SCARF JOINT ORIENTATION AND TESTING FOR LOWER FRAME MEMBERS

[bob mitchell]

SCARF JOINT TESTING FOR LOWER FRAME MEMBERS

In scarfing a new piece of oak onto the lower frames that connect to the keelson, it may be easier to cut a scarf in a "vertical plane" (plane intersecting perpendicular to the hull plywood) instead of the "horizotal plane" (sloping down towards the plywood). I am sorry about the labeling - not quite perfect, but humor me. Clearly it would be easier to draw the scarf line, set the depth, and make the cut in the vertical plane than to try a cut slanting down towards the plywood.

But would the vertical scarf be as strong, or even close? I decided to run some tests on very short scarfs (60 degrees) on 1 1/8" x 1 1/8" x 30" spruce using West System epoxy. Yes, I know scarfs should be 12 to 1, but that's not what I was testing. The first test set failed miserably because I did not appreciate just how much epoxy the end grain would suck up. Although I did brush each surface, and then wait a minute, and brush it again, only one joint out of the five was anything but a joke. Good Experience, but it is hard to test the strength of a scarf with the scarf plane oriented vertically versus horizontally when all the scarfs are worthless. The next set was from spruce 2x4's ripped to 1 1/2" x 1 1/2" x 30" . This time I gave the epoxy plenty of time to soak in, and KEPT BRUSHING UNTIL ALL DRY SPOTs AND LINES WERE GONE, AND MADE SURE PLENTY OF EPOXY CAME OUT OF THE JOINT WHEN LIGHTLY CLAMPED. All except one of the joints turned out to be good - one even passed the 200 pound hop test - i.e. in keeping with my SOP (seat of the pants) testing I tested all scarfs systematically up 225 pounds (not all made it that far), but at that point I ran out of methods to increase the weight - so I applied a "hop", and a "hard hop". There were some finish nails in the joint to hold them while they were curing, but with that said the vertical plane scarfs out performed the horizontal plane scarfs. The advantage of a vertical plane scarf is when you are cutting it in place you just draw the angled line on top of the frame, set the saw depth, and make the cut with plenty of clearance. Not so with the "horizontal" plane scarf cut that runs into the hull sheathing. I am not sure what an experienced scarf maker would say to all this.

The next set of scarfs I did were 6" scarfs on 1 1/2" x 1 1/2" x 30" inch yellow pine. These felt very strong, and all passed the 225 pound test, and most had to have the hard hop test to fail. So now I have some first hand experience - which I certainly reccommend to everyone. One for sure conclusion was you better have your clamping plan in place because just slapping on a C-clamp will NOT get it. I was sloppy because I had so many to do. But if you are going to make a serious scarf you need alignment boards on each side with wax paper in between so they won't get glued. Clamp them away from the scarf first to prevent the scarf from sliding when you clamp on the actual joint.
A week or two has gone by, and I realize that I have not done any testing with white oak, which is the wood I will be scarfing. Also the finish nail influence was unclear. And finally, I really did to much "seat of the pants" to be satisfied with my results. So for my final scarf test set I will use high quality 3/4" square well seasoned, 30", white oak blanks. Mostly I wanted to make sure that seasoned white oak behaved in a reasonably predictable way. In fact I used the left overs from my first floor that goes under the mast step. I free handed 6" scarfs on my bandsaw, knocked the high points off with a couple of strokes with a plane, leaving surfaces still pretty ridgey, and then mixed the half blanks up so that no pair would be a perfect fit. I slathered west system epoxy on a couple of times, then I mixed some filler in the epoxy to ketchup consistency, and slathered them one more time. My clamping was significantly easier because I put two stops 30" apart to prevent the two sides from sliding away from each other. The stops eliminate the need for fasteners. There is a piece of plastic underneath each to keep it from sticking. Doesn't take much clamping pressure. The grain, looking on the end of a blank, was 45 degrees more or less (not vertical,not horizontal) which minimizes influence that grain orientation can have. Now they are drying, will test in the morning. Remember, the point here is to make sure that a scarf plane that is oriented vertically is a valid option, because they would be much easier to cut. Just draw your scarf line, set your depth, run your cut.

OK!!!!!! All white oak 6" scarfs passed vertical tests while failing equivalent weight "horizontal" tests. I used high enough seat of the pants (SOP) quality control to make the statement that I think vertical scarfs (relative to hull sheathing) are stronger than scarfs with their plane sloping down towards hull sheathing (harder to cut). You will have to present data to convince me otherwise! I included a glue up photo, an in stress photo to give a sense of the tests (130 pounds), and then the final data. I started the tests at 40 pounds and moved up in increments of 20-25 pounds. It is noteworthy that the first blank to fail was clearly the worst looking joint. P130 H + V P150 V F150H reads "pass 130 horizontal and vertical.......... pass 150 vertical........ fail 150 horizontal" (all units are pounds)