Pour foam

Gday,

I need to finish the bow sections of my catamaran build project. I need to shape the bows from solid foam and was thinking i should approach it by making a small mold and using PU pour foam to get the approximate shape i need, then glue them to the 1st hull bulkhead and final shape them by hand from there.

Ive never used pour foam before, is there anything i should consider before approaching it like this?

Also, can i use the pour foam as an adhesive to glue the molded blocks to the 1st bulkhead, by mixing it up and applying it to the foam block, and then holding it in place until it expands and cures to form a no gap bond line?

 

You can use pourable foam, but it needs to be structural foam, not buoyancy foam. This requires at least a 5 pound per cubic foot foam. You can use the pourable foam to bond other blocks or sheets of foam together if it's convenient for you, but the laminate is what bears the load, so though it'll stick (the foam) the fabrics are what does all the work. Technically, the foam has a role in compression. Foam does best if the pieces to get bonded, are simply held in location and the gap filled with foam and allowed to cure. If you press the blocks together, it's not going to expand much and you'll not have a good bond.

 

You won't be able to use it as an adhesive, as it will expand as it reacts, and push the two pieces apart. I would go for 2kg/cu ft mix, at least, the 1kg stuff is very friable. Have a good paint mixing tool in a power drill to mix the two parts thoroughly, hand mixing is not fast enough.

 

Looks like PAR beat me to it ! I assumed groper meant using it as an adhesive held flush.

 

This must be your design ? or the plans would tell you what to do I imagine.

For that small a section, I would be inclined to get some low density Divincyell, and glue contour cut sections onto the bow with epoxy. ( see attachment)
http://www.pwt.net.au/Feature Articles List/Build of the trimaran - KISS.pdf

That way, you have 90% of the accurate shape, and 90% of the symmetry solved without constant reverence to templates as you cover yourself in foam granules.


If you pour the stuff, you get approximate shapes, and varying density, that is a bugger to fair up and get identical.

These guys do pretty good job with small blocks, but long skinny non rectangular pours would be a bit awkward
http://www.dillon-racing.com/dillon-sportc/plans/sportc-page-09.htm

 

Hmm...

I do have a bunch of divinycell off cut bits, but they're all random sizes and shapes... The thought of piecing them all together to resemble something close to the shape I need boggles my mind... I wonder if I could some how throw it all in a trough lined with plastic, then use pour foam to stick it all together... Ha,that sounds like a mission also... Gluing all the divinycell with epoxy also sounds like a **** fight, as shaping it will be difficult with all the hard glue lines coming through...

I know there's a million ways of skinning a cat ( there's a pun ) I'm just looking for the easiest way ...

 

Forget about glueing bits of divinycell, imo, you will find it a nightmare to sand, especially as the glueline will be harder than the foam. You could use some kind of syntactic foam plastered onto the nose of the boat, then sanded when it sets.

 

The foam IS hard to sand, which is why I said to use it as a pattern - a bit smaller than the actual exterior.

While you paint a layer of 'straight' epoxy onto the mating surfaces, the 'adhesive' is a mixture of microballoons and epoxy, as is the outside surface.

This epoxy mix is easy to sand into shape, but most of the accuracy of the hull shape is created by the underlying accurately cut shapes.

Besides the poured foam being awkward, it tends to absorb moisture in the long run through unavoidable voids created during the pour.

And dont think the pouring foam is much cheaper either. It works out pretty steep, especially after all the bits that stay in the mixing bucket, and spill down the sides.

Whether you could use cheaper foam is as Par said - a matter of load. Is the skin going to hold the water out on impact, or do you want the foam to absorb some of the load.

Theoretically, a better foam will save on skin costs too.

 

I'm not concerned with accuracy at all. This is one of the reasons I wanted to use solid foam. A fairing batten can simply be used, running it off the existing hull and out onto the foam block. Keep checking it with the batten at various locations as the foam is planed down, then finally sanded to its final shape. It's the most accurate way of shaping the whole thing and getting it married into the existing hull.

I'm not so concerned with cost either, time is more important to me. So I'd rather get this done easily and quickly rather than cheaply... I could potentially buy a large block of solid foam from a local supplier, however I was concerned about such a large amount of waste. This led to me to consider pouring my own blocks, which were much closer to the actual size and shape I need.

 

Groper,
surely you can just set up some "formwork" in situ using melamine sheet strait on the job say to a 40-50mm flat nose ... wax & release before screwing on & pour...pop off.... while later cut the crust off with a zec disc & board in to fair off & round off the front then maybe a qcell mix to fill porosity then wrap with a few DB 450 glass to cover & bond back 50-100 mm to hull sides... then sad/ fill & board to satisfaction....

Jeff

 

Perfect...

How much pressure does this stuff create when it expands? As in how strong does the formwork need to be?

 

Depends on whether you have an open side to the box. If you were filling a closed box, a lot of pressure, potentially some tons. When fridges are blown, they are in a solid concrete 5 sided 'box' with a bolt down steel lid. Obviously they are blown with a known ammount of expanding foam...otherwise....

 

leave the top open............use a bit of 90x45 pine as the "stem" with a bit of packing tape on it.... lap the melamine maybe 100mm onto your hull sides & shoot a few particle board "grabber" screws, don't worry if it blows out a little, the top foam might only be 300mm & you're going to sand a bit any way, if it's a bit over lightly cut using a hard point saw using the topsides to guide as a reference.

 

Ok sounds good. Don't suppose you have a good source for the foam do ya Jeff?

 

I'd try FGI/Nuplex or Fiberglass Australasia or even Whitworths stock it.... might be 10 years since I've used any.
Looking at your pics, is there continuity of fiber from your bulkhead at the bow to topside laminate, I'm sure it would be fine with just the internal but it's hard to see if there's something external... would be good to have that structure complete with say 50mm lap to the topsides & the foam plus its laminate as an addition with 100mm lap or vice versa to make tapering & fairing easier.
Jeff