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Canoe finished, but too much flex?

Discussion in 'Strippers, Stitch-n-Glue, and Other Wood Composite' started by helgin, Jun 9, 2009.

  1. helgin

    helgin Curious about Wooden Canoes

    After many many more hours of work than expected, but lots of fun too, I finished my first canoe. This forum was invaluable BTW. I'm pretty happy with it overall - looks great from 6 feet, which is good enough for a canoe I want to use. Took it over to my folks pool just for a fun maiden "voyage" before a week at the lake next week. I was a little surprised how much the floor flexed as the family was loaded in (maybe an inch of movement in the middle center). Especially since the canoe feels so stiff to me when moving it around.

    Anyway, I was hoping that flex was a common property of strip canoes, but in searching the forum, it sounds like floor flex like this will eventually lead to fiberglass cracks or worse. I built this "laker" canoe (plans from the Gilpatrick book) for family use on flat water, so I figured using single layers of 6oz inside and out should be fine. Until now I didn't consider how the flat, wide bottom (36") of the design makes it probably quite a bit weaker than a continuously curved design.

    So, seems like the prudent thing to do would be to use my extra cloth and epoxy to add an extra football shape inside and out. I'd love to hear experienced opinions saying that isn't needed, but my big question is: should I go ahead and use the canoe on a lake a few times next week, or play it safe and leave the canoe home until it is reinforced? We don't get a whole lot of family time at the lake and summers are short, so I'd really like to get on the water unless I'm just asking for cracks.


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    Last edited: Jun 9, 2009
  2. Canoez

    Canoez Paddle Bait

    Which Gilpatrick design is this? I know that he does use a "football" of fiberglass on the outside of the hull before covering over the whole hull in his book. From the use of single layers that you describe, it sounds like you omitted that extra stiffening layer. With a wide flat floor, I think you would be smart to add the extra layer.
  3. OP

    helgin Curious about Wooden Canoes

    It's his "Laker" design. I knew he always used a football, but I recall he said it was for abraision, not stiffening. I also read Canoecraft which seemed to be the gold standard and only mentioned 6oz in/out regardless of design (although it might assumed a more rounded canoe). Elsewhere I've read that the only way to stiffen is to put equal amounts of glass on both sides of the sandwich. So seems like I'd need a football shaped glass piece in and out to really reduce the flex.
  4. Todd Bradshaw

    Todd Bradshaw Sailmaker

    An extra layer on the outside of a canoe will do virtually nothing to help stiffen the bottom in normal use. As water pressure, or that occasional rock just under the surface pushes up on the bottom, the outer layers are put into compression. Fiberglass offers very little compression strength unless you use a whole bunch of it. It will flex inward to a point and then crumple and fracture if you go any farther. Extra layers on the outside do increase abrasion resistance somewhat, but it's pretty much just a matter of providing more hard stuff that would have to be worn through before the core gets damaged. Even so, fiberglass isn't that hard to wear through, compared to some other materials. A couple layers of six-ounce only make a skin about as thick as a plastic milk jug and you can prove how relatively easy it is to wear through such a skin with one hand and a piece of medium sandpaper.

    The inside layers, when subjected to the same forces, pushing up on the bottom, work quite differently. The force puts the inside fibers in tension. Fiberglass resists tension pretty well, so these layers do far more to stiffen the canoe in normal use than the outer layers do. I am a firm believer in balancing the layup on both sides with double layers on both sides of the bottom. I've seen too many single layer boats crack or come apart to ever believe that skimping on the glass layers is worth the durability loss for saving a few pounds on touring canoes.

    I really believe you would be wise to double up the bottom on both sides of a boat that size. On the outside it will help resist abrasion and impact damage to the core and on the inside it will help prevent the boat from splitting along any weak grain lines or softening up even more as it ages. A small number of fiberglass fibers are damaged every time the boat flexes. They don't grow back and you have a limited number of them available in the hull to do the job. The more it flexes, the fewer you will have left to try to keep the hull in shape.

    In addition, on a wide flat stripper bottom, you might benefit drastically by adding some half-ribs on the inside of the bottom. These can be cedar, or even balsa - maybe 1" wide by 1/2"-3/4" tall, half-round or similar with tapered ends and long enough to span the flat. You epoxy them to the bottom every 18"-24" and cover them with a couple bias-cut strips of 6 oz. cloth. You can see a couple of them in the bottom of this canoe. We used glassed-over tubing for this one as an experiment. It turned out to be a pain in the butt to work with, compared to some nice, neat wooden ribs, but half-ribs really do help stiffen the bottom on big boats.

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  5. Canoez

    Canoez Paddle Bait

    I took a quick look at Gilpatrick's recommendations for the Laker in the book and there was one stating that a keel was highly recommended for that design. That may have camoflaged the flexing of the hull on his builds a bit. The additional layer of thickness will add to the stiffness of the hull - think about laminated fiberglass leaf springs, they add stiffness by adding thickness. Still a 6 oz cloth layer will not add much thickness and will not come close to doubling the stiffness. Don't forget that for a composite, you need to add both reinforcing and the matrix material. (glass and epoxy) Painting on resin will only add weight, not appreciable stiffness.

    Another thought came to mind - if you thinned out the wood excessively while sanding the hull, you bring the inner and outer layer of the stressed skin construction together, reducing the effectiveness of the stiffening of the hull. As Todd notes, that's why the addition of balsa cores is effective because it adds additional distance between the fiberglass skins.

    I would still add the extra layer - outside and perhaps inside.

    Todd's idea of adding ribs is good, but I'd be sure to use the cedar (Northern White or Atlantic) , I've not seen good results with balsa - if you have any pinholes in your fiberglass, they seem to rot quickly. If you do put ribs in, see if you can bend them in like a cedar and canvas boat. The only reason I suggest this is that the structure of a short rib that ends at the turn of the bilge tends to be a "hard point" and the rib works the hull there if you don't have a large overlap of fiberglass there. If you do have a short rib ending near the turn of the bilge add a good overlap of fiberglass - 4" or so - up the side. If you look at canoes that have been constructed this way, the fractures seem to be in the hull area at the end of the half-ribs as the stresses are concentrated there. It may also have been that the manufacturers of these boats used insufficient reinforcments in this area.
    Last edited: Jun 10, 2009
  6. Dan Lindberg

    Dan Lindberg Ex Wood Hoarder

    I couldn't agree more, Todd explains it very well.
    See the other string for a supporting story.

    "I've seen too many single layer boats crack or come apart to ever believe that skimping on the glass layers is worth the durability loss for saving a few pounds on touring canoes."

  7. OP

    helgin Curious about Wooden Canoes

    Thanks for the great replies. I'll start fashioning some ribs today.

    It's probably true that some areas are a fair bit thinner than 1/4". If I had a keel and the extra outer layer, I probably wouldn't have ever thought it was a problem. So I'll be another poster child for a newbie deviating from the directions to the canoe's detriment. Luckily it's not too late to reinforce...

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