Birch plywood

[jadranko]

I found very interesting topic on some forum, there was very intense discussion about which plywood is the best for boat building, and majority of professional boat builders sad that birch is the best of all, durable, light & strong.

Also in my area I found very affordable birch plywood.

So what u think ?

[Brian.Dixon]

There are many different kinds of birch and some rot very easily.  I cut down a birch tree here in Alaska that poured water and rotten wood out of it like an open faucet once.  Light weight often means the wood doesn't have as much impact resistance, e.g. light weight wood may have lower density and not resist abuse from beaching, docks, and rocks as well.  My personal preference is to use a high-density (heavier) wood for the bottom panels, chine flats, and side panels, then anything you want inside the boat (as long as it is an exterior grade plywood designed for home construction or better).

Brian

[jallii]

In general the best plywood (or any material) for boatbuilding is the one that has the best strength / weight ratio, especially for a boat is designed to travel at higher than planing speed. For birch plywood this ratio is extremely good, so because of this birch plywood is one of the best materials available. Birch plywood is the stuff that airplanes were made of before they started making them from aluminium.  For a boat that is designed for less that hull speed the extra weight does not matter so much as the extra power requirement for additional weight is so small. But in general the faster you want to go the more weight matters.

The basic problem with plywood and wood in general is that many times the plywood manufacturer does not know, or does not tell the exact values to be used for calculations for sufficient strength. Basically you cannot calculate how thick plywood to use if you do not know the strength of the material you use. The strength of wood depends very much of the direction of fibers, so the composition of plywood veneers is important. Also because the strength of wood depends highly on the moisture content of the wood, the manufacturer would have to have good control of the whole process of manufacture and storage in a consistent manner in order to be able to be sure about the quality and specifications of his product. That is not possible without enough knowledge and quality control of the whole manufacturing process. For plywood suitable for boat building you also need to use glue that holds its strength in constant moisture and does not contain voids. There are several standards that contain definitions for these matters and also for the quality control.

Because there are so many things that affect the strength of plywood smaller manufacturer dare not publish their specs for the plywood they manufacture so they cannot be sued for making unsubstantiated claims. I am posting here the specs for Finnish birch plywood. I know that there are a few big
Finnish manufacturers that export to USA also. Here is more info http://www.woodproducts.fi/wisa-birch-plywood. Without specs based on proper study the strength of wood is pretty much guesswork. Thats OK also if you use big enough safety margins. Without better knowledge the general rule for wood strength is that the heavier the wood the stronger it is. But this a big over simplification. 

Also consider the following:
Birch Plywood has about the same strength/ weight ratio as Aluminium. That is about 5 times better that for Steel and about twice as good as for Glass Reinforced Polyester that most boats are made of.   

[jallii]

Sorry, the above should say about twice that of aluminium

Let me put that a bit better...

Here is a table of relative strength...

[Ed Snyder]

I agree with all the above.
Ply here in  a is of Indonesia and they sell it as a marine grade but I have seen a little of it behave like interior where it has been left outside even though it has a marine 'stamp' on it.
I've put that down to where the manufacturer hasn't used timber that has correct moisture content either as a veneer or perhaps where the area of the factory has been in tropical cyclone conditions.
I have ply off cuts thats been through 2 winters outside, got that weathered look and otherwise still is in good condition