Cold molding with thin strips
 

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?).

If I used sheathed strip (with 1/4"-1/2" or greater core), I might not
be able to obtain the desired curvature. I could probably obtain the
desired shape with cold-molding using veneers, but I think I that I
would have to make a decent mold to support the veneers during
construction, and it would still be awkward.

I read some posts about people using multiple layers of thin wood
strips for cold molding, instead of using veneers. I'm going to have
some exterior framing surrounding the tub, and I think that I could
simply attach the first layer of wood strips directly to this frame
(probably via staples). Then add a second layer of strips, diagonally
oriented from the first, working from the inside. Then a third diagonal
layer, with the opposite orientation. The strips would probably be 1/8"
each. I'm not sure exactly how many layers I would use, I would gauge
the stiffness of the structure after 3 layers and go from there.

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? 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.
Keep in mind that I would be edge gluing the strips together, and also
gluing each layer to the previous one.

Thanks for the help.

Shamus

Cold molding with thin strips
 

If you can take the weight it would be a good application for ferrocement.
I've seen government programs for making water and grain storage tanks in
"developing" countries that way.

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Cold molding with thin strips
 

wrote in message
oups.com...
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

Cold molding with thin strips
 

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.

Cold molding with thin strips
 

Here's another thought.

Picture the profile of the tub you want.
Cut this profile into a ply rib, and repeat every 6 inches or so.
Then cut a profile of the tub at rt angles to the first every 6
inches, with slots to fit in the other ribs.
2 inches depth would be stupendously strong, with an upper and lower
glass skin.

Brian Whatcott

On 18 Oct 2005 15:39:52 -0700, wrote:

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?).

If I used sheathed strip (with 1/4"-1/2" or greater core), I might not
be able to obtain the desired curvature. I could probably obtain the
desired shape with cold-molding using veneers, but I think I that I
would have to make a decent mold to support the veneers during
construction, and it would still be awkward.

I read some posts about people using multiple layers of thin wood
strips for cold molding, instead of using veneers. I'm going to have
some exterior framing surrounding the tub, and I think that I could
simply attach the first layer of wood strips directly to this frame
(probably via staples). Then add a second layer of strips, diagonally
oriented from the first, working from the inside. Then a third diagonal
layer, with the opposite orientation. The strips would probably be 1/8"
each. I'm not sure exactly how many layers I would use, I would gauge
the stiffness of the structure after 3 layers and go from there.

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? 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.
Keep in mind that I would be edge gluing the strips together, and also
gluing each layer to the previous one.

Thanks for the help.

Shamus

Cold molding with thin strips
 

Hi Brian,

I wish there was a standard way to draw diagrams on the net. I'm not
quite sure if I've understood correctly. Do you mean that the first set
of ribs would be mainly vertical (for a "conventional bathtub", they
would look like U's), and the second set would be longitudinal? ie, a
see-through grid of ribs. If so, would you recommend attaching a
sheathing of thin ply before applying the glass skin?

Also, by 2 inches, do you mean the width of the ply strips?

Sorry if I'm missing the obvious, I haven't yet learned much about boat
building, and probably don't see some of the things that are clear to
others.

Cold molding with thin strips
 

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.

Cold molding with thin strips
 

wrote in message
ups.com...
Hi Brian,

I wish there was a standard way to draw diagrams on the net. I'm not
quite sure if I've understood correctly. Do you mean that the first set
of ribs would be mainly vertical (for a "conventional bathtub", they
would look like U's), and the second set would be longitudinal? ie, a
see-through grid of ribs. If so, would you recommend attaching a
sheathing of thin ply before applying the glass skin?

See http://www.gartsideboats.com/jessie.php
This is a could moulded design, where temporary stringers are laid fore and
aft, to hold steam bent frames. This is covered with two layers of wood, one
1/4" thick, the second 1/8". When you add a 3rd layer, you can probably
leave the frames out. BTW, this boat is not covered with glass.
If you build the same boat with strips, you need glass inside and out but no
ribs are needed.

Meindert

Cold molding with thin strips
 

See http://www.gartsideboats.com/jessie.php
This is a could moulded design, where temporary stringers are laid fore and
aft, to hold steam bent frames. This is covered with two layers of wood, one
1/4" thick, the second 1/8".

When you add a 3rd layer, you can probably
leave the frames out.

Leaving the frames out would be good - otherwise I'd have to put a
layer of wood on the inside too (for comfort reasons - it is a hot
tub).

BTW, this boat is not covered with glass.
If you build the same boat with strips, you need glass inside and out but no
ribs are needed.

I think sheathed strip would be the easiest construction method, if I
could get it to work for the desired curvature. Could I use very thin
wood strips (1/8" or so), and rely almost entirely on the fiberglass
for strength? Basically the only purpose of the wood would be to avoid
making a mold (as would be needed for 100% fiberglass construction).

Meindert

Thanks again

Cold molding with thin strips
 

On 19 Oct 2005 13:45:10 -0700, wrote:

Hi Brian,

I wish there was a standard way to draw diagrams on the net. I'm not
quite sure if I've understood correctly. Do you mean that the first set
of ribs would be mainly vertical (for a "conventional bathtub", they
would look like U's), and the second set would be longitudinal? ie, a
see-through grid of ribs. If so, would you recommend attaching a
sheathing of thin ply before applying the glass skin?

Also, by 2 inches, do you mean the width of the ply strips?

Sorry if I'm missing the obvious, I haven't yet learned much about boat
building, and probably don't see some of the things that are clear to
others.


This is a speculative idea, rather than the fruit of long experience.
Yes I am thinking of ply ribs in two directions at right angles,
Whose faces are all vertical, so that in plan view, they would look
a noughts n crosses game of lines say 1/4 in thick, and in side view
they would be 2 inches deep and wide (at the sides.)
An internal and external skin would then provide a strong stiff light
structure. The crucial detail is making the inside skin fair and
smooth. This is left as an exercise for.... :-)

Brian Whatcott