Cross-grain Scarfing?

Ben M

Ben M

Active member
In order to make best use of the plywood I have, I may need to nest my cuts in such a way that a cross-grain scarf is necessary for the hull. (The grain would be vertical on a couple sections of plywood rather than horizontal.) Should I avoid that altogether, bite the bullet and buy another sheet of plywood? What do you think?

Having end-grain on the chines will make rounding them off before glassing difficult I know, and if I were going to leave the grain exposed, I would never even consider this option.
 
In order to make best use of the plywood I have, I may need to nest my cuts in such a way that a cross-grain scarf is necessary for the hull. (The grain would be vertical on a couple sections of plywood rather than horizontal.) Should I avoid that altogether, bite the bullet and buy another sheet of plywood? What do you think?

Having end-grain on the chines will make rounding them off before glassing difficult I know, and if I were going to leave the grain exposed, I would never even consider this option.
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Cross grain on what? The individual plies, as in the scarfed on piece the top ply is 90 degrees from the first, but the plies are in the same plane.

I wouldn't be super concerned with that for a strength issue on most boats, but I'd be concerned on any boat if the panel made needs to be bent, especially a complex bend. Bending the panel with such a hard spot and the unpredictable response you may get from the panels across the scarf may give you something weird. Especially if the scazrfed on peice is different on the different on the two sides of the hull - you may end up with a different bend on each side. I don't know your boat well, but I have seen pictures and if it is hull panels, I'd be concerned given that there is some bend to the panels.
 
Cross-grain as in the top ply is 90 degrees from the first. The scarfs would be in the same place on either side of the hull.

I've never scarfed plywood before. You say the scarf joint is stiffer than the surrounding wood? My scarf would end up about 5ft from back from the bow, right before some significant bending starts. I'm positive I'm going to end up strip-planking once the plywood reaches the end of its bend-ability.

The plans call for butt-block joints. No scarfing. But, I'm trying to save a little weight where I can and avoid adding more wood/adhesive to the boat. Plus, the designer did tell me that scarfs would result in a fairer hull. But if you're telling me your experience is that scarfing where there is some bending is going to give me weirdness . . . maybe I ought to just stick with the butt-block joints . . . Butt blocks would be faster. I can just cut, lay them in and roll on rather than spending all that time creating the scarf joints and then waiting for adhesive to dry.

I guess the added weight of butt block joints is pretty negligible, but every ounce counts! Trying to keep her under 100lbs . . . Spira says that's the way he designed her!
 
Cross-grain as in the top ply is 90 degrees from the first. The scarfs would be in the same place on either side of the hull.

I've never scarfed plywood before. You say the scarf joint is stiffer than the surrounding wood? My scarf would end up about 5ft from back from the bow, right before some significant bending starts. I'm positive I'm going to end up strip-planking once the plywood reaches the end of its bend-ability.

The plans call for butt-block joints. No scarfing. But, I'm trying to save a little weight where I can and avoid adding more wood/adhesive to the boat. Plus, the designer did tell me that scarfs would result in a fairer hull. But if you're telling me your experience is that scarfing where there is some bending is going to give me weirdness . . . maybe I ought to just stick with the butt-block joints . . . Butt blocks would be faster. I can just cut, lay them in and roll on rather than spending all that time creating the scarf joints and then waiting for adhesive to dry.

I guess the added weight of butt block joints is pretty negligible, but every ounce counts! Trying to keep her under 100lbs . . . Spira says that's the way he designed her!
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Yes a scarf joint is a hard spot. It is often quoted that scarfs are stronger than the origional wood and from those that I've torture tested that is the case - so that strength has to come from somewhere - the joint must be stiffer than the surrounding wood if the wood breaks, no thte scarf. Long thin strips fo plywood with scarfs when bent in a curve clearly show a flat spot at the scarf. I don't know what would happen if the grain changed direction within the scarf. I have scarfed cross grain as you describe, but not on anything that wasn't flat or subjected to a lot of stress.

You boat is built over frames, so you aren't going to change the overall hull shape like you would in a stitch and glue boat, but depending on what you are trying to get the panel to do, you may get a weird bend in the panel (hook/cup/bump/etc...). Given that you are talking plywood strip building part for a fairer hull, that indicates that the panel in areas will have complex bends. Butt blocks will give you a hard flat spot as well, so I don't know what is best. I wouldn't worry about weight of the butt blocks, that is for sure.
 
I have been building plywood and epoxy cold molded sportfishing boats for close to thirty two years. Have scarfed plywood both on the four foot end and the eight foot long side. never had a problem either way. the grain direction changes with every layer in most plywood, so you are always going to scarf cross grain. Hope this is what you mean by cross grain? Rich.
 
I have been building plywood and epoxy cold molded sportfishing boats for close to thirty two years. Have scarfed plywood both on the four foot end and the eight foot long side. never had a problem either way. the grain direction changes with every layer in most plywood, so you are always going to scarf cross grain. Hope this is what you mean by cross grain? Rich.
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Most ply boats that I've seen have all the grain running longitudinally--bow to stern. By "cross-grain" I mean having a horizontal-grain piece scarfed to a vertical grain piece. Or, scarfing a 4ft edge to an 8ft edge.

I see your point: all the plies in a sheet are cross-grained (and that's where the strength comes from). So in a sense you're always scarfing cross-grained . . .
 
Ben, If you decide to use butt blocks you better give them some shape. They will be a lot stiffer than an epoxy scarf. Rich
 
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