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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