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4mm Plywood vs. Cedar Strip Planking - Durability

Posted on August 15 2012

Plywood vs Cedar Strip PlankingWhile Bear Mountain is known for developing a user friendly method of building small craft using a strip-planked wood core between layers of glass and epoxy, I have also written a book on building with plywood. (Kayaks You Can Build) To my knowledge, there is no test data that compares these two core materials - we need someone like yourself with structural engineering knowledge to do some testing and then share it with the rest of us. We did have impact and deflection testing done by the WEST System lab on samples of 3/4" cedar core with 1 to 6 layers of 6 oz. cloth. The test results were published in EpoxyWorks and are probably available online. If you are using their products, they do have a tech support department that is there to answer your questions.

A few subjective opinions

Stiffness is related to the space between the layers of glass and epoxy - think 'I' beam - the deeper the beam, the stiffer it will be. This would suggest that a 1/4" core will be stiffer than 4mm.

Failure happens when the core caves in and the panel folds or shatters. This would suggest that the denser plywood would take a greater impact than cedar before caving in - the multi direction of the grain should also contribute to holding the core together after glass failure. When failure happens, it is usual the inner layer of glass (tension side) that lets go first, suggesting that more reinforcement on the tension side is desirable.

Some of the strength of a monocoque structure comes form it's ability to spread the shock of an impact over a large area - used to advantage on a strip-planked, soft chine hull. Failure on a hard chine hull often happens where two panels come together. The shock spreads to the joint between the plywood panels and concentrates.

Design

The 'Prospector' was a series of wood/canvas canoes built by the Chestnut Canoe Co. The Prospector in Canoecraft is the 16' model and the Nomad is a 17' Prospector-type modified to reduce the displacement of the original 17' model - it was a very big 17 footer. The Prospector series has had more than 100 years to prove itself. It was build as a working canoe where load carrying capacity and safety were imperative. As a recreational canoe, it is seaworthy and has good balance between tracking and turning. (Bill Mason made a great movie about paddling Lake Superior in a Prospector - some fantastic big water shots.) It does need some weight to reach it's full potential. Visually, a hard chine canoe does not suit my eye. I did grow up where canoes have always been soft chine - from bark and dugouts to Kevlar and carbon fiber. Multi chine kayaks do look right to my eye because skin kayaks were hard chine.

Choosing a Design

After defining how you will use the boat and the weight it's intended to carry, the design statistics will give you design displacement as well as the optimum displacement. Also look at weight to immerse and stability factor. After that, look for a profile that suites you eye. Boats are always a compromise - the reason we build our own is to get the balance of compromises we can live with. Hope this helps to narrow down you choices.

Regards, 
Ted

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