11 replies to this topic

[RC Bill]

As a newbie, maybe I can get away with asking a dumb question and hope someone else has tried it.

 

Last year I had some extra epoxy so I coated a piece of regular 1" building construction styrafoam.  I did one area with resin alone.  As you could expect, the surface was much stronger but still flexible.  On another spot, I used fiberglass mat with the resin.  That appeared to make the foam about bulletproof.  

 

Now the question: Could you use foam planking over your conventional frame in building a boat - or at least use it building a lightweight cabin top & sides?  This question has been bothering me for a couple of years and if it's feasible, I'd like to try it.

[Oyster]

There are many issues that you are dealing with. A foam core boat of proper building technigue requires a batten contruction or grid system incorporated into the hull with bulkheads, even when you use quality structual glass. Even open skiffs will require thwarts at the minimum working along with the rails or covering boards.

 

. People use the blueboard house foam for hardtops where you are not planning on walking on it.  But you still need some form of a frame because glass and epoxy is only as good as the surface in which its applied to.

 

In structual applications, a grid system with house foam core as fillers needs to be sandwiched inbetween door skin plywood. Bonding blue board is not that good and the single biggest issue is that it has a low compression strength, that is if you hit it directly, the foam will collapse and stay put. If you attempt to glass it, you will also loose a bond.

 

People have attempted to build using the home type boards, but normally you have to over build the hull and its not worth the effort and the quality materials as a supplement to make the boat sea worthy. Use plywood in any boat up to 12 foot and then if you wish to build a foam core boat of sizes over that, then look into using Divinicell or Klegacell type foam, even though that material is high dollar. Make sure that you also choose the right density, which varies depending on the application.

[PAR]

You don't have to use a structural grid of wood, though it's commonly employed this way. The basic problem with these construction grades foams is it's density, which is usually quite low. Because of this, it's not a structural foam, will tend to sheer from it's sheathings and must have the laminate (or something) as the structural element, which is self defeating, as the whole idea is a light weight panel.

 

If you use a real structural foam, the core (foam) also serves in a load bearing capacity, so the laminates can be thinner, hence lighter. I would consider a minimum 5 pound density for structural foams. The Lowe's/Depot stuff is usually 1 to 2 pound stock. This is an insulation, though can make a moderately stiff panel when sheathed, can't be considered load bearing.  This is the type of foam that would require some sort of frame system (laminate, wood, metal, etc.) for it to work in a structural application and probably why it's seen so often.

 

So, as with most things, it's application specific. You can employ these low density foams, but they'll need support and can't tolerate much abuse, be the core delaminates from the sheathings. As Oyster mentioned, they do work for a cabin roof, if not highly loaded or will be walked on. Grab rails or other hardware on the roof would need structural support, but it does work, particularly with small craft.

 

I know a few that have made small skiffs, canoes and kayaks from this type of foam, but to get a product stiff enough to work, they end up using as much 'glass, as they would on a single skin 'glass hull, so there's no real weight savings. Lastly, sandwich core builds, require a fair bit of laminate engineering, if you want to get the best from the materials involved. In small craft, it's very difficult to rival plywood, in terms of weight and cost, but in larger sizes, you can make big in roads on the weight end, but as I mentioned, it does require some laminate design skill, if you expect to benefit.

[Oyster]

One of the key reasons for the grid system especially when using the cheap and low density house foam is that this gives you a margin of security upon compression or stress in high impact areas. This transfers loads while the cheap foam is nothing more than to form the shape. You bond sucks long term when using the stuff, in my own personal observation.

[RC Bill]

Thanks for your responses.  I learned a lot and won't waste my time.  

[PAR]

Oyster's correct, in that using the low density stuff is just using the foam as a mold or jig. In this vain, you can use your aunt Millie's mash potato salad as the stringer (or whatever ) forms. Once the goo and fabrics are cured, you could remove the foam (or potato salad) as it doesn't serve any useful purpose anymore. A wooden frame can help to a degree, lessening laminate requirements, kind of belt and suspenders approach as mentioned. Redundancy is often a good thing, but in small craft it usually just means heavier than it needs to be, which isn't desirable.

[hokeyhydro]

Lemme see - built numerous model airplane wings with foam, hot wire cut to airfoil desired, sheathed with thin balsa or thinner plywood (comes in 1/64"). Very strong structure, used beadboard. Also built a outboard hydroplane, 9' 6" long. I hot wire cut the hull airfoil shape, skinned the bottom with 3mm ply, skinned the deck with 6 oz fiberglass. Foam was 2 lb/cu ft beadboard. Ran a 15 c.i. Hot Rod on it, speed 65 mph plus. Only problem I had, dang boat came out so light (65 lb)I had to add lead to reach the required class weight, boat, engine, driver, of 350 lb.

Later I used some foam/wood skin/glass skin parts of my race boats but remanded the main hull structure to wood. Yes, picking the right foam (higher density as previously mentioned) can save weight but not $$$. Sheathing foam in glass eats up many epoxy $$$.

[Howard]

Paul:

 

To clarify, lets say a person wanted to build a coachroof on modest sized boat.......the pink or blue stuff could be used between a sandwich of light (say 3/16th's plywood) panels......above and below, with cabin beams spaced in between.......say on 12 to 15 inch centers. The exterior glass covering would be applied to the thin topside plywood.

 

Used in this manner, the foam is more insulation than structural, but does start to take on the characteristics of foam core construction, unless any deck flexing would result in sheer over time. As long as the cabin beams were as thick, or slightly thicker, than the foam, I'd think the sheer force would be carried over to the inner and outer plywood skins and beams......the foam....more or less......along for the ride. Our box stores cary a 25 psi product (Owens-Corning Foamular 250) in sizes down to 3/4 and 1 inch thick. They make higher psi stuff to in the range of 40 and 60 psi that are still 1 inch thick, but these become hard to find.

 

Presumably this is the type of structural grid you were referring to and would be a good way to build a coach roof?

[Oyster]

Personally I am not opposed to using foam as a hull material or using foam as a material to seperate laminates, creating a structual component. But to get back to the original question and OP, there are reasons why both cost and work to achieve a small boat hull makes it not worth the effort that is required to end up with a safe hull long term f you attempt to go cheap. Of course in some of the smaller hulls, they are sometimes a stepping stone to a bigger hull, and really does not matter, even though you spend a lot of money in the learning curve too with the same product making the hull structual with the cheap core.

 

On my cabin boat I incorporated quality foam for the core, which insulated the full sized cabin and created a strong and liteweight bulk component. I will organize a reply that shows a pictorial of what I did, even using solid wood skins and post it later, in particular for Howard.

[Chick Ludwig]

Low density foam has been tried for years in different ways. I've used it to build "plugs"(forms) to make fiberglass molds over for small boats and various other parts. Remember the small sailboats and outboards made of foam---some with a plastic skin, and some not? How about the old Boston Whalers? A thin glass skin shell pumped full of foam. Of course this was a high density foam. I've repaired a lot of Whalers that were waterlogged and broken.

One use it is really good for is to build a small boat to test design, make hull panel patterns and such. Carve it 'til it works, make patterns, throw away...

A number of years ago, there was a low density foam that came in sheets that had slits all over them. They were used as a core material much like balsa blocks, but it wouldn't rot. The idea was to tie the skins together structurally by allowing resin (polyester back then) to soak between the blocks. The resin tended to break up and allow the skins to separate.

[Oyster]

I did a nice insulated cabin, both on the cabin roof and the foward trunk. I also used the same stuff for the sides. This was white cedar sawn to 1/4" and installed on structual but liteweight beams. this gave a nice setup that allowed me to walk on the top but also did not add excessive weight for the bulk structure. Around the borders and up foward, I created a solid layup of several veneers of plywood. In the middle of the top I added small wooden strips that also reinforced the structure  and transfered some loads to the beams between the layers of divinicell. The inside maintained a nice temperature, both in the winter and in the summer time.

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[Howard]

That was helpful.......and WOW.......what lovely work!