scarf joints

[joe oski]

How does one do a long scarf joint , as in joining plywood sheets.
Ive seen pics, seems simple concept, but dont see myslf flpping sheet over and having 4 feet line up

[Jim Belair]

Wouldn't a half lap joint work just as well and be simpler to make?

[jamie shard]

Boat builders do this a lot to maintain strength/smooth bending across the seam. Mostly I've seen belt sanders and planes, including power planes, and radial arm saws used to make the scarf. You might want to see if googling "scarf joint boat building" gives you some leads.

[Steve Baumgartner]

Boat builders do this a lot to maintain strength/smooth bending across the seam. Mostly I've seen belt sanders and planes, including power planes, and radial arm saws used to make the scarf. You might want to see if googling "scarf joint boat building" gives you some leads.
+1 Get out any book on modern boat building (e.g. John Brooks', Ian Oughtred's, or Greg Rossel's) and you will find detailed info on how to scarf plywood into longer sheets.

[Jim Ritter]

It really isn't that difficult. How thick is the plywood you want to scarf? Are you doing 12 to 1 ratio? I wouldn't do less. If you are using 1/4" ply measure 3" and draw a line parallel to the edge. Knock the bulk off with a power planer if you want, but in the end I use a sharp hand plane. The seams of all the different plys give you visual guide as to how you are doing. Try it on a scrap first and you will see what I mean. The last panel I did was just over 3" long and 1/2" ply for a boat bottom. I'll see if I have any photos.
Jim

[Brian Thornock]

Nothing beats a hand plane to clean it up before gluing, in my opinion. I've never done one as long as a boat panel, but I've done several on my guitars.

[Jacob Reverb]

I usually lay out my scarfs as Jim described, but then I stack the two pieces like stair treads, and clamp such that the end of the last piece is just over the edge of the bench or table saw, before planing. That way, as you plane away the waste, you end up with twice as long of a scarfed surface for the sole of your plane to ride on. Much harder to describe than to actually do, but it makes things easier in my experience.

[russell lusthaus]

Jacob is describing the easiest way to scarph by hand. The ply lams actually help keep your scarph straight.

[Jim Ritter]

You are right Jacob, I forgot to mention to stack them. Good catch. Also if the glue lines between the laminations curve toward the edge that area is too high if they curve into the panel it's lower that the rest. You want those glue lines straight across.
Jim

[joe oski]

I guess watching the glue lines sounds promising. Will have to get brave and try it. If I mess up to bad, will have to add a backer board.

[Steffen Dahlberg]

Just a quick addon from the sidelines, the world of solid-wood boats (non-plywood, not actually solid). Like this one: http://no.wikipedia.org/wiki/Nordlandsb%C3%A5t
We use the scarf joint too, but it is not glued, it is riveted with fibre and pine-tar in the joint for sealant and preservative. We value it because it enables us to "hinge" parts on the joint, and make the necessary adjustments before riveting the part in place. This feature is especially useful when mounting stems. The boards are made quite a bit oversize along its top edge, so that it may be given its shape after it is fixed in place, therefore the joint does not need to be overly accurate as to match the boards shape along their lengths. Also this joint allows for water to run over it without hindrance (the forward board is laid on last), and it allows some pressure to be put on it during mounting, where a half-lap would just split.

I go about a scarf by marking it's length, placing the board on its edge between my knees with the scarf on a stump, and chopping it with an axe. The last mm or two is planed away with a smoothing plane, checking with the sole of the plane that it is straight and true. When it is, I hollow it in the center with one pass of the plane to make room for more tar and fibre. Scarfing stems to keels require more elaborate marking based on centrelines, as well as a couple of attempts (going from oversize towards goal) to line them up. They also need more hollowing in more specific areas, and more sealant since they are more rigid.