: One of the things that struck me about Sam McFadden’s test results of Nick
: Schades’s Strip-Built panels (this board, 23 August) arose from the large
: difference in strength between samples stressed with the wood grain as
: compared with those stressed across the grain. While confirming what I
: intuitively ‘knew’ as a woodworker of sorts, the results highlighted the
: contribution of the wood itself to the strength of the final composite.

: The question that popped up, then, was: How much does the grain orientation
: within the individual strips have to do with that? For my Strip-Built
: projects I’ve been cutting my strips so that the grain is vertical, as
: most builders do. In their fine books, both Nick Schade ( The Strip-Built
: Sea Kayak at p. 36) and Ted Moores (KayakCraft at p. 49) seem to prefer
: this orientation as well, but seem to determine this preference by
: workability considerations rather than final composite panel strength. (To
: Nick Schade and Ted Moores: Please correct me if I've misunderstood or
: misrepresented your text.)

: Any ideas or thoughts, then, whether we could build stronger boats by using
: flat-sawn strips rather than vertical-grain strips?

: Beyond that, could we improve strength by additionally building the hull with
: the wood grain oriented vertically from keel to sheer rather than the
: traditional horizontal, bow to stern orientation? If so, this may have
: some application to my experimental hybrid ‘Strip & Stitch’ boat
: projects, as one could more easily make full boat-length panels cut out of
: rectangular sheets made of strips oriented across the short dimension of,
: say, a 4’ x 18’ panel.

: It’d probably look a little goofy, maybe, but I’d give it a try on my next
: hybrid if it makes structural sense to some of you marine engineers out
: there.

: Pete in Snohomish, Washington

Pete,

It is easy to look at the grain orientation effects we saw in the bend test samples and assume that there is something to be corrected in our approach to building kayaks. This may not be the case. The first approximation to describe loading on a hull due to buoyancy forces considers the whole boat as a simple beam of hollow cross section. When we do this, the largest stresses are oriented parallel to the axis of the boat. Therefore, it makes sense to create a structure with most of its strength oriented along the same axis.

When we wish to consider other types of loading, like impact with a rock, the stress distribution is different. In the case of impact, it makes sense to have a more uniform distribution of strength. With this in mind, I think that it would be best to maintain the longitudinal orientation of strips, and investigate the effect of tailoring the glass fiber orientation to increase impact resistance, if that is what concerns you.

I am not sure about the advantage of flat versus vertical grain strips. When growth rings are present, like in cedar, it stands to reason that there would be a difference in the compressive strength between the two orientations. Higher across the grain compressive strength would increase the panel strength in the across grain orientation. I think the Gougeon brothers may have some data on this in their book, but I would have to check. I took a quick look at the Wood Handbook (see link) but did not find data for the two orientations.

Sam