> A quick question - does using 3 mm ply in a stitch-and-glue, hard-chine
> boat's hull make a really significant difference in the strength of the
> hull? I'm considering building either a CLC North Bay or a Sea Kayaker
> Greenlander, probably with the same modifications as listed below, but
> would like to make it as light as possible. How much strength would be
> compromised? We'd have to consider impact resistance, puncture
> resistance... etc. Any thoughts?

> I guess when it comes to equipment like this it's better to be safe than
> sorry, but I'm still wondering. Thanks in advance,

> Nathan

Any thoughts? Yeah. The material is 3/4 the size of what is specified. I would say that is significant.

It would be reasonable to assume that it is NOT as strong as the 4 mm.

The unasked question, and I assume you implied it, is: ''can I really get away with this?'' The answer is: Probably.

The work by West Systems and (the unfortunately oft-maligned) George Roberts is important here. In one of their books on boatbuiliding, West shows that when comparing 1/8 th inch cedar strips covered on both sides with a single layer of glass cloth and resin to 1/4 inch cedar strips that are similarly covered, the difference in breaking strength is marginal. The thinner strips deform far more before they break, that is, being more flexible, they bend a lot more rather than snap. George has been a very vocal supporter of improving on the glass layup, using multiple layers of fabric (glass and other fibers) over thin strips to provide greater strength while still saving weight and maintaining flexibility.

If you work from this base of knowledge, you can assume that in this range of wood thicknesses, the strength comes more from the glass on the outside than the thickness of the wood inside. So, if you are careful with chosing and applying the glass layup you should be able to use the thinner plywood. Or at least this is how I read things.

Hope this helps

Paul G. Jacobson