Measuring the amount of canvas stretch

Just1moredave · Apr 24, 2019

The two common books on canoe restoration don't have much detail on how much to stretch the canvas. Both mention wrinkles and puckers. Mike Elliott talks about the sound of a tenor drum. (Find YouTube videos on tenor drums if you, like me, are clueless on musical instruments.) Since I maybe didn't get it right the first time, I thought a lot about how to quantify the stretch. My idea is borrowed from a bandsaw tension gauge. I put down two pieces of blue tape and measured the distance between them when the canvas was slack.

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Then stretch the canvas and see how much the distance increases. I started at 22" when the canvas was slack, and ended up at 22 3/4" or ~3.4% more. That's a 19' #10 canvas. Easier if the canoe is upside down.

It's not useful as one data point, except to release tension for adjustments and get back to the same tension. I don't even know if it's the correct tension either. But it's easy to measure and might be useful for people doing multiple canoes.

jva74 · Apr 24, 2019

I canvassed upside down and stretched it until it was really drummy and then even more. It stretched another half inch and another one, until I could not move that short lever. Canvas fits excellently. As someone told here, you seldom overstretch.

paddler123 · Apr 24, 2019

Has anyone ever measured the mechanical tension applied? You could do it with a (suitably heavy-duty) spring scale attached in-line with your come-along. I think the most physically meaningful value would be the total force divided by the width of canvas, to give the surface stress. This would be related to the stretching by the elastic modulus of the canvas.

Just1moredave · Apr 24, 2019

I would have done that if I had the spring scale. There's some research online about stretching canvases for paintings which suggests force values for that purpose.

Usually if I try to apply maximum force to something, I break it first. I'm good at that.

MGC · Apr 25, 2019

The rule of thumb for canvas stretching is to stretch to .5 pound before it tears.

Dave Osborn · Apr 25, 2019

I’ve measured the distance my come along travels a few times. It works out to about an inch of stretch per foot of canvas.
A canvas on a 16’ Canoe stretches about 16”.

dtdcanoes · Apr 25, 2019

I usually stretch til I see the center thwart begin to bow and have found that when the carry thwart starts to bow the canvas begins to pull out of the end clamps at the come along. Then I back off a lot of clicks and do like Rollin does .....stand back and stare at it haaad !

Andre Cloutier · Apr 25, 2019

Has anyone ever measured the mechanical tension applied? About 8-10 cranks of the handle on my come along. Its all feel - too tight and you will never lose puckers or creases after you do the ends; too loose and it sags.
Dave, you 're brave stretching those 100+ year old dry boats till the thwart bows! please take some video haha

Rollin Thurlow · Apr 25, 2019

Different pieces of canvas seem to all have their own amount of stretching. I just go by the rule " when its tight at the center thwart, its time to stop pulling."

Howie · Apr 26, 2019

On a similar note, do you let the canoe sit for a while under tension before cutting it down & tacking the ends? I've been letting it hang for a few hours with the thought that there's a hell of a lot of friction between the canvas and the sides and that the canvas might like some time to get comfortable.

Just1moredave · Apr 27, 2019

Can't hurt. The first time I tried canvassing, it was January and I was trying to get it all done in one day of halfway tolerable temperatures so I could have my garage door open. We can get days here with highs in the 60s but it's a trap; the short days means it doesn't last very long. That definitely contributed to stress and whatever mistakes I made. Redoing it, I left myself a lot of time for each step. I took a week to do the same steps that I first did in two days.

MGC · Apr 27, 2019

I'm still chortling over the idea of bending the carry thwarts....now that's funny!

Gil Cramer · Apr 29, 2019

Personally, I only stretch the canvas enough to conform to the hull shape, and remove the wrinkles. Occasionally, I will hand canvas a canoe as a demonstration.