Scarf joint questions

[BWCA66]

I'm looking for thoughts on where to place a scarf joint on some new ash gunnels. Is there an ideal location on the canoe to put the scarf joint, in the middle or toward one end or the other? Also, would you place them at the same place on both sides or offset them? Thanks in advance for the opinions!

 

[Alan Gage]

By all accounts a well made scarf joint shouldn't be a weak spot. But I still don't like to put them in high stress areas. I envision a high stress area as usually being towards the center of the hull and far from the thwarts. I don't imagine the gunwales get a whole lot of stress unless something goes wrong (dropped or pinned).

I think the strongest location would be at a thwart.

Alan

 

[Mihun09]

Inner or outer gunwales?

We often have to scarf outers since we just cannot get lumber long enough. We do our scarfs in the middle where there is less stress, no real bend. We have tried in the past to have it nearer the ends but when trying to make the sweep upwards we have had them break, even when glued with epoxy. I too don't see a scarf joint as being weak, in fact it could be stronger since it is a glued joint and not dependent on just the wood.

 

[Jim Dodd]

As Alan stated, a proper scarf should be strong enough to go anywhere on the hull.
Mine always seem to be in the middle, or close, because of the length of my Ash.
I do try and offset the inwhale from the outwhale, at least a few inches.

To date, I haven't had any fail, including the one that came off my truck rack at 60mph.

Jim

 

[pblanc]

A scarf joint may not flex exactly the same as unjointed wood. If you place a joint in a location where there is a lot of curvature at the sheer line, such as near a stem, it might create a flat or "hot spot" where the wood does not quite conform to the hull. I think it is fine to have joints on both sides of the canoe line up at the same spot, but if you need to scarf both inwales and outwales, I would recommend offsetting the joints on each side of the boat by a foot or more, if possible.

I know that many rec​ommend using long scarf joints of 1:7 or 1:8 for strength, I have found that shorter scarfs of 1:3 or 1:4 work perfectly well. I think long scarfs are necessary when jointing thin panels together in order to get a sufficient bonding area for the adhesive or epoxy, but gunwales are thick enough to provide a very satisfactory bonding surface with shorter scarfs.

I try when possible to avoid having gunwale screw holes, or the holes for machine screws fastening thwarts or suspending seats from going through the scarf joints, although I have seen some recommend having a gunwale screw go through the center of the joint. If you use long scarfs you are probably going to have to accept having at least a gunwale screw go through the joint.

 

[Jim Dodd]

My scarfs are in the 10:1 or 12:1 category, may be overkill to some, but they work for me.

Jim