question: Polystyrene & fiberglass

Hi everyone. Completely new to boatbuilding but I'm buying books and reading threads here with lots of great information.

Question - Ive seen some threads with good answers on similar subject but this is my concept so far just need to know if anyone has tried anything like this. I'v seen lots of info on male and female mold used to shape foam materials designed for the task and then covering with fiberglass, and I've also seen everything that says use proven plans and don't try to design your own.

My idea is to use the 2" thick pink polystyrene foam from home store and build a 15' flat bottom "carolina skiff" like shape - very simple shape - gluing polystyrene together with epoxy then covering the entire structure with fiberglass fabric and epoxy resin so that the entire structure is fully encased.

Please give me any positive feedback, ideas, or related experiences. I know it may be hair-brained but this is an idea in its infancy and has a long time to grow before I build - so be gentle.

PS - Yes this is a poor man's redneck experiment. Just want to find out if it can be done.

 

not sure but i think polystyrene may melt or go out of shape when you put resin on to it.

can you get hold of a foam like cellatex or kingspan. its a sheet foam available in big sizes.

i see your in america so it may have a different name. i use it quite a lot and its perfect for making shapes with and you can glass over it with no problems at all.

 

Not sure. I'll see if that material is available. I did read in one book that you have to use epoxy resin with the polystyrene because it wouldn't melt but I will test it before I try, Thanks.

 

i only use polyester resin and it melts polystyrene.

its spelt celotex. not cellatex.

its used as an insulation on buildings over here so you may have an alternative over there.

 

Epoxy will not melt polystyrene, though styrene based resins systems will and very quickly too. No one that understands the physical properties of this type of foam, uses it as a core. It just doesn't meet the minimums necessary as a structural element. PVC foam is a much better choice and also available from the big box stores. Polyurethane is another choice, though I'm not sure of availability in the big box stores.

 

check cost

Polyester resin will devour polystyrene, but before you consider expoxy check the price. When I was building expoxy resins were very expensive vs polyester. Before pouring money into what might be an experiment, you might consider cheaper. Polyurethane is a great product. Any large insulation company should stock. Remember, neither foam provides any structural strength other than acting as a mandrel for your design.

 

Epoxy resin at retail pricing is costly, but there are wholesale options that bring it down to the cost of good polyester. I pay about $30 a gallon, though I do buy quantity. If purchasing small amounts (3 gallons or less) expect $55 a gallon pricing and up.

It's incorrect to assume the foam has no structural role. It's not a "mandrel" but a core and it must have fairly specific physical qualities or it will fail, leaving you with two flimsy 'glass skins that will buckle under load.

 

Awesome responses this is what i need. Let me pose this then: what's your opinion of strengthening polystyrene if that were all i could use hypothetically by laminating a layer of thin plywood to the outside of structure prior to glassing?

 

Mandrel core, tomatoe tomato

My point was that something additional has to be done with the foam in order to provide the structural integrity needed for a boat. One or two plies is not going to make for a safe boat. Strengthening at critical locations will be required. This is easly accomplished by adding additional plies in critical areas.

If proceeding with your boat, I have used Luan extensively and bonded it to polyurethane with good ole white glue...like Elmers. Luan is cheap, thin, will soak up resin for a good bond and readily available at home centers.
DO NOT LET THE GLUE EXTRUDE FROM UNDER THE LUAN ONTO THE FOAM. IT MUST BE WIPED UP WHILE WET.
If you are set on polystyrene foam, check whether white glue will bond/eat it up first.

Unless things have changed there are two types of polyester resin. One has wax in it, bonding resin, this promotes curing. The other is wax-free. Use wax-free as your final coat. It will make final finishing so much easier.

Just remember, if Columbus would have had diesel, he would have come over on a Hatteras

 

You can think of the boat as a large windsurf board. EPS core, a tougher core on the outside, and then the laminate.

 

The polystyrene foam you'll get from Lowe's/Depot doesn't have the compressive strength nor the shear strength necessary to work in a sandwich core situation. You can make it work by using overly thick laminates, but why bother, as you're just making a double skin panel that's equivalent to a single skin, at twice the effort. Use the blue polyurethane foam from the big box stores. It's still not very good, compared to foams designed for core use, but far better then the pink polystyrene they sell.

No, PVS's (Elmer's, TiteBond, etc.) will not melt polystyrene foam.

 

By the way, over here we sell a polyester resin that will NOT melt EPS foam. We call it EPS-Coat (Ce-Sense). Lot of artists use it, but also technical products are made with it.

We can buy several densities of EPS foam. 15 kg/m3 (1 lb/ft3) is too soft, and is what they sell at most DIY stores. Perfect for insulation though, except for the fire properties.
The higher densities are usable in structural products (35 kg/m3 (2lb/ft3), 80 kg/m3 (5lb/ft3).

 

5 pound would be my minimum recommendation for a core foam choice.

 

It can totally work out. My company has just made a mold in your thought. It seems great, and save money.
We use 5 axial CNC to cut EPS, then apply 3 glass fiber layers with epoxy. After the epoxy cured, we put on 5mm thick putty and use 5 axial CNC to cut it in a right shape.

 

That's the technique used for windsurfing boards (with epoxy, not polyester that eats polystyrene), but the foam is used just for "filling the void" and shaping, not as a sandwich core foam, PVC foam is used for the sandwich skin.