: 4mm will work and so will 3mm. IF it's quality BS1088 okoume. I have a kayak
: that is entirely 3mm ply that's been around for 10 years and it isn't even
: covered with glass.
: You can build kayaks from the cheap stuff but it usually turns out to be too
: flimsy, if unglassed, to guarantee a moderate life span and it may not
: tolerate anything more turbulent than a farm pond.
: The first thing you want to know is if the middle veneer has any gaps in it.
: Gaps cause structural weaknesses. You can scan a sheet of thin ply with a
: 250 watt spotlight applied in a dark room. The voids will show up as an
: orange glow. You can test for waterproof glue by boiling a piece. It
: should last for an hour without delaminating.
: Or you can use your head and pay the price for marine quality ply. $20 a
: sheet ply is never going to be as good as $70 a sheet goods.
: A hull for someone your size has to be bigger than one for someone 100 pounds
: lighter but, otherwise, everything else is about the same.

Try again with this

There is alot of literature around the web, some of it mine, saying that it's ok to build boats that don't live in the water all the time out of exterior grade ply. I've done it. Have a pair of dingys that I built like 20 years ago from a/c exterior grade ply done S&G with polyester resin and bondo for the filets that are still going strong and likely will last for another 20 years. But they only occasionally get wet and were built of much better exterior ply available 20 years ago.

Would I do it that way again? NO, the available materials are lousy in comparison and the cost of the materials arn't worth the amount of work you put in the project. Spend a few extra bucks up front and have a boat you can comfortably use for a lifetime

Ok, you might be able to get away with the British stamped exterior ply, it's about the equal to the stuff we used to be able to buy from the big box stores.
It is about 1/2 the price of marine grade from what I've seen. Still for the average project you might save $50, not sure that it's worth that little a savings.

Bill H.