Help! Fuselage Frame has a corkscrew twist after taking it from the strongback

[kayak_building_with_fir]

Hello,

the title explains it already pretty well. On the strongback, the frame was level and square. Now I took it out of the strongback and it developed a rotation along the long axis.

 

I took some pictures to show it. I can not really tell what is causing the twist, my guess would be the bow. 

Stringers are spruce and the weight of the frame is 8.4kg, overall it feels rather flexible. I tried to fixate the bow and rotate the stern clockwise, and it helped a little bit but I think it will go back.

 

Is there a way to fix it? How do I find out what is causing it?

 

Thanks!

[lattenkracher]

Hi,

 

please measure for all edges (lashings) the distances beetween the frames - they must be equal for each segment!

 

Are they all equal ?

[Kudzu]

It will take some digging but usually problems like this are a piece wood that was forced to bend into an odd shape and then when released it starts to return to it's original  position. 

 

The gunwale is the largest and would be where I would start. It has more influence on the shape of the boat then you might think. I suspect one side is trying to return to it original shape.

 

I remember one that did something similar to me. I know it was the gunwale but I don't remember what I had to do it fix it. Maybe it had a bad twist in one end and I cut it off and scarfed in a new straight section?

[kayak_building_with_fir]

Today I check all the frame distances between each other and also with respect to a "zero" point, which is a nail in the middle of the keel in the cockpit. Still, all the distances are symetric. Which makes sense, as I double and triplechecked while it was still on the strongback. Also, the gunwales were always level, and also the frames were vertical when checked with a waterscale.

 

When installing the stringers I always tried to match left and right stringer so they needed an equal amount of force to fit to the frame.

So my theory is, that one diagonal opposing pair (e.g. top right gunwale and bottom left stringer) is not so well balanced (one is bending inwards too easily, where as the other is resisting this much more). Don't know though how to find it, and also I don't know what I would do with it if it would fit.

 

Anyway, now I put it back on the strongback and level the stern. Then I measured carefully with a waterscale going forward one frame a time and it seemed like  most of the twist happens where there is long distance between frames where the cockpit will be. Doublecheck this by leveling the bow and measuring backwards, and voilà, I ended up in the same spot. Now I applied some torque with some improvised levers and bungee cords as shown on the picture, and wait for a few days. 

 

@lattenkracher: Doublechecked, was fine.

@Kudzu: Thanks Jeff for the hints, this brought me in the right direction. Actually I now think it was the faulty part is one stringer that was not scarf in the straightest way, which is causing the bend.

 

Let's see how it goes, if it does not settle (my garage is about 10°C or 50 degree fahrenheit) I can still try a warmer place, or pour some (hot) water over it.

 

If somebody has a similiar problem, this is what I found:

Here it explains how to check and then just use force to straighten it.

straightening an old canoe

some more discussion

This is what I have tried:

Are all the distances between frames equal, are the frames vertical (check with a waterscale)? Also check looking from top for symetry of each frame.

Did you notice that some stringers took more/less force than others to bend them into shape?

Check the scarf joints! Are the extensions straight, or point down/up?

Where is most of the torsion happening? More in the front, the back, over the whole boat?

 

Thanks

 

Edited October 25, 2020 by kayak_building_with_fir
added some hints

[labrat]

 

I had a similar problem with my Curlew. I believe it was due to uneven stresses in the stringers.

 

This is copied from my post at the time:

 

"The entire assembly corkscrewed when I took it off the strongback. I spent weeks trying to figure out why it sprang that way when not held in place. I can only assume that a number of the stringers had bends that aligned and pushed it that way. In the end I used brute force to twist it back past straight to corkscrew in the other direction and held it that way for some days. It is still not 100% but is as close as I can get it."

 

I suspect that twisting it back to exactly straight and holding it there will not entirely fix it. Like everything, it will have a slight tendency to spring back when released so forcing it back past straight will help. I like the idea of wetting the frame too. Some people use heat guns to bend timber but I am not sure if that would work with timber that is the thickness of the stringers. It's something you could look into if all  else fails. 

 

You probably noticed that my post says it did not come out perfectly straight. It turned out OK though - paddles fine with no tendency to go off course that I can pick.

 

Good luck with yours.

[kayak_building_with_fir]

Quote

 

I believe it was due to uneven stresses in the stringers.

 

 

@labrat: This sums up my thoughts in one sentence, thanks! English is not my first language. If I look carefully at my gunwales, the port side is nice and has straight grain, but the starboard side has a funny pattern in it behinde the cockpit.

 

One week of applying quite some torque did not help at all, did not try with steam though.

 

Today I spent some time trying to figure it out again with a friend. We tried multiple things, e.g. opening the gunwale lashing in certain frames and bending it out (with a shin), but whatever we tried did not match what we predicted. It's kind of tricky, because there is always translation and rotation (=levering) and that in both directions of the change).

 

One more thing worth mentioning maybe, is that most of my scarfs were not good enough. They will hold becaues they have a neat fitting, but the are not as straight as they could be.

 

Also, all these changes did not have a lot of impact on the frame, which is good as it shows the frame design is rather robust. Thanks for the design @Kudzu!

 

In the end I opened the bow gunwale lashes and planed away a bit of the port side gunwale. This helped a little bit, but I did not dare to go any further as it might skew something else.

 

Anyway, as I am out of ideas proclaim my curlew frame as ready for skinning! As I am out of ideas mostly. If it turns out to not track well, I can still add a skeg to make it go straight.

 

What do you guys think?

 

[Kudzu]

I think it time to make a new Gunwale. Yes it is extra work but if you don't you will always regret it.

[kayak_building_with_fir]

Alright this took me a long time to figure, to eventually I decided to just replace the suspect gunwale, and it reduced the twist significantly. It is not gone completely, but I decided to declare it straight enough for skinning.

 

After inspecting the bad gunwale, I realized there is a scarf at the height of the seat, which was not perfectly straight but shifted to a slight angle during glueing.

 

This must also have caused some misalignment to the rest of the frame while it was still on the strongback, at least that would explain the remaining corkscrewing in the frame.

 

So next step is skinning, hope that goes a bit smoother than the frame

 

[Kudzu]

Glad to hear it.  Twisted wood forced into place it almost always the cause the problem.

[kayak_building_with_fir]

So next time I would make sure that all gunwales and stringers have the same grain orientation on the cross section, and also check that both sides have an equal flex.

here is a video about kayak wood selection, I linked the part where he talks about the stringers.