William R. Watt wrote:
The foam boatbuilding I've seen described requires a building frame to which
a special boatbuilder's foam is attached. It's then covered on the outside
with lots of fibreglass for strength. Then it's femoved from the bulding
frame, turned over, and covered on the inside with more fibreglass. All
that fibreglass makes it heavy.

That's an odd thing to say. Fiberglass/foam core or sandwich
construction is usually described as very light & strong.

However, it's not cheap. The cheapest type of foam insulation is not
suitable for core material as it does not have a good bonding surface
and it's shear strength is comparable to lumpy oatmeal.

Luan underlayment is probably the cheapest per square foot for boat
building material that has a chance of success. It will probably have
durability & longevity issues unless it's given very careful work during
building & good maintenance during use.

Here's a thought- has anybody tried building a stitch-n-glue boat out of
OSB?

Fresh Breezes- Doug King

William R. Watt wrote:

The foam boatbuilding I've seen described requires a building frame to
which
a special boatbuilder's foam is attached.

That's one way, probably the easiest.

It's then covered on the
outside
with lots of fibreglass for strength. Then it's femoved from the bulding
frame, turned over, and covered on the inside with more fibreglass. All
that fibreglass makes it heavy.

What absolute BULL ****.


Lew

Lew Hodgett wrote:

William R. Watt wrote:

The foam boatbuilding I've seen described requires a building frame
to which
a special boatbuilder's foam is attached.


That's one way, probably the easiest.

It's then covered on the outside
with lots of fibreglass for strength. Then it's femoved from the bulding
frame, turned over, and covered on the inside with more fibreglass. All
that fibreglass makes it heavy.


What absolute BULL ****.


Lew

Don't hold back, Lew.
As a data point on the weight of foam sandwich construction, my current
project is a 29' sailing trimaran built in core-cell foam, knitted glass
and epoxy. The main hull, with interior, tanks and some fancy cockpit
joinery weighs about 900 lbs.

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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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.