Scarf Joint

Hi all,
Excited to be here and find the site very interesting.
I'm trying to build a small duck boat out of plywood, via stitch and glue.
Just tried my first scarf joint and used a power planer to do it. My question is:
The ends of the plywood sorta shattered or became tattered from running the plane over them to knock off the "stair steps" Though I think the joint looks good, is this common and does these joints usually require a lot of dressing with sandpaper after then are cut. Using 1/4" ply.
Thanks for any help,
Daniel

 

You should lay the edge of the plywood on a board to prevent breaking it. Also, use sharp blades on the planer. It is not necessary to use any sandpaper, a plane should give you a good enough surface. This is an example of a scarf I did for a repair with a plane:

 

Yep, when you do a scarf, place a piece of sacrificial wood under it, and clamp both down tight. This will help save the edge, but a power plane still will tear out wood along the "feather edge" side. When you get close, you're best switching to a tool with much less blade speed, like a hand plane. Lastly, if it's under paint, then the edge doesn't matter much, it can be filled and faired. Under primer and paint, no one will have a clue how much cussing was involved. If under a varnished finish, you should probably use a nibbed or notched scarf.

 

Thanks for the info.
I'm not concerned with the look of the joint but the structural integrity of it. The thing actually looks good and fits nicely. How thin should the feathered edge of the joint be, and how much epoxy and fiberglass do you guys use on the joint?
Thanks again,
Daniel

 

I don't use any fiberglass on the joint. Epoxy with the consistency of mayo is all you need. Thicken it with fumed silica (cabosil). The edge of the wood is almost transparent if you use a sharp plane.

 

I done a practice joint and glued it up yesterday. Gonna see how it is later this afternoon.
I wetted out both surfaces of the joint and let the un-thickened epoxy stand for about 5 minutes. Then I mixed up some epoxy with wood flour about as thick as mayo and put it on. Then I put a weight on it. So we'll see how it turns out. From what I read scarf joints are better, stronger, joints to use than butt joints. Is this true?
Thanks, Dan

 

i understand it is a question of surface contact, so it is making each layer continuous and make certain the joints are stagered, 3 layers of 3mm plys is suprisingly strong. we will see how the new nanno wood will be developed, stronger than carbon fiber ? !
I am looking at after vaccuuming the air, i could wrap (i call it the boa mold) the hull with strong bungies so i dont have to run the vac for so long. This would be best for a tube hull with the stringers and all structural parts pre-done, hence no more screws and ties, but good scarf joints pre-done. Okoume or Meranti ? probably hybrid.

 

Yellowcat, what are you trying to say about the Daniel's desires for a scarf?

 

Dan: good idea to do a few practice joints! Did you use actual wood flour or wood dust from sanding activity? The wood flour is finer and would give a smoother looking joint, but a thickener such as fumed silica would, I believe, produce a mix that would flow more easily when the joint is made.

A properly made scarf joint has the same strength as the material it is joining and you can't do better than that. A butt joint (see correction) is similar strength-wise but changes stiffness resulting in a flat spot if the resulting piece is bent. Either way, it is best to keep joints away from sharp bends and to stagger joints in adjacent planks.

I haven't had much luck using power tools for scarf joints due to edge splintering either, but a sharp hand plane does the job so easily that is the way I go. The only time I use a power tool for cutting scarfs is for stringers and other long, relatively thin members. For those I use a bench-mounted power disc sander, it's fast, accurate and leaves perfect, sharp feathered edges, but it only handles small stuff.

The nibbed scarf joint that PAR mentioned is virtually invisible when bright finished if the grain and color of the pieces are matched; I find the best tool for those is a router if the planking material is thin, but an accurate jig is essential to control the cut depth precisely. Other guys may have other ideas of course. However, some strength is lost: a 10% step loses 20% of strength, and that's about as small a step as I can cut on 3 mm ply.


Correction! By butt joint I was referring to a butted joint backed with a butt block.

 

Sleepyweasel,

The scarf should be as strong as the plywood.
A butt joint will break under much less force.

This is important enough that you might make a little test. Make two test pieces, 2 to 4 inches wide each. clamp each to a table and lean on them. watch out you don't hurt yourself falling down. If you want to be scientific then drill a hole in the free end and load with weights until one cracks.
Very cheap, easy, and a great way to be happy with what you have on your boat.

PS 2 to 4 inches is just easy and not too hard to break, no special magic. go big if you want to.

Edit: just read the suggestion about butt joints being equal strength to scarfs. Not in my last mistake. I was joining 3/4 x 3/4 cedar for stringers on a kayak (Home Depot wood limited to 8'). I thought "epoxy is really strong and will work as a butt joint". Not a chance. The joint broke in the glue when I tried to bend the stringer around the boat frames - a very gentle bend. So I tried a 2/1 scarf - same thing but it broke with more bend (just not enough). 6/1 scarf bent twice as much as the boat needed without breaking. It might have been different if I was going to glass both sides of the stringer, but I wasn't.

 

A butt blocks and butt joints are different. A butt block is actually stronger then a scarf if bent with the butt block on the outside of the radius, but considerably weaker if bent with the block on the inside of the curve. A butt joint is a very weak technique, unless it's a Payson butt joint. If so, this joint can be stronger then a scarf, if directional fabrics are used. If not about the same as a scarf. A 2:1 scarf is useless anyway you attempt it. A minimum slope would be at least 5:1, with 6:1 and 8:1 being common and 12:1 in parts needing bend around a significant radius.

 

PAR,

All true, I am not sure if you were commenting on my post.
I did not use a butt block.

It would be a very strange stringer for a skin on frame boat that had a butt block on the outside of the curve.

It is also true that high strength composite panels which get a hole punched in them can be repaired by grinding the material around the hole away at a 8/1 slope then relaying the material in the sloping hole, then curing (aircraft - at least).

 

sorry, i forgot to point out that i use 8/1 ratio, and that in general, scarf joints are stronger than butt joints, but i like to use the scarf joints perpendicular to the short curves so that it doesnt try to open when bending, the pushing out layer ply scarf must be inside (epoxy better not in tension) .
Zigzag zipper joints could be interesting, i have some left over plys 6mm , i'll give it a try.

 

Yellowcat,

Strong enough is strong enough.
Epoxy can be on the opening side, it has been done for at least 30 years and works fine with a shallow enough scarf angle.

If you want to make it over the top strong then put glass over the joint like PAR suggested and Payson made popular.

KISS is a really good idea in this case.

 

Yes, this is what i did on a boat repair where we had no choice.
Have you tried Gflex on a scarf joint ? I find difficult to work with scarfed ends, especially with 3 mm ply. A long time ago, we repaired and built small boats and my grand-father scarfed and tong and groved pieces on one axis, glued them and did the day after the other axis, so that we had an easier time to manipulate the weak ends.
Now we use fiberglass boats and fancy boards, we removed all the wood plys on a 24 ft small cruiser, what a mess, it was full of insulation (i called it sponge insulation) we had to replace with corecell and my friend (the owner !) did not listen and played with his grinder near the skin, trying to scarf on site ... you guessed it, he went right thru it , we had a good laugh because we had just assessed the danger 5 minutes prior. It was hot under the florida sun and he was in a hurry to finish, he had bought a 21 ft new project (to his wife dissaproove) .
Sometimes it is hard to KISS when working in existing conditions.
By the way, we switched the motor and shaft from the 24ft to the 21 ft and he planned to install the volvo twin counter rotating props on the 24 ft.
Have you used thinned epoxy on scarf joints ? I am not confident about thinning epoxy, it is a way to reduce the cost for sealing the wood at the sametime as easing the epoxy into the end grains, everywhere for that matter.
Got to run, another stinky project awaiting.
Thanks for your inputs.