: . . . However I only attempted scarfing ply on the first. A couple
: of the scarfs failed during building, so I backed them up with a ply strip
: epoxied across the inside of the join, which is what I did with subsequent
: boats. Not pretty, but strong.

Since yo have already tried the idea, why not build on the experience -- and this time use a better adhesive. the big reason scarf joints fail is that the combination of glue and surface area is not strong enough for the stresses. If you trust your glue, then make a longer taper to your scarf joint to increase the surface area of the bond. If you don't trust your glue, then get a better one. There are plenty of epoxy brands out there, as well as white and yellow carpenters glues, jsut to mention a few. With epoxys you have to be sure to give them plenty of time to cure before handling them. Some brands take a long time to come up to their maximum strength.

: Is scarfing such thin ply practical, and does anyone have any ideas for
: prettier scarfless joints?

It is certainly practical.

If you are looking for similar joints, consider using an electric plane to remove exactly 1/16th of an inch of wood on a two inch wide swath on both pieces. This lapped joint should be plenty strong, and you won't have to worry about getting down to a whisker-thick edge on your tapers.

Another type of joint is a butt joint which is reinforced with fiberglass instead of a block of wood. A 4 to 6 inch wide strip of glass cloth is applied ot both sides. Once the boat is assembled you can sand throgh most of it before you put glass over the outside of the boat. That tapers it in, and removes any need to fill in the weave.

there are two variations on the reinforced butt joint which go along the lines of the joints used in drywall. the panels are set next to each other and secured in place so they dont move. Then, with a router or a belt sander the ends of each panel are thinned but removing the top veneer layer of the plywood for a few inches on the ends of each panel. Usually fiberglass is laid into this trough -- and it can later be sanded smooth. This gives a lousy "look" as you see the coloration of the plywood core. You can modify this idea by using a piece of veneerto bridge the gap, fitting it into the area where you've removed they top veneer layer of the plywood. If you cut the trough with a belt sander or a grinder it will have rounded edges. Gluing your veneer in will cause the edges of the veneer to rise above the top of your plywood. Once the glue has hardened you can sand this raised area off, leaving a perfect patch of veneer bridging the panels. Carefully flip the panels and do the other side.

As for cutting 1/8th inch ply wood with a jig saw: WHY BOTHER?

3mm and 1/8th inch ply are thin enough that you can cut them with a utility knife just by making multiple passes with a sharp blade. By the time you score the top veneer to eliminate splintering (which is a very good idea) you have already cut through that 1mm thick layer. Two more passes cuts the 1.5 mm core, and a final pass cuts the veneer on the opposite side. What could be simpler or faster? Using a knife you are done just about the time someone using a jigsaw is half done. In tight curves you have more control, there is no wasted kerf, no sawdust, no power required, yada, yada, yada... At least try using a knife on a scrap. Just remember to keep your fingers behind the direction you are cutting so if you slip your blade will cut away from you, and not into you.

: My first visit to this excellent site, can't understand why it took me so
: long.

Welcome. It's good to hear from you. Just curious: How did you find us?

PGJ