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Scarf Joints


wolfman

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The joints are real smooth, I think the PL premium doesn't cure real well in 24 hours, good general construction glue, but not direct force like that. Just glued up samples with Titebond 3 and JB wood weld will check those tomorrow Sent from my iPhone using Tapatalk

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I was a little concerned about the strength of my scarf joints  

only because I used the same jig (angle) for all the scrarfs

  the 5/8th scarfs were a little long but the 1 1/2s were a little short

I used titebond 111

I centered the scarfs all under the lashings and pulled them really tight 

I am confident they will all hold   I tested a 1 1/2 stringer with a 5 inch scarf

the wood gave not the titebond

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  • 2 weeks later...
  • 4 weeks later...

"Both the epoxy and Titebond 3 held great, so Titebond it shall be!"

I was curious about the type of wood you used for your tests. I was planning on using Western Red Cedar for one of my boats and heard from several sources that the oils in the wood make it tricky to glue up. Anyone have thoughts on that? I know that Jeff has glued up WRC before. What do you use?

Thanks,

Richard

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I don't do plywood enough to make one Paul.  But even so it doesn't quite work for 12mm plywood, just 6mm and 9mm.  Unless you have a 10" or larger circular saw.

 

This jig was initially intended for cutting 1/4" and 3/8" plywood. On 1/4" it makes a 2.5" scarf (10:1) and on 3/8" a 6 2/3's:1 slope and on 1/2" plywood about a fat 5:1 slope (1/2" ply is less than 1/2" thick). I've had little difficulty with 5:1 scarfs on panels not highly bent. Of course on pieces that do need a good bit of twist or radius, you'll want a little more slope, but I've never seen a 6:1 slope fail, though some if tightly bent will have a hard spot, with 8:1 or larger solving this issue, with less fairing compound.

 

TiteBond III needs to be carefully looked at before committing. If it gets wet, it will soften (a lot), but it does seem to dry as hard as it was if the moisture is removed. Second is "creep". TiteBond will creep under load, especially if in a moist environment. It's not considered a structural adhesive (actually it's a glue) for this reason. I use quite a bit of it, but never on anything that will be highly loaded, splashed or immersed. It's great on cabinets, trim and other things, but I wouldn't trust it to hold stringers, masts, etc.

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Does anyone think pinning a scarf joint is required? If you put a small dowel in perpendicular to the stringer it might give it some more stability? I've seen a few videos that pin and/or glue their stringers to the ribs too. It seems like overkill but is there a disadvantage to doing that?

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Lots of traditional joints and scarfs use pins and or hooks, but no glue.

The makkusiineq scarf of Greenland, for example, uses hooks, bird's mouths, dowels and lashings, but no glue.

Also, ribs are pinned into gunwales, and deck beams are sometimes dowelled into place. Lots of dowels and/or treenails are used, but not in conjunction with glue.

These are SOF kayaks, I mean.

Modern glue scarf done right should need no mechanical backup.

Resorcinol, as far as I know, is the only glue deemed totally waterproof by the Navy, because it will pass the boil test.

If you use TBIII, don't boil your boat.

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Lashings are so effective because they handle flex better than anything brittle (glue) or that weakens the wood (fasteners).

Could you lash a scarf joint? I would think you would have to drill a hole into the stringer or maybe cut a notch for the sinew to bite into. Any advice on how to do that? I'm guessing there would be a need for many wraps around the scarf. Although I trust the opinions of this esteemed panel I like experimenting with new (or in this case old) techniques. I really wanted to be able to boil my boat but don't want to use Resorcinol. (lol)

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For a lashed stringer you need to cut a hooked scarf. The tips of the scarf are cut into bird's mouths that fit matching bird's mouths cut into the other piece. Harder to do than explain or visuslize.

Lots of careful measuring and cutting.

And throwing away "practice" scarfs.

The lashing serves merely as suspenders. The joint itself does the bulk of the work.

Yeah, the first builders were genius...

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Lashings are so effective because they handle flex better than anything brittle (glue) or that weakens the wood (fasteners).

Could you lash a scarf joint? I would think you would have to drill a hole into the stringer or maybe cut a notch for the sinew to bite into. Any advice on how to do that? I'm guessing there would be a need for many wraps around the scarf. Although I trust the opinions of this esteemed panel I like experimenting with new (or in this case old) techniques. I really wanted to be able to boil my boat but don't want to use Resorcinol. (lol)

 

 

For what it's worth, boiling a boat does not make it taste any better or make it tender. I've made a nice stew from a bare frame, but be sure to remove the skin first and use lots of onions. The hardest part is finding a large enough pot.

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Lots of traditional joints and scarfs use pins and or hooks, but no glue.

The makkusiineq scarf of Greenland, for example, uses hooks, bird's mouths, dowels and lashings, but no glue.

Also, ribs are pinned into gunwales, and deck beams are sometimes dowelled into place. Lots of dowels and/or treenails are used, but not in conjunction with glue.

Lots of techniques have been used in the past before modern glues and epoxy, that doesn't make them as good as a simple glued scarf, especially for bent under pressure applications.  Every time you drill a hole in a jointed end you weaken the wood, period.  Is it strong enough?  Maybe, but why settle for less than the best?  If it amuses you, then do what ever you like.

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