I wouldn't worry about the seam (5x7 vs 4x8). Scarfing isn't hard, but this
would probably be the place to use the technique of just putting fiberglass
tape on each side of the plywood along the joint. Your structure makes up
for any lack of strength and you can fair in the lump easily.

I'd think CDX was the way to go. HDO is much nicer with fewer voids and
more plies, but you don't need it and it's pricier. For boats, it takes
bends with fewer lumps.

Too bad you didn't buy the lumber before Katrina hit.

Roger

http://home.earthlink.net/~derbyrm

wrote in message
oups.com...
I've eliminated some options. I'm going to either use:

- exterior grade (ie CDX) or high density overlay (HDO) plywood, with a
coat or two of quality epoxy, and make a 5'x7'x3' box.

I don't think I'd even glass it, just caulk the seams and put a liner
in. One problem with this method is that if I used 4x8 sheets of
plywood, the 5'x7' bottom would have to have a horizontal seam. I'm
guessing I'd spend $150-250, plus the liner.

- cold-mold a tub with more comfortable contours and curves, using 3-5
layers of approx 1/8" fir (because I have an enormous free supply)

I've read in an article (not sure of the date) that cold-molding cost
about $3.30 per pound, at 1 pound per square foot, including the cost
of the wood. My tub would be about 107 sq ft. So by that person's
calculation, it would cost just over $350 for the raw materials. I'm
wondering how much of that cost was due to the wood, and how much for
the epoxy. I have a good amount of free time, and would love an excuse
to learn this method, so labor isn't an issue.

- I've heard that there was an article in Home Mechanix (somewhere
around 1990, I haven't found it yet) that described plans for building
a tub by fiberglassing over masonite (hardboard). Sounds interesting,
but I'd like to find the article first. Masonite is very cheap, and
that "supply of fir" I have includes 2x4's, which could be used for
additional framing.

Thanks for the continuing help.

wrote:

Hi,

Thanks for the replies. What grade of fiberglass would each of you
recommend (with plywood, or without plywood)? I was planning on buying
fiberglass cloth on eBay, and I've seen anywhere from 2oz to 45oz. The
shipping is usually more than the bid price.

Terry,

When you say "Took about 20 thin coats brushed on.", do you mean 20
finish coats of resin? How many layers of actual fiberglass cloth do
you recommend? Is it better to use a lighter grade glass, and use many
coats, than to use 1 large 45oz layer?

Thanks for the input.

My firt 8' dinghy job was formed over a saggy old wooden skiff. A
coat of wax paper, cloth, 2 or 3 layers of mat alternated with light
cloth, all done while green except for a break in the middle cured
overnight, rough sanded and washed with acetone in the morning,
followed by many coats of clear resin with like, 7 or 8 drops of
yellow pigment per mix. Just kept on mixing a coat's worth in a cup,
colour first, MEK next, paint it on, mix up, again, again,,, got the
speed mixed right to keep it all tacky, flowed on with an old brush
stiff and yucchy, squeegeelike, built up to about 1/16 thick self
levelled gelcoat, in an open garage in summer humidity. Filletted
inside corners with bog and a layer of cloth. Strong as a cast iron
bathtub, stiff, with a little hogging between the centre and each
end, hardly noticable. Planed at about 15 kt with me and a Suzuki
3.5 o/b on the flat. I steered "with my chin", hung over the front,
leaning to steer. Great fun. Traded it away, wish I hadn't, now.

I don't think it matters much what weight cloth, it was all ad hoc,
I had some scrap and bought some at the local race track f/g works.
I watched them make apple barrels and learned to keep an airtight
ammo box acetone bath to clean tools, especially a washer roller I
made up from threaded rod and loose alternated 2 sizes washers with
locked nuts to squeeze out air between glass lams.

The boat was about 1/4" thick or less and included a luan floor to
hold down 1.5" styrofoam, 3 plywood seats sitting on wooden 2x2
clamps, screwed from the outside. The gunnels were old plywood
strips, one in, one outside, screwed together and capped with a
layer of glass. Shoulda been 2 or 3 layers. Coulda been nice
mahogany, routed to fit over the rough edge. It got tore up dragging
it over stones. I whittled knees for the top of the transom and flat
stem, leaving holes for rope, forming handles screwed from the
outside, and painted all inside with leftover beige latex. A plywood
motor mount doubler for the transom incorperated into the stern
thwart. Space under the seats for my "golf club" bilge pump clamped
onto stiff plasic pipe 4 feet long, using a bag phone battery and
flex hose discharge. Putting the discharge under the water outside
the boat formed a syphon that meant the pump needed not work to
actually lift the water, it only worked to move the water sideways,
out of the boat. I used to go around the marina and pump out dingies
after a rain until the battery went flat, just for the hell of it.

They never found out it was me.

Couple of hundred bucks, then, altogether.

Fun.

If you cut holes big enough to pass curved through hull "nozzels"
with hosed clamped on and 5200 permanently glued to the inside, you
could route hoses to a "T" manifold assembly and valves, with pump
controller, or use the big nuts and clamp hoses on later if you have
ample room behind. Make sure hair, etc can't get sucked into a
recirculating drain and drown someone. Keep the pump intakes high on
the sides, and grated to protect your biggest pekker too, or use a
passive, nonsuction overflow collector, maybe perforated drain pipe,
possibly under a decorative, removeable side deck grating. Add a tub
/ shower surround kit, maybe some mirror tiles, and Bob is your
uncle. A timer on the pump would ensure eventual release of anyone
snared too far from the power switch. People have been drowned, dare
I say even worse, by recirculating spas.

I never heard of osmosis blister in fresh water, especially if the
tub is not always full.

Terry K