Question On Stretching

[DGuertin]

I've not bought any of the many fine books I see posters referencing so the answer to my question may be in one of those.

I see the photos of come-alongs and drawings for non-slip clamps and one page where the action is described, the writer states "...you will know when it's tight enough..." (paraphrased) but...is there a more "scientific" explanation to all this?

Why does the canvas have to be stretched in the first place to such a degree? I can understand you don't want it so loose it'll wrinkle, but I also think the tighter the canvas the larger the void between the fibres. The larger void would certainly offer more space for the filler (a better bond?) but conversely if there smaller spaces there's less filler required, so what's the trade-off; what's the optimum tension, or optimum void?

Has anyone tried less stretching, or by-hand stretching? What was the result? Would a less-tightly stretched canvas last 10 years less that a more-tightly stretched one would?

Dennis Guertin
Aylmer, Quebec

 

[Dave Wermuth]

Dennis

What Gil did not mention (he's so humble) is that he canvassed a duck boat and an Old Town Sport boat last Saturday BY HAND ONLY at the Quiet Water Symposium and they both turned out great. I Hand canvassed a Thompson take a long last winter and it turned out really well too. For a canoe Gil has done the same thing at the QWS in prior years. I use a come a long for canoes. My rule of thumb is tighten it enough to take out wrinkles. It really seems to be one of those things that are done by experience. I wonder if anyone has ever put a scale between the clamp and the anchor point and how much stretch they registered in pounds/kilos. As far as the spaces in the weave----hmmm. I should think that a stretch in one direction would tighten the spaces of the weave parallel to the direction of pull and wider for the threads perpendicular to it. But I am not sure on that one. In that case it would average out.

 

[dboles]

I will step out with my view on stretching.I use the upside down method and hand stretch on saw horses.Works very well.I do not know why some stretch.The canvas only serves as an impermeable seal to keep water out the boat.Upside down doesnt require all the other apparatus that stretching does.
Works for me.Maybe the term "whatever floats your boat" came about from wood canvas folks discussing the pros and cons of this same debate.!

 

[Rollin Thurlow]

The canvas does seem to relax just a bit after it has been tacked so there is always more danger of having the canvas be too slack on the boat if its not pulled tight enough to begin with. Of course there is a very wide range of what "tight enough" really is.
The canvas can be applied by hand but it certainly is the most labor intensive way to apply the canvas. There is a lot of going back and forth and redoing areas that already were done once or twice before. There is also the greater possibility of not enough tension in the canvas.
The upside down method is less labor intensive because there is a mechanical device doing the stetching instead of your arm. There is more set up time and you have to work a lot down low and on your knees.
The right side up method is the easiest, labor wise, you get to work at a comfortable heigth and its the easiest method if your working on a lot of canoes. It also takes the greatest amount of set up, at lest in the beginning for the first canoe. With the great mechanical advantage its easy to do the most amount of stetching to get the canvas to conform to the complex curves of the boat, maybe even more than is really needed.
In almost all cases there is much more trouble for the builder if not enough tension is put on the canvas. While its possible to overstrech the canvas ( most often when its really cheap canvas) it seldom happens. The filler seems to more than compensate for any "extra" the canvas weave may open up. In thirty years I have not experienced any problems due to over stretching, that I know of, but I do know of probems that I have had due to not enough tension in the right direction in the canvas. Pulling in the right direction is just or even more important than the total amount of tension that you put in the canvas.

 

[DGuertin]

Thanks for the Info, Fellas

My brother graciously offered me what he described as a 13' Tremblay "no-seater" that he's never gotten around to finishing to do a "practice" on.

Looking at all the text and pictures it seems to me that while there may be a (relatively) strong tension stem to stern-wards, one could never equal that with canvas stretchers by hand pulling perpendicular to that, so the canvas will already be stretched more one way than the other. <shrug>

What I think I may do is mark off the canvas in lines length-wise and see how much actual stretching goes on side to side; if nothing else I'll be able to see that my lines remain parallel (as much as I can around the curves) and prove to myself I'm tensioning it evenly.

Thanks again, and I'll try and document as much of the "trial" as I can. Aren't digital cameras wonderful, eh?