Pre-shaping stringers, gunwales, and keelson

[Richard Peter]

When bending the stringers into place I was wondering if they need to have the tension on them for this method of construction or can a person pre-bend the stringers (and gunwales and keelson too for that matter) so they are the correct shape before attaching them to the frame. They could be bent by soaking them in water for a while and then clamped into the correct shape to dry so that they will have a smaller force trying to straighten them out when you assemble your boat.

Is it an unnecessary extra step, or one that would have a negative impact the the build? I've never seen anyone do it so I don't know if the force trying to straighten the wood is actualy a part of the boat's overall strength?

Your thoughts?

[Hirilonde]

The loads of the opposing stringers (trying to straighten themselves out)  is a major factor in keeping the shape of the boat  true when removed from the strong back.  It also makes the boat less wobbly, or twisty, as there is little rigidity in the unbent stringers.  That, and why would you want to fix a process that already works by making it more complicated?

[Richard Peter]

Good to know,

My original worry was that I would break the stringers when bending them into position. I thought that by giving them some shape beforehand it might make it easier to install.

[P Doug (WA)]

The size/length of the stringer will give no problem bending it into place.

[Kudzu]

There is one designer that recommends this, but some of his boats have 'hogged' You have to prebend stringers for his or the boat will deform and hog.

 

I ran into that on my Stonefly Canoe and I set out to figure out why it hogged and I did.  I work very hard to design my boats so that the wood is not forced into a shape it doesn't want to take. Sometimes wood does funny things, so I am make no promises, but if you have good wood mine should assume a proper shape.

[msaxton]

After doing my break test when experimenting with glues, and seeing how well the titebond 3 held, I did not see any reason to bother reinforcing the scarfs or making sure they landed under a lashing. and as far as bending, all of my stringers bent really well and held their shape with just a little tension at the bow and stern. Although, at the bow and stern where the stringers attach, I did lash, glue, and screw them there.

[Richard Peter]

I tried my first test bends today and all went well. I'm not affraid of bending the stringers anymore. I'll just be sure that the bends I'm putting into the wood are smooth natural curve so I don't end up getting hogged.

Thanks all!

[DURRETTD]

I used a chine stringer that bent in all three dimensions, but once it was fastened in place it took the designed curve and behaved itself nicely.

 

If you're concerned about the boat changing shape after you remove it from the strongback, leave it on the stongback for a while. If it still changes shape after you release it, put it back on the strongback and let it sit for a while. 

[msaxton]

I was really surprised when I removed mine from the strongback, thought there would be a little bit of shifting, but had none at all.

[Hirilonde]

. Although, at the bow and stern where the stringers attach, I did lash, glue, and screw them there.

Using a combination of fastening methods adds nothing to the strength of the joint.  Screws weaken it.  Glue will not handle flex very well.  The strength of these boats comes from the fact that they flex a little which dissipates the forces that would brake the same materials in a situation that would not flex.  Lashings are a key part of the over all system.

[msaxton]

I had screwed the ends with brass screws to draw them in tight as there was a lot of tension and it was difficult to just lash them(didn't have any clamps that would hold good on the angle). I intended to take the screws out after lashing them, but decided to just leave them in since they were brass, mistake maybe? time will tell. I've lived my life learning from mistakes,,, or maybe not so much