Completing the butt joint on the deck.

Underside of the deck, coated with 2 layers of epoxy.

In Glen L. Witt's book, Boatbuilding with Plywood, in the chapter on Plywood Planking... there is a full-page sidebar titled "Making Long Length Plywood Panels The Easy Way."

What it describes is a variation of a butt joint, used to join two panels of plywood. Instead of using a plywood butt block to overlap the joint, (held in place with thickened epoxy and screws), this variation simply uses a strip of fiberglass tape, wetted out with epoxy, to straddle the joint and hold the panels together.

The weakness of this joint is on the side without the fiberglass tape. In other words, the fiberglass tape can become nothing but a hinge if the panels are flexed in that direction. However, the book describes the joint as being adequately strong if flexed the other way.

(You can also add a second piece of fiberglass tape on the other side, to make the joint stronger. This is often referred to as a "Payson Joint," named after Harold "Dynamite" Payson.)

The simplified version described in Boatbuilding with Plywood is what I had in mind for the centerline seam on my deck. 

At first, I had planned to simply cut a strip of scrap fiberglass cloth to do the job. However, I didn't want to deal with all the frayed ends, which can create a real mess. So, I got a roll of 4 inch fiberglass tape. The edges are woven on fiberglass tape, making for a neater edge. The downside is that the woven edge is thicker than the rest of the cloth, and will need to be sanded away once the epoxy cures. Of course, the frayed ends would have to be sanded away, too.

Sanding. You can't escape it. This is boatbuilding, after all.

8 oz fiberglass tape (top) and 6 oz fiberglass cloth (bottom). Note the neater edges on the tape. The frayed edges on the cloth make a real mess.

Tape rolled out on the centerline seam.

Waiting for the epoxy to be rolled on.

I wetted the tape out with System Three Silvertip epoxy, and rolled it on with a thin foam NAP roller. I coated the whole deck, and meticulously squeegied it.

While waiting for the epoxy to cure, I attached the forward bulkhead panel.

After the epoxy cured

The fiberglass tape holds the seam together nicely as the panels are bent to conform to the crown.

Soon, I will be sanding down the edges to help blend the fiberglass into the finish. I may wait to do that until after the deck is attached.