Zip: Scarf Joints, Half-Lap Joints, and Floor Battens

When I went back to the lumber yard to buy more mahogany, they only had 8-foot boards available. So, I bought a couple.

When I got them home, my son helped me rip the wider boards into 2-1/4" widths to use as floor battens on the Zip.

The next challenge was to make 8-foot boards ten to twelve feet long.

The preferred method for this, without question, is to use a scarf joint to join two pieces together. The longer the scarf, the stronger the joint. A 1:12 ratio is optimal.

However....

To accurately make these cuts for a scarf joint on my table saw, I would need a lot of room to maneuver and support these 8-foot boards. In order to make that room, I would need to move quite a bit of stuff around, build a sled for the table saw, and enlist my kids' help in supporting the boards as I made the cut.

To me, it seemed so much easier to simply clamp the boards to my work bench, and cut away sections for a half-lap joint with my router.

The Case for Scarf Joints

Scarf joints are simply stronger than half-lap joints. In many places, I've seen them described as being almost as strong as solid stock. They have a particular advantage of withstanding bends better than half-lap joints, which will fracture much more easily.

The Argument for Half-Lap Joints

My thinking is: the joints in these floor battens will be located in the aft section of the hull, where there are NO bends. Not only that, but just because a scarf joint is stronger, does not mean the half-lap joint is weak. The question is, will it be strong enough to do the job?

In this particular case, I think it will be. As I said, there are no bends involved. Besides, part of my plan includes adding a strip of 1/4" plywood backing over the joint, just as with my laminated keel. The joint, therefore, will be sandwiched between 2 layers of plywood: the backing (or butt block), and the hull itself.

The Jury is Still Out

I decided to go ahead and try the half-lap joint. 

It was easy enough to do with my router and a 3/4" rabbeting bit. The boards themselves were rough-cut, 5/4 mahogany, so it took more than a 1/2" bit to do the job. 

I placed an 8-foot board and a 3-foot board lengthwise on my work bench, aligned & butted them up against each other, and clamped them firmly into place. After deciding on a 4" overlap, I clamped a stop guide for the router across the top of the boards. Then, it was simply a matter of cutting away with the router.

4" half-lap joint, after sanding away all the epoxy mess.

I glued the pieces together with thickened epoxy, and clamped them firmly together. Once the epoxy cured, I was honestly surprised at the strength of the joint. However, I still wanted it to be stronger than it felt, so I continued on with the plan to add plywood backing.

For the butt blocks, I used 1/4" Douglas Fir marine plywood. I've found that the Douglas Fir plywood is considerably more rigid than the Meranti, and in my opinion it's better suited to this type of application. The butt blocks are 23-1/4" long, centered over the joint.

Once these butt blocks were epoxied into place, I was very happy with the strength of the joint.

After re-reading the section on floor battens in Glen L. Witt's Boatbuilding With Plywood, I also came away with an idea for how to reinforce these joints even more. It depends on how I want to build the floor. I'm still considering my options. For now, it's time to move on to notching these battens into the frames.