For a short while, I was looking for an alternative to the expensive 'marine grade' plywoods. I picked up a 4x8 sheet of '1/4"' birch, which actually measured a little less, about 5mm. The grain is very pretty, and birch is relatively hard compared to say, cedar.

What I found was that the birch was 50% heavier, even though it was only 25% thicker. My informal calculations indicated that my 40 pound boat, using the 4mm ply, would weigh 50 pounds using the 1/4" birch. If the materials were the same, I'd expect a gain in strength as well, but who knows in this case where the core wood is unidentified?

I'm pretty hard on equipment, and I'm not a marine engineer, so I take refuge in the generally accepted 'common' wisdom in this case. Lots of boats have been made with the fancy 4mm, and failures are apparently very rare and have more to do with construction technique than material failure. Without some kind of test data or reports, I'd always have this worry in the back of my mind about whether using something else is really a good idea.

On the other hand, if I contemplated paddling only on warm shallow lakes with sandy beaches, I'd be much more open to experimenting: Those 1/8" birch doorskins are also pretty. But around here the water is cold and the 'beaches' are rocky.

So consider what you'll be doing with the boat you want to build BEFORE you buy the materials. It's been made pretty clear here on the bbs that a boat CAN be made from just about anything that's thinner than it is wide.

It may also affect your boat's saleability, if that's a possible endpoint to all this. At least with the 4mm marine grade you can point to it as the industry standard, so to speak. A buyer can feel like they can make a comparison. If you start using alternatives, without any sort of backup documentation, that puts the burden on the buyer to find the proper way to compare boats.

Have I made things muddier? If this is your first boat and maybe the only one you contemplate ever making, why add 'materials applicability' to the list of stuff you need to learn about? It's been pointed out in this bbs before, I think, that the cost of the wood is a fairly small part of the total cost of the boat, so I've concluded the uncertainty factor isn't worth the relatively small money savings.

For now anyway. I've built exactly one boat, and that was a kit. Maybe after my first dozen, I'll throw caution to the winds and try some of the wierder things I've read about here. For now I have to remember I take my boats out where no 5 minute 911 response exists, so I want them to handle anything that I can reasonably anticipate, such as barnacle-covered rocks or heavy seas pounding the hell out of us.

Quite a while back, there was an extended debate here on the best glass layup protocol to use. It got so complicated I lost track of it somewhere around the 'scantlings rule' whatever the hell that is. Can anyone out there say what conclusions, if any, were arrived at?

Was there ever any similar debate on plywoods for S&G construction?