Help support the WCHA Forums by making a tax-deductible donation!

fiberglass layering?

Discussion in 'Strippers, Stitch-n-Glue, and Other Wood Composite' started by Steven, Dec 13, 2004.

  1. Steven

    Steven Curious about Wooden Canoes

    I am fiberglassing my sailing canoe.
    Which would be stronger....1 layer of thicker weight cloth ie 6oz or 2 layers of a thinner wt (3 oz).....I guess what I am really asking is laminated fiberglass layers the same principle as plywood?

    Poorly worded...I hope it is understandable.
  2. Dan Lindberg

    Dan Lindberg Ex Wood Hoarder

    In compression, 2 layers of equal weight are stronger then a single layer.

    I don't know if this holds in tension.

    Another benifit of using lighter glass is that it is thinner with a tighter weave, meaning that it will take less resin to wet/fill.

  3. Todd Bradshaw

    Todd Bradshaw Sailmaker

    Either of the layups above would be considered pretty fragile on a paddling canoe, in that the potential for damage or problems and the possible need for future repair may well offset the value of the reduction in hull weight. Three ounce cloth does take fewer filler coats, but it's often the thickness of the filler layers which protects the glass layers from abrasion. Scratches in filler coats are pretty easy to fix and remove. Scratches which go down deeply into, or which bruise the cloth are both more serious and nearly impossible to completely hide with repairs, so it's again a trade-off with durability getting the short end of the stick.

    Such a layup on a sailing canoe crosses the border of "fragile" and would much more likely be considered "flimsy". Sail rigs, with their various parts pushing or pulling and twisting on the hull and the very act of sailing the canoe, where you may be moving twice as fast as you could paddle with the boat heeling and possibly pounding a bit on the waves puts far more strain on the hull than just sitting upright and paddling at three or four knots.

    Light weight is all well and good, but those "I built my boat ultra-light and it broke - what do I do now?" threads are quite common on the strip-building sites. If you ever sail this boat hard, I suspect you'll be making one of those posts. If this is your first stripper, I'd suggest a heavier, more general-purpose layup. You'll still end up with a lighter-than-average canoe and it will give you a substantially more durability, whether paddling or sailing. The minimum I would suggest is either a full layer of six-ounce with a double layer over the bottom (inside and out) or two full layers of three-ounce with a third reinforcing the bottom (also inside and out) - and don't fall for the classic beginner's mistake of thinking that you can skimp on the inside layers to save weight because the rocks hit the outside of the boat. Those inside layers do more to hold the boat together than the ones on the outside do and the failure to understand that concept has spawned it's own endless string of "my boat broke" threads.
  4. OP

    Steven Curious about Wooden Canoes

    Layering fiberglass

    Thanks Dan and Todd.
    With my previous strip kayaks I used 2 6oz layers on the bottom one in the inside and another 6oz on the inside deck and a 4 oz on the surface of the deck. I have never had a problem with them....but you are right paddling isn't sailing. I have never built a sailing canoe before (or even sailed in one).
    I had initially planned on using 2 6oz on bottom 1 6oz inside hull and 1 layer (6oz) on both sides of deck. Do you think this will be enough?

    I putting in 4 bulkheads and will be bracing them to each other with shafts from broken carbon fiber hockey sticks (Easton). The two masts will be attached at the bulkheads. As well I am hoping the fiberglass coning as well as the bed for the center board will provide structural strength. I was considering using 4oz cloth instead of the instead of 2 6oz layers use 3 4 oz layers.....I understand now that the benefits may not be worth the effort.
  5. Todd Bradshaw

    Todd Bradshaw Sailmaker

    I seem to be on a one-man campaign lately, on several forums, to teach strip builders that a weak inside layer makes for a weak boat. You're building a sandwich which acts somewhat like an I- beam. If one side of the sandwich has half the strength of the other, or if the top plate of an I-beam is only half as strong as the bottom plate, the resulting construction is nowhere near as strong or as rigid as it would be with equal strength on either side of the core. This gets even worse when you apply it to the bottom of a canoe or kayak where the vast, vast majority of the destructive forces it may encounter come from the outside, pushing inward, compressing the outer layers of fiberglass and trying to tear apart the inner layer, usually breaking the core in the process.

    In normal situations, the outer glass layers on a stripper don't do much more than keep the rocks from getting into the core. The sturdier and thicker you make them, the harder you can hit a rock before having to do a serious repair. The inside layers, on the other hand, don't get abraded like the outside may, but their function is far more important - they keep the boat from breaking! - whether from impact or just the force of the water pushing up on the bottom, trying to oil-can the hull.

    A strip core is extremely unidirectional and has very little cross-grain strength. Upon impact or when the boat hits waves the outside glass layers are not in the position where they can do much reinforcing. You can split the core wide open, blow apart the inner layers and the outside layers will have gotten so little strain that they may barely even show a fracture. Whether or not the core splits and whether or not the inner layers rupture is strictly a matter of how strong the inner layers are. If in doubt, always balance the layers, inside and outside and be very aware that reducing the layup on the inside of a canoe bottom makes for a much weaker boat. Why build a boat that you know is weak? - to save a few pounds? - Not worth it in my book.

    Bulkheads are a double-edged sword. They are exceedingly practical, especially on a sailing canoe if they're helping to form watertight flotation chambers or reinforce mast partners, centerboard trunks, etc. They do tend, however, to make stiff spots called "stress risers" in the bottom of the boat which can tend to focus abrasion and/or impact damage in the area around the bulkhead. On a sailing canoe the stress riser part of the equation probably isn't worth worrying about unless you plan to do some "whitewater sailing"....

    A single layer of six-ounce is probably fine on the outside of the deck and will yield maximum clarity on this very visible part of the boat. I think I'd probably use the same inside, but back it up with some glassed-in ribs or deck beam structure in case anybody ever sits or kneels on the deck.
  6. OP

    Steven Curious about Wooden Canoes

    Thanks Todd for the makes a lot of sense. (the I beam analogy is fitting) I will balance the inner and outer layers on the hull. (and your book is a good book).

    As well it does make sense that the bulkheads would provide a "fulcrum" point for stressors. I am considering constructing the base of the bulkheads wider (fiberglass filleting?) to spread the forces over a greater area.
  7. RHaslam

    RHaslam Curious about Wooden Canoes

    Very interesting conversation....I had direct experience with this last summer....I had always been building my strippers heavy, but decided to lighten up my 18 foot Quetico. Went with six ounce cloth in and out, but only put the wet-out coat on the inside. Has a job clearing ports this summer, and at one point got hung up on a log across the river, about 2/3 of the way down the canoe. Was loaded VERY heavily, and I was also very tired, so just poled over the log. The inside layer of glass split for about 20 inches, longitudally, but the outside hull retained its integrity, well, sort of.

    Anyway, building a new work horse now, a 20 footer, and I have been wondering what to do...this is my ninth canoe now, and I use them pretty hard...It was suggested that perhaps I go to 8 ounce cloth inside and out. I was thinking of just sticking to the 6 ounce cloth but going heavier on the fill coats, andd definately filling the inside too.

    Weight is not a problem for me, I can easily manage anything up to 100 pounds.

    Any thoughts? I would be most interested to hear from the pro's.


  8. Todd Bradshaw

    Todd Bradshaw Sailmaker

    I'd either use heavier cloth or go to three layers of six ounce (two full, one extra on the bottom - inside and out) for a 20 footer that's going to get hard use. I've used fabrics in the 7.5 oz to 10 oz. range for big strippers and had reasonably good luck. Other than grunch-patches with lots of layers built up on lower stems and the like, clarity is generally pretty reasonable. If it has the typical beam of most 20' workboats and freighters, you may also want to add some half-ribs in the bottom to stiffen it up. Filling the inside cloth weave is nice if you're afraid to look at fiberglass cloth texture, but it actually weakens the layup by creating a lower glass-to-resin ratio on the inside layers, making them more brittle, less flexible and less able to survive impact (as well as heavier).
  9. RHaslam

    RHaslam Curious about Wooden Canoes

    Thanks Todd, I'll take your advice into consideration.....I'm glassing the last of two canoes for the school this weekend, so should be able to start on my new one next week.


Share This Page