Compare total cost: cheap exterior covered with glass and resin often
cost more than decent marine plywood.
It is also heavier and require more work.
Jacques from bateau.com

On 6 Jan 2005 07:22:51 -0800, vaguely proposed a theory
.......and in reply I say!:

remove ns from my header address to reply via email

But this assumes that you would not cover the marine ply with glass as
well? I would. A glass sandwich is so stiff, and so bump/scrape
resistant, it seems impossible not to. I would go down a size in ply
for weight.

Compare total cost: cheap exterior covered with glass and resin often
cost more than decent marine plywood.
It is also heavier and require more work.
Jacques from bateau.com

Old Nick wrote:
On 6 Jan 2005 07:22:51 -0800, vaguely proposed a theory
......and in reply I say!:

remove ns from my header address to reply via email

But this assumes that you would not cover the marine ply with glass as
well? I would. A glass sandwich is so stiff, and so bump/scrape
resistant, it seems impossible not to. I would go down a size in ply
for weight.

Careful when doing that. The scantlings don't always translate that way.

--
Matt Langenfeld
JEM Watercraft
http://www.jemwatercraft.com/

please note that we are discussing replacing a jon boat here, a small
boat. foam core gets "light and strong" as the hull volume increases.
for small boats like this jon boat it is heavier than plywood.

laun underlayment is okay for small boats that don't have to last a long
time and preferably can be stored under cover. I have two small boats made
of laun underlayment. The only finish on them is a 2 coats of exterior
grade latex house paint. I go over the finish a couple times a year. Laun
underlayment is 1/5 inch thick. Although the wood is rot resistant it's
only 3 ply and the face ply is thin. It weighs 17 lb per 4x8 sheet
compared to dougals fir at 22 lb. I'd use it for a small lightweight
cartop boat stored under cover when not in use, with some hull curvature
for strength and no long unsupported panels. I use skids on the bottom to
give strength (photos on my website below). There is a better grade of
underlayment called "meranti" that has replaced lauan here in Ottawa the
past 2 years. Personally, for a cheap jon boat which might get some rough
treatement I'd use painted douglas fir plywood and cover or store it out
of the sun so the surface doesn't check. I don't think anyone makes spruce
plywood any more. I have a few scraps of exterior grade spruce but have
not seen any for sale for years. Now they use that chip board and oriented
strand board to sheath houses.


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Check out instantboats.com for some inexpensive easy to build skiffs.


"Ookie Wonderslug" wrote in message
...
What is the cheapest foam sheeting that one can buy that will not
melt when you apply polyester resin to it? What is the best source for
this foam? How much does it cost? Would it better than using luan as a
core?

I need a bigger jon boat. Mine is 40 years old and starting to fall
apart. I am going to build one, but money is a deciding factor in
this. If I had the cash, I would just go to Outdoor World and buy one
of theirs for $500. But since I don't have that kind of green I guess
I can build one. I would like to use foam as a core because it is
light and easy to form. I could see glueing a boat together out of 1/2
or 3/4 inch sheet foam and then glassing it. It just feels like it
would be easier than building a frame and then attaching the luan and
then glassing that. However I have never built a boat. I have rebuilt
a couple, but never built one from scratch.

Any advice? Pointers? Suggestions?

IMO a jon boat is perfect for plywood or aluminum construction and not a
foam core boat
which can be an expensive complicated project that leaves you with an
inferior boat.