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View Full Version : Splining Joints VS Scarf or Carnell/Payson Butt Joint



Hal Forsen
07-06-2010, 05:50 PM
Tried a little experiment the other day.
I used a "biscuit" joiner to cut a 1/2" deep mortise along the end of two 4" wide X 2' long pieces of crappy 3/8", 3 ply, plywood and then used System 3 T88 to glue 'em together end to end with a spline of plywood. After overnight drying, the high tech "put the joint on your kneee and yank hard" test showed, as I had predicted, the joint held and the outer veneer of the ply failed on the tension side.
I wouldn't try it on anything thinner but if it worked that well in junky construction 3 ply it would have to be even better in quality 5 ply stock.
Anyone see any problems using this technique to hang lapstrake planks?
Seems like a lot less muss and fuss.

Thorne
07-06-2010, 06:17 PM
Anyone see any problems using this technique to hang lapstrake planks?

Too many to mention, really. Why do you think that a scarf/joint 1/2" wide would compare to either a standard 12:1 scarf or a butt joint with a 6" block? And how do you cut a 4/8" mortise in 3/8" ply? I'd think you'd need both a plank stretcher and a gronicle mount to make that happen..

TerryLL
07-06-2010, 06:37 PM
The plank might hold together with that short splined joint, but you'd never get that joint to take a fair bend. The beauty of the 12:1 scarf is that the joint has the same flex as the rest of the plank. The spline introduces a hard spot in the plank, which will show up like a boil on your nose when that plank is bent on. Fine if you're happy with a lumpy boat.

Hal Forsen
07-06-2010, 06:47 PM
There is as probably as much glue area in this spline joint as a 12/1 scarf if not more + the mechanical advantage of the spline. After the planks are hung, they can't go anywhere anyway.. If the joints hold where's the difference?


And how do you cut a 4/8" mortise in 3/8" ply? :confused:



The spline introduces a hard spot in the plank, which will show up like a boil on your nose when that plank is bent on.

Do you know this to be true from experience or are you speculating?
If the spline is the same ply as the strake how can it possibly form any harder a spot then any other glue joint?

htom
07-06-2010, 07:07 PM
Take two four foot long planks, join with your method.
Take two more four foot planks, join with Payson overlap.
Take two more five foot planks , join with a 12-1 scarf. Trim this one so it's also eight foot long, with the center of the scarf in the center.

Let them dry. Weight the ends, letting 7'6" hang free. Observe the differences in where the ends are, and the shape of the curves.

This is not about the strength of the joint; it is about the fairness of the joining.

TerryLL
07-06-2010, 07:07 PM
Do you know this to be true from experience or are you speculating?
If the spline is the same ply as the strake how can it possibly form any harder a spot then any other glue joint?

Don't take my word for it. Glue up some planks both ways and learn for yourself.

Hal Forsen
07-06-2010, 07:21 PM
I'll defer to better minds.

Tom Lathrop
07-06-2010, 10:19 PM
There is as probably as much glue area in this spline joint as a 12/1 scarf if not more + the mechanical advantage of the spline. After the planks are hung, they can't go anywhere anyway.. If the joints hold where's the difference?

:confused:


I've known builders to use a spline joint on plywood and I have done it myself but never on a joint requiring strength or highly curved surface.

You are missing the important element of strength in a joint. It is not the glue area in a lapped joint but the strength of the wood joined by the glue that determines joint strength. Of course, this refers to a joint of sufficient contact area. A splined butt joint is weakest in this regard.

12:1 is overkill in almost any scarf other than a mast or other spar where it must support severe bending loads. 8:1 has proven to be adequate in all plywood scarfs over many years.