weekend.

 

[missouri]

sick! I've been brain storming a similar build for some time now. They say you can use Montana Gold spraypaint on the foam not sure about after glassing. That was a tun of rib sections to cut and glue. im looking at something like these boats using longer sheets instead of glued ribs. less material used less time to construct (maybe) so how are things now? is the top of the boat flat like a surfboard? cool build. how much did you sink into her? got a motor on it? thanks

 

[PAR]

Welcome to the forum.

The pink foam from the big box stores isn't a structural foam, so the laminate will have to do all the work. These foams are usually in the 2 pound or less per cu. ft. range, while structural foams are 5 pounds per cu. ft. or more. This means you don't need the foam except as a mold to apply the fabrics over. As a foam cored laminate, the core will separate (delaminate) from the inner and outer skins, making it useless, except as built in floatation and moisture containment system. I'm not sure why you elected to employ 5 layers of 1708, but this is a whole lot more than necessary in a real foam cored build.

Yep, all epoxied surfaces that will have UV exposure will need to be protected, with paint (the best) or varnish. Of course, painting the foam then applying goo and fabric means, your bond to the foam is entirely dependent on the paint to foam bond, which compared to epoxy is really weak.

 

[LP]

After I finish my first layer, would it be a bad idea to test it in the lake? I am worried that it might be bad to have unfinished fiberglass get submerged. If I did put it in for a test at that point, it would just be for a weekend.

Just make sure you seal your fiberglass with an extra coat or two of epoxy to insure no water penetration occurs and keep it the sun off of it while you're not using it and a couple of days on the water shouldn't impart any ill effects. I have done many "sea trials" on incomplete kayaks with bare epoxy. I think it makes sense for piece of mind. If all goes south, you can cut your losses. Maybe, you'll find something that needs changed and it will be easier to fix before it's fully laminated or it will work perfectly and you can drive on without hesitation.

It sounds like you haven't laid up all of your laminations. Maybe a senior member familiar with this type of construction could offer a lighter laminate schedule.

 

[gonzo]

1708 has mat in it and is designed for polyester or vinylester. It is a bad choice for epoxy. The mat absorbs resin (a lot) and does little for strength. Bi or tri-directional stitch cloth would be better.

 

[alexbarker2]

Progress update

Thanks for all of the advice. I did get to test it out after finishing my second layer and I am pretty happy with the design. As a test I sat in it and used a ski rope to pull it behind my jet ski. I got up to about 30 mph and didn't notice any issues. My reason for doing 5 layers was that I intend to put an old jet ski engine in it, so it will be going pretty fast, and will need to have similar strength to a jet ski hull. I am not experienced with fiberglass, so my estimate of how much to use might be off. the top of the boat slopes slightly to both sides, but is mostly flat, but there is a seat built in that is kinda like what you would see on an ocean kayak. the foam cost about $150-200, and I have spent roughly 400 on glass so far. I did not intend for the foam to be structural at all. It was intended to be there just as a form for the glass. I will be looking into other types of glass as well. The plans for how it will look when I finish are above. current pictures soon.