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Hi,

A number of weeks ago, I posted a question on this group about building
a composite hot tub by "borrowing" some boatbuilding methods. I've been
looking at cold-molding vs sheathed strip (is this synonymous with wood
cored glass?).

snip

My question is this: If you are planning on using this type of "cold
molding", and plan on encasing everything in glass in the end, what
type of glue/epoxy can you get away with for the layers of wood in
between?

Epoxy.

I've read that a lot of the sheathed strip kayak builders get
away with using yellow wood glue (some even say they've used hot glue),
knowing that the fiberglass is going to provide a moisture barrier.

With sheathed strip, there is only one single layer of wood, which is kept
together by the glass on both sides. Glueing the strips together is only
necessary to keep them together until the glass goes on. Nails could be used
as well.

However, with could moulded, you have more layers of wood which will only
keep their shape when glued together very rigid. So in this case you MUST
bond the layers together very well.The glass is not providing any or much
help to retain the shape. Could moulded can even be used without glass
sheathing, as Paul Gartside does.
And since it is impossible to privide the necessary clamping force needed by
ordinary glues to get a decent bond, you'll simply have to use epoxy for
this which does not need any clamping force at all.

And as said earlier, don't go blind on the cost of epoxy, it will only be a
fraction of the total cost of the boat.

Meindert

Meindert Sprang wrote:
And since it is impossible to privide the necessary clamping force needed by
ordinary glues to get a decent bond, you'll simply have to use epoxy for
this which does not need any clamping force at all.

Make that "thickened epoxy", as it will have to be applied thickly
enough to fill any gaps and irregularities. Sounds like a huge pain the
butt to me.

Brian Nystrom,

Yes, I'm starting to think that myself. However, due to the need for a
custom design (the shape I need isn't available off the shelf), my
options are somewhat limited. I could learn to build a "real"
fiberglass foam mold, but from what I hear, it would be a long learning
process, and I'd probably mess it up the first time. The "fallback"
option is to use plywood sheathed in fiberglass, with straight vertical
walls, and add seperate benches.

Meindert Sprang,

Thanks for the advice. I've found some relatively cheap sources of
5-gallon thick epoxy kits on eBay. They're not suitable for wetting
fiberglass, but probably fine for this task.

William Watt,

I haven't actually calculated how much cement I would need to use, but
I have a feeling that it would weight too much. Maybe I should look
into it, because the water will weight around 4000 pounds after all
(though there may be less water if the tub has a curved bottom and
seats).

Let's say the tub is 5' x 6' x 2.5', and for now, it is just
rectangular. The surface area (including bottom and 4 sides) would be
85 square feet. If the cement was 3" thick (I'm not sure if this is
reasonable), the volume of cement needed would be 85 ft^2 * 0.25 ft =
21.25 ft^3. Let's see, 1 foot is about 0.3 meters, so 1 cubic foot is
about 0.027 cubic meters. So 0.57 cubic meters. At 2300kg/m^3, that
would weight about 1367kg, or a little over 3000 pounds.

I think that would be too heavy for my application, though it all rests
on my guessing that the walls would be about 3" thick.

Thanks for everyone for the help.