I was looking over a website selling Marine plywood. After reading the introduction, I wondered why they felt it necessary to exaggerate the qualities of what is, in fact, a superior product. The following (in quotes) are some of the statements made on their website; the True/Flase replies are mine.
Small boats:
"While small stitch and glue boats can be successfully built with inexpensive exterior plywood, there are many advantages to the use of quality marine plywood."
- "Marine plywood provides greater protection: a small scratch through the resin or glass, if not repaired, may expose the plywood to water. Inexpensive plywood may not last long in that case."
- False: The differences between Marine plywood and exterior are structural. Exposed to water, woods like Douglas-fir will outlast okoume. The ability of wood to stand up to weather is affected by the glue used and the wood species. The glue is essentially the same, whether Marine or exterior plywood. Interior plywood should never be used in a boat.
- Note also that in today's market, while Marine plywood continues to be sold in "net" thicknesses, exterior plywood is generally sold as "scant" i.e. approximately 1/32" thinner than its advertised thickness. Thus you would not want to mix the use of Marine and exterior plywood on the same working plane as there would be raised ridges where one type of plywood met the other.
- "Marine plywood strongly increases the resale value of your boat."
- True... or at least should be. Marine plywood is structurally superior.
- "Marine plywood is stronger and has consistent mechanical properties: no voids."
- True, more or less. In structure, Marine is superior; no interior voids. As a lumber, Douglas-fir is stronger than say, okoume. If a panel of equal thickness of Douglas-fir exterior and okoume marine was stressed, the Douglas-fir might fail first, if there was a void at the point of stress. If there was no void, okoume may well be the first to fail.
- "Marine plywood is much nicer and easier to work with."
- True. If comparing fine grained woods like okoume or meranti to exterior Douglas-fir, this is true. If comparing Douglas-fir marine to exterior, there would probably be no noticeable difference, other than having to be aware of voids in the interior veneers.
- "For small boats, quality 4 mm plywood is a good substitute for the 1/4" exterior ply often specified."
- Maybe. Would depend on the design. I'm not sure what "small" means. The Flying Saucer (12') and Zip (14') are built with 1/4"... 4mm would not be a good substitute.
- "Marine plywood (except for Fir) will not check. That feature may by itself pay for the difference in cost. To get a good finish with Fir or Pine, the hull will require extra fiberglassing and large amounts of fairing compound. This is not necessary with Meranti or Okoume."
- Yes and No. I would question part of this and disagree with part. I have not done extensive "check" testing on Meranti or okoume, but I understand it will check, though not nearly as badly as Douglas-fir or pine. It is harder to get a good finish with D-F or pine, but additional fiberglass would not serve any purpose. It may be necessary to use fairing compound, especially if Douglas-fir is over sanded.
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The thing is, Marine plywood is always a better choice for hull construction, but it is important to understand why this is the case. It really is counter productive to make up reasons to use it. When people realize claims are false, they tend to doubt the legitimate reasons for using a product as well.
