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When Should I Use Marine Plywood?
- Jan 11, 2019 -

Some feel ONLY Marine plywood should be used for hull planking. But I think this is too rigid a rule. The important quality separating Marine grade from Exterior panels is the quality and soundness of the inner plies and their construction (both are made with waterproof glues as noted above). With the Marine panel, there should be no major voids or surface defects, and inner veneer joints (if they occur) should be tightly fitted.
That said, using Marine plywood is most critical in my opinion where panels must conform to rather extreme curvatures (the bow in most boats, especially the bottom), and for boats built with “stitch-and-glue” methods. Another case where Marine plywood would be highly advised is on boats intended for high speeds and/or severe service.
Here’s some reasoning. You might be able to save some bucks by using Exterior plywood for planking curved areas. And experience tells me that once such a panel has been bent into place, if it doesn’t fracture in the process, it will probably stay that way forever without incidence.
But the problem with lower-quality panels is that you may not always see the voids. And if your panel does break while you are installing it due to hidden voids, you will have to replace it with another, thus making the cost about comparable to what you would have spent on the better Marine panel in the first place. And there’s no guarantee that the next panel won’t fail as well.
As for “stitch-and-glue” construction, a void-free panel for planking is important so that the wire ties at joints don’t pull out under tension, which is likely to occur if stitching holes need to be located coincidentally with a void that suddenly crops up once the panel has been cut to shape. Such panels are also easier to work with for the same reasons when used for internal members and bulkheads on “stitch-and-glue” boats.

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