plug materials... dow Styrofoam?

[gates]

Hi all, this is a great website. anyhow.
I'm planning to make a plug/prototype for a small canoe/kayak this summer and am looking into materials. I've previously used the cheap white styrofoam from home depot (beads), but I want to look into something that will shape better.

I've come across extruded polystyrene foam, as it's used for epoxy surfboards. I also discovered it's nothing more than dow Styrofoam.. the blue stuff used in construction for insulation. Has anyone used this? Where can you purchase the stuff? (I've searched google, pretty unsuccessfully... the dow site says to call, and they're not open now)
Do you have any other suggestions?

Also, in my previous project using the white styrofoam, I laminated 2" boards using 5:1 marine epoxy. The epoxy became a problem in shaping since it was so much harder... not to mention the thicker spots burned holes into the material. Would you have any suggestions of glue to laminate the foam?

Thanks

[marshmat]

Between the boats and the solarcars I've seen a whole lot of plug materials tried... polyboard, Renshape, various polystyrene foams.... generally, the denser ones work OK while the light stuff (white styrofoam) doesn't. The blue poly foam you mention (it also comes in pink from OwensCorning, exact same stuff) can be found somewhere behind the lumber/plywood aisles in most Home Depot or similar building stores.
Some fellow students used the exact same blue sheet foam for a canoe plug this year... they cut the foam to the shape of the transverse sections of the boat, probably around 100 slices of foam, and glued them together from bow to stern... then they trimmed and smoothed it, worked great.
Ordinary white carpenter's glue will bond this stuff just fine, just make sure you have a lot of it on hand. Its texture is close enough to the foam's that it won't be as big a problem as the epoxy.

[pjwalsh]

Some co-workers and I just finished a one-off marathon style racing canoe using a male mold of trapezoidal section styrofoam strips over transverse frames which we then covered in a thin layer of plaster. Basically we followed the techniques described by Jim Moran in his book "building your kevlar canoe". Have attached some photos of the mold.

Some things we learned:

Space your transverse frames ~12" apart - the styrofoam strips have little strength and will bend inward between 18" frame spacing during fairing.

You can use hot glue to fasten the strips to the transverse frames but a can of great stuff expanding PU foam works much better for glueing the strips together along their length - you can glue the strip to the frames with hot glue then squirt the PU foam into the gaps between the strips from the back (interior) of the surface after they are fixed. This stiffens the strips greatly which aids in fairing later.

Use the sandable durabond 20 or durabond 45 plaster as the finishing surface. This, like the styrofoam, is available at the lumberyard. It is a a 20 lb bag of plaster of paris and calcium carbonate plus a polyvinyl Acetate binder. It is mixed with water to the consistency you desire. It is much sturdier than the stuff that comes premixed in a 75 lb bucket and unlike the premixed stuff will dry overnight when applied in a thick coating - this is the reverse of Moran's reccomendation. We found the premix impossible to work with in our 50 F shop. Be sure you get the sandable stuff.

Jim Moran suggests using a thin coat of melted paraffin wax applied using a roller as the release agent - very much a pain in the arse, and realy dangerous to melt 5 lbs of wax over an open flame. we found that a coat of latex paint over the sanded plaster surface with PVA release over the painted surface was much easier to accomplish and worked just fine. If you are a belt and braces type (like us) you can apply 3 coats of paste wax over the PVA. Our hull did not stick to the mold at all.

This approach made a male mold serviceable for a wet layup but not for vacuum bagging as mold's strength was low enough that cracks formed along the styrofoam plank lines destroying the integity of the vacuum (some cursing went on around this time, but we regrouped, pulled the bag off and were able to proceed with a conventional wet layup). If we ever take this approach again we will glass the plaster surface to allow use of a vacuum bag to consolidate the laminate.

Just a quick comment on cost: we spent about $150 for all the mold materials: lumber, styrofoam, plaster, hot glue, paint and PU foam. It is definitely a low cost way to realize a mold, gotta be satisfied with a lower grade exterior finish though.

[gates]

thanks marshmat.

pjwalsh, what was your timeframe for that project? seems like a fairly delicate process to lay in all the strips. How big were the strips you used? Were they full length strips?

I strip planked an wooden canoe, and that definitly took longer than expected.

Let me try to explain how I was thinking of doing it for this project, and maybe you guys to let me know if it sounds reasonable. Basically I was gonna cut the stations out, probably every foot as you suggested, and then just glue foam blocks inbetween each station, and shape down to the stations. That way I'd get an accurate shape and the mold would be solid. And I can just go at with a saw at first, then a planer, and then longboard. And then I would probably glass it to make the thing indestructable..

I do really like your price point though, I think my method will be quite a bit more.

[pjwalsh]

setting up the stations and planking the mold took us 3 weekend sessions of about 6 hours each. The strips are 8 feet long and ripped on a tablesaw to 1.75" width with a 5 degree bevel such that on side of the strip is narrow and one side wide - this to accomodate the curvature of the station molds and minimize gaps between planks. The planking part is quite straightforward - you don't need to take as much care fitting strips as in building a wood strip boat because you will be coating the whole surface with 1/4" or so of plaster - little gaps make no difference and actually help the plaster to stay fixed to the foam.

your idea of solid foam seems reasonable, it will certainly use a good bit more foam. We only needed 4 2' x 4' sheets and had nearly one sheet left over. I find that the foam doesn't really fair that well though - even with a 60xx long board we were ripping up good size chunks of styrofoam. The strip plank approach has the advantage that the shape is already pretty close and less massive stock removal is required. We ended up rough shaping the foam with an 8" grinder with 36xx paper.

Some students here at Umass built a mold for a concrete canoe using the technique you describe and it worked for them, though their design, in my opinion, was a pig. One advantage is that with a solid foam plug well glassed over you would not need to worry about pulling a vacuum on it when laminating your hull.

[Packeteer]

the best stuff is pva wood glue

doesn't eat the foam, plenty of work time, gives nice strong bond and its sandable

[icetreader]

Originally Posted by gates

Where can you purchase the stuff? (I've searched google, pretty unsuccessfully... the dow site says to call, and they're not open now)

gates,

If you want 4" thick boards and even thicker ones to carve from you can find them in special building insulation materials stores -Look in books like your local Yellow Pages.

Yoav

[gonzo]

Wood glue (white or yellow) works best. We use to make surfboards with styrofoam. You can buy it at any building supply-Home Depot, Lowe's, Ace, etc.