Grain direction in butt straps

[MiddleAgesMan]

I'm building a Goat Island Skiff (designed by Mike Storer) and have a general question that doesn't seem important enough to pester him about.

I cut out the butt straps for the side panels without referring to the plywood layout drawings. My plywood is several inches longer than standard so I had a piece left at the end of the side panels that was big enough for both straps. The layout drawings show them coming from another sheet.

After cutting, I checked the drawings and see MIK places them where they would all have long grain. In other words the grain in his straps would run up and down along the side panels but my left over piece had the grain running fore and aft, matching the grain of the sides.

I checked Payson's instructions (in Building the New Instant Boats) and he says to run the grain fore-and-aft on sides and athwartships on bottom panels. He doesn't say why this is important but my side panel butt straps would be correct per his instructions.

I hate to keep pestering MIK with every little thing and this doesn't seem to be a major issue anyway.

Do any of you know why it would be best to have the grain direction running in a certain way? Visually I think it will look best if it matches the grain of the pieces you are splicing.

If the splice needs to take a bend it seems you would choose the grain direction that bends the easiest but MIK's butt straps are only 75mm wide so any possible flat on the outside at the splice will be minimal and should be easy to fair. (Payson's butt blocks are much wider and I think he specifies the grain direction for best bending or resistance to bending depending on where it is going.)

[Andrew Craig-Bennett]

I have noticed my local professional high end wooden boat yard (Robertsons, Woodbridge, Suffolk, UK) making their butt straps out of two thicknesses of wood with the grain at 90 degrees, epoxy glued before fitting.

So, I reckon you are OK fitting a plywood butt strap either way round!

[Thorne]

I'm confused. This is marine ply we are talking about, yes? As far as I know, you've only got one sort of "grain" in ply, and that's the surface layer's orientation for visual beauty or matching the other bits of ply's surface grain orientation.

You can cut ply on the bias, that is, 45 degrees across the sheet -- or you can cut it with the layers, either across or up and down the sheet. Cutting ply on the bias has been recommended on this forum for some uses, like centerboards and rudders, to increase the strength.

I'm sure that others will know more on this issue.

[pipefitter]

I have been told that using ply straps that have equal thick veneers and an odd number of plies, with the grain running with the direction of the seam creates somewhat less of a flat spot on curved panels. Strength wise,I don't think it matters with plywood.

[MiddleAgesMan]

Yes, it is 6mm okoume 5-ply marine plywood. The inner layers are not quite the same thickness. The center layer is slightly thicker than the other two. The exposed layers are slightly thinner than the rest. I believe the varying thickness of the layers was chosen to provide equal amounts of wood running in each direction. In theory I guess that means it should bend in either direction with about the same effort.

Thanks to all. I'm fairly comfortable using what I cut, with the grain running fore and aft to match the sides.

[MiddleAgesMan]

I have been told that using ply straps that have equal thick veneers and an odd number of plies, with the grain running with the direction of the seam creates somewhat less of a flat spot on curved panels. Strength wise,I don't think it matters with plywood.

That would make sense, especially if the layers are equal thicknesses. Three of the five layers run along with the splice while only two layers have grain that spans the splice.

The plywood manufacturer used three different thicknesses in mine so the bending characteristics are similar in both directions.

I never thought about cutting plywood parts on the bias but if it adds strength to fiberglass it would seem to do the same for plywood. Nice trick.