scarf or butt-blocks

[dan-marques]

I'm still shopping around for designs, and (thanks to a response to my other post) am looking at some of the designs at Boat Plans OnLine (http://www.boatplans-online.com/).

Their designs (at least for the small boats) seem to use butt-blocks to form the full-sized plywood panels. How does this compare to designs that scarf the panels together? How much easier is it? How much weaker is the joint?

Thanks.

Dan

[Ian McColgin]

The butt block approach will work as indicated where the bend is gentle and no twist.

It looks tackey.

Use a low angle plane on the plywood and abundantly described in other posts for the scarfed joint.

Your call.

[Nicholas Carey]

Putting the panels together with butt blocks isn't necessarily stronger or weaker than a scarf, but the butt block introduces a hard spot in the completed panel. The butt block also puts a kinda ugly hunk o'wood in the middle of your panel.

Depending on the amount of bend or twist introduce in the panel, the hard spot will spoil the fairness of the curve. It also concentrates stresses at the edges of the butt block. If you want to see this concentration of stress in action, tack a piece of, say 1/4 inch plywood, maybe 4 inches wide and 4-5 feet long. Glue the moral equivalent of a butt block, 4 inches wide and maybe 6 inches long in the middle of the panel.

Now start bending it (with your "butt block" on the inside of the bend) until it fails. The failure will probably be fairly sudden and it will fail right at the edge of the block.

Scarf the plywood.

It's much easier than it sounds. Plywood, comes, after all, with built-in guidelines (the plys themselves) for cutting the bevels. The bevel is uniform if the exposed plys are of uniform width.

A razor sharp hand plane or a power plane and you'll be good to go. Search the archives for scarfing plywood and you'll find more than you want to know.

Strike a line to indicate the top of the scarf (if you're plywood is, say, 1/4-inch and you want a 12:1 scarf, the line will be struck at 3-ins (12 * /4 = 3). Stack up your plywood with the edge of each aligned with the line on the one below it, so you have a "staircase". Tack them down with brads or screws as you go. Start planing (you probably want scrap underneath, plus the bottom panel and scrap piece should prolly overhang the edge of the bench/table a inch or so to allow the plane's sole to come down past the bottom edge of the bottom board. The scrap underneath should protect the feather edge on the bottom piece from blowout. Keep the stripes made by the individual plys at a uniform width and the bevel will be absolutely uniform.

I've also heard of obsessive-compulsive types who do a "staircase scarf". They set up their router and individually route each ply so that it forms an N:1 staircase. So if you want to do the same 12:1 scarf in 1/4", 7-ply wood, you'll route an [extremely] small rabbet along the edge of the plywood. The depth will be the thickness of the topmost 6 plys, the width of the rabbet will be 1/6 — 1/(N-1), where N is the number of plys — of the total length of the scarf (3 inchs, in this case). So the width of each rabbet will be 1/2 inch (3 * 1/6 = 1/2.

The depth of the next scarf will be the thickness of the top 5 plys. Repeat. The depth of the last rabbet will be the depth of the topmost ply. The width in all cases will be L/(N-1), where L is the scarf length and N is the number of plys.

The argument is that you avoid the feather edge and get a nice crisp seam to it. Kinda silly if you ask me :D But jigged properly, with a good setup, it might be a pretty fast way to cut a bunch of scarfs.

There you go.

[George Roberts]

For most purposes both form a hard/stiff spot in the panel.

If you use a butt block, taper the edges. It looks nicer. A 8:1 taper feels nice.

[NormMessinger]

Butt blocked joints are for people that are too danged lazy to do it right. Scarf the panels. You'll like yourself in the morning.

[edsr]

I have used both butt joins and scarfed joins.

Both work and what everyone else said.

There is one other consideration and that is with the butt joins the scabed piece protrudes and it can be a water stopper when used on the boat's bottom, your limbers may not fully remove the bilge water. The constant wet surface speeds up the rot process.

edsr

[ishmael]

One other consideration. Some designs use the pretty darned near the full length of the plywood. Scarfing shortens the finished plank by the length of the scarf. Something to consider before you change a design with butted planks to scarfs.

[warthog5]

I gotta agree about the blocks. They are UGULY and stopping water could be a consideration in the design too.
I like many that have never done one before, it's a little unnerving [sp?]
About 3yrs ago I bought a Mikita 4 and something inch power planer. Not the small one. I contacted John Henery in Spanish Fort, Al. about his attachment.

Well Spanish Fort is about a 40min drive from me. So this past Feb I jump in the truck and ride over there to see him and wanted him to mount the attachment and demo it for me.
He's a pretty cool guy. His wife helps him out some and his basement is FULL of sawdust. HeHe
This planer and the attachment don't come cheap.
Show me a good tool that does?

Anyway it works like a champ. 3 quick passes on 3/8 and it gave me a 4 1/2wide scarf. If I remember right. No fuss no muss! It get's the thumbs up award.

[Dan Cavins]

Hey Dan. I think it really depends on the boat and what you want. I've done both, and they both work. My sharpie has a tame curve on the hull and I butt blocked it. If there is a hard spot I can't see it. As to ugly? Maybe, but mine is in the cabin in the middle and, well, I don't care. Use more supple wood of course and I've even planed and sanded the outer edges of the contact side -slightly - the help the curve. Works fine.
That said, scarfing isn't that hard and is kind of cool I think. If your boat has a sharper curve scarf it for sure. If you don't like the butt look, scarf it. Your boat your call. Dan Cavins.

[NormMessinger]

There is the third alternative when one needs the full length of both sheets. That is to grind a very shallow "U" on each side of the joint and lay in two or three plies of glass, say a 2", a 3" and a 4" wide piece to bring the "U" back up to grade. A scarf is probably faster since you don't have to wait for the glass to cure first on one side, then the other.

[DickB]

To add to all this: I've done both, and find that the butt block works out fine especially if it is positioned so that it will not be seen (as under floorboards or behind seating). Stop the butt block shy of joints for water passage, butter edges of block at 45 degrees with epoxy to keep water moving away from plywood end grain. I put them down in epoxy with many ringed silicon bronze nails (did this on shop floor with polyeth. sheeting underneath to break bond to floor). Scarfed joints are best where joint is exposed to the eye. No hard spots on either types in my use -- soft curves to be sure.

[Bruce Hooke]

Just to make sure one point is clear: even if the plan call for butt blocks there should be no reason not to scarf, as long as it will not make the panel too short -- an important point noted by edsr.