Core Suggestion for Moldless(?) Hull

I'm a senior mechanical engineering student building a double-hull solar-powered boat for a design competition. I've been doing a lot of reading and I am leaning toward a moldless layup using kevlar and fiberglass and im curious about what the best core material would be. I've read that polyurethane foam is the way to go, but I am wondering if regular rigid pink insulation foam will be good enough. I've read its more friable and not quite as good to work with, but can we make it work?

Also, is there a strong reason not to be doing a moldless construction? It seems to me that since we're planning on having foam-encased fiberglass hull, we'll be ok making the moldless hulls. I realize ill be sacrificing consistency between the two hulls and we'll have to do more to finish the surface, but are these reasons to change my plan?

I'd appreciate any insight. Thanks.

 

Not sure what the pink ins foam is made out of. Are ya using epoxy resin or polyester? Poly will eat most foams up. Epoxy on the other hand is compatable with alot more species of foam. Have you considered balsa? I've used stuff from both Baltek and divinycell.

http://www.baltek.com/products/prodintro.html
http://www.fiberglasssupply.com/Product_Catalog/Core_Materials/core_materials.html
http://www.diabgroup.com/

I have minimun exp with foams truthfully, above are some links that may help.

 

Sure, for a quick-and-dirty boat with a very limited service life you can use styrofoam. Polyester resin will melt the foam, so use epoxy.

Start by rigging up a flat table big enough to make the largest panel, and skin the rough-cut foam panels with glass on one side. Fit and fasten the foam panels to the jig with the glass skin facing inward. Fair the joints between the panels to your liking, then laminate glass over the entire outside surface. Fair the hull, pop it off the mold, and tape over the joints on the inside.

Wouldn't recommend straying too far offshore, but for testing ideas on the cheap it's a good construction technique. If you want a more durable boat, substitute the styrofoam with a proper core foam.

Good luck,
Yoke.

 

Pink foam's usually used for flotation; I've seen hull plugs made from it but I've never seen it used as a core. Yoke's suggestion sounds reasonable and workable. You must use epoxy resins, and be very careful about your cleanup solvents. A core foam like Corecell, Airex, Divinycell etc. would last a lot longer though, as well as being easier to work. So long as your hull consists of developable surfaces (no tight compound curves), your plan should work.

 

Hey guys, thanks for replying. I think we may be on different wavelengths thought. I am thinking of having a solid, completely encased hull rather than an open style (like a conoe i guess). Here is a link to a website detailing the kind of process i was thinking.

http://homepages.paradise.net.nz/garyd/quikama.html

Is there a major downfall to this type of contstuction?

Also, as I am planning on doing kevlar, I will definitely be using either vinyl-ester or epoxy resin so we shouldnt be melting any foam.

Yoke, it sounded to me like you were describing a different process since you mentioned a mold. What did you mean by skinning on one side?

The main reason I was asking about foams is that I've come to understande the benefits of sandwich-composite construction and wanted to make sure if I had the entire hull filled with foam, it wouldn't crack inside.

Thanks for your suggestions and please keep them coming.

 

I looked over that site.... I've seen that done before, generally with good success. Epoxy is a must, and be careful to get all the acetone off your layup tools when you're done cleaning them. If you're laying up directly over the foam block it should not crack; it will, however, probably not be as long-lived as a more conventional construction. Make sure your foam-to-skin bond is good as it can easily delaminate if you aren't careful.

 

Solar,

I was describing a method for building a sandwich-panel boat. A sheet of core-foam with laminate on only one side is not much stiffer than the foam by itself. The magic increase in stiffness occurs when you add the skin on the other side, thus, it is often easier to laminate the inside skin before assembling the panels into the shape of a boat.

The term "mold" is unfortunately a bit imprecise, in this case I was referring to a series of properly spaced transverse plywood "slices" ("stations" in boatbuilding parlance) that you fasten the foam panels onto to produce the desired 3D shape.

Practical consideration: Vinylester, or any other styrene-based resin will not work, (styrene-styrofoam, see what I'm getting at?) all you'll get is a gooey mess, might work as a napalm substitute though.

If the shape you are planning to build is similar to the one in the link you posted, then shaping from a solid block of foam is most likely the most expedient, since the hollow sandwich construction would require a lot of panel segments. Perhaps you can figure out a way to hollow it out afterwards, maybe even take all the foam out and pressurizing it a little bit, foam won't crack if it ain't there.....

I'm not sure the extra expense of kevlar will give you a lot of benefit. Although its density is low, (kevlar 1.4, glass 2.55) its compressive strength is atrocious, (28% of tensile, glass is 50% of tensile strength) so the long and short of it is that S-glass is actually more efficient per unit weight in compression than kevlar.

That'll have to do for now, I don't want to do all your homework.

Good luck,
Yoke.

P.S. Shape the two hulls at the same time, and do it by cutting/sanding facets, (flat spots with ridges separating them) that makes it much easier to quantify the shape, then round off all the edges last.

 

Thanks for all the suggetions guys, appreciate it. Its hard to become a boat builder in my spare time over one semester so excuse my lack of vocabulary, etc.

Yoke: Ive considered popping the laminate off the foam, but then we would have to build some type of cap for the top which complicates connecting the two hulls. Also if its hollow we'll have to rig up a bilge system. Pressurizing the hull sounds complicated but I like the cocept and I'll look into it.

I was thinking that a foam-laminated, contained hull would act like a sandwich panel since it has the core material with FRP on each side everywhere. I just wasnt sure if the increased ratio of foam-to-laminate would cause problems in the core.

Since building our first boat, we're trying to keep it as simple as possible, which is why I was leaning toward the type of construction described in that link.

The cleaning, laminating tips are extremely helpful as well as the suggestions on shaping foam. Ill probably have more questions to follow.

 

Solar,

No doubt do you want to keep this project simple. I just got to thinking about the possibilities, when you have a structure that would benefit from a circular cross section anyway, and you want to use kevlar, wich is weak in compression but very strong in tension, then a pressurized single skin structure (no compression loads in the skin) would be a rather elegant and efficient solution.

"Any car wich holds together for a whole race is too heavy" -Colin Chapman

For getting the foam out, all you need to do after laminating the skin is to drill a hole and pour in a bit of acetone, slosh it around until all the foam is melted, then pour it out again.

Yoke.

 

For getting the foam out, all you need to do after laminating the skin is to drill a hole and pour in a bit of acetone, slosh it around until all the foam is melted, then pour it out again.


I have built custom gas tanks for motorcycles with cheap foam and epoxy resin layups.
Old battery acid also works well to clean out the interior.

FAST FRED