Lightweight Canoe

Discussion in 'Fiberglass and Composite Boat Building' started by Bluegoose, Jan 11, 2014.

  1. Bluegoose
    Joined: Jan 2014
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    Bluegoose Junior Member

    Hello everyone. I’m looking for some advice on constructing a lightweight canoe. I purchased some plans for a 15’6” long by 36” wide stitch-and-glue canoe designed to be built with plywood. This is a “regular” shaped canoe, nothing exotic. I plan on using it to fish the local creeks. It’s not going to run Class IV rapids or be used on remote Alaskan fly-in adventures.

    The plans gives 3 different options for the hull material: (1) ¼” plywood and fiberglass tape the seams only, (2) ¼” plywood for the bottom and 3/16” plywood for the sides and fiberglass tape the seams only or (3) 3/16” plywood but covering both sides and seams with fiberglass cloth.

    The plans don’t list the finished boat weight, but I’m guessing it’s going to push 65lbs-70lbs by the time the seats and all the trim are installed. That’s not terribly heavy, but I’d like to get it down to about 50lbs because of the difficultly in carrying the canoe to some of the places I fish.

    I’m trying to reduce the weight by using a foam core sandwich in place of the plywood. I’m looking at Corecell or Divinycell (maybe 3/8” thick and 5 lbs/cu ft density) for the core material and about 10oz fiberglass cloth on both sides for the skins. Most boatbuilding forums and websites make a very general, blanket statement saying that plywood is lighter than composites for any boat less than 20’-25’ long. I understand the thinking behind this statement, but I’m not sure I buy it 100%. I’m sure it’s simpler and cheaper to build small boats out of plywood. Plywood is both fairly stiff and puncture resistant, but I’d really like to use something lighter in weight.

    What foam composite panel lay-up would be equivalent (in stiffness and puncture resistance) to ¼” plywood? It’s very difficult to find this type of information. (I’ve found several opinions, but few seem based in anything substantial.) Since many, many small boats specify ¼” plywood for the hull, you would think someone would have an “equivalent” composite lay-up. I’d prefer for this “equivalent” composite to use standard fiberglass cloth and epoxy resin (no Kevlar or carbon) and be made using a “regular” hand lay-up (no vacuum bagging or infusion processes.) I’m sure the composite panel can be made stiff enough to be equivalent to the plywood, but I don’t have any idea how much fiberglass cloth is required to give a similar amount of puncture resistance.

    Also, does anyone know where to find accurate weights (per sq. ft) for composite panel lay-ups? If I knew the weight of the composite panel (per sq. ft), I could compare that to the weight of plywood (per sq. ft) to get a relative difference between the two. I’ve seen some websites estimate the resin to be equal to the weight of the fiberglass cloth, but does the finished panel weight come out that way?

    Sorry this question turned into a short story, but I’d really appreciate some help with these questions.
    Thanks.
     
  2. gonzo
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    gonzo Senior Member

    What you are asking for is a complete re-design. There is no simple answer when it comes to substituting building methods. Also, to get a lighter weight than thin plywood with a foam/fiberglass sandwich construction you will have to be extremely careful and skilled on the lamination. Also, the puncture and chafe resistance will be much lower due to the thin skin. As for the weight of laminates, it is whatever the plans require. 70 lb seems heavy for a canoe built of thin plywood. 1/4" is about 18 lb and 3/16 about 13 lb in okume. If you trim everything in light wood like pine, cedar or spruce, it can't add up much more.
     
  3. Bluegoose
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    Bluegoose Junior Member

    Gonzo, thank you for the quick reply.

    Last spring I made my first "boat". It's a pirogue based on Uncle Johns's plans. (You can find it on the web easily.) It's no work of art, but it's a good place to start to get your feet wet. (I know - that pun was terrible. sorry) It's made of 1/4" plywood (not marine grade), about 15'6" long by 34" wide and ended up weighing about 65 lbs. I covered both sides of the plywood with 6oz fiberglass cloth, and I'm sure I went much bigger than necessary of the lumber for the frames, trim and seat. If I were doing it over, I'm sure I could cut some weight out of the boat. That's how I arrived at my 65-70 lbs weight mentioned in the first thread.

    I'll be the first to admit that I'm not a composites engineer, but I still think it's possible to specify a foam core sandwich panel that's "equivalent" to 1/4" marine plywood. The plans for the canoe I purchased gave options for building the hull - 1/4" plywood OR 3/16" plywood and tape the seams. Why can't there be another option, a foam core sandwich panel that's roughly equivalent in strength and puncture resistance to 1/4" plywood? For example, would 3/8" marine foam (~5 lb/cu ft density) covered with 12oz fiberglass on both sides be as stiff and as puncture resistant as 1/4" plywood? It should be possible to specify a composite panel that's very similar to a plywood panel without knowing all the specific details of the boat.

    Maybe there's a better way to say what I'm asking. Say I made a composite panel as stated above and send it to a test lab along with a panel made of 1/4" plywood. The lab runs their test to determine the stiffness and impact resistance of both panels. If they happened to have very similar results, couldn't you say that the composite panel is roughly "equivalent" to the plywood panel and could be substituted for it in it's place? (Remember, I'm not building a 100mph speedboat here, just a canoe.)

    Since 1/4" plywood is so commonly specified on small boats, I'm very surprised someone doesn't offer a composite alternative. I can't be the first person to ask this question. Also, commercial boatbuilders have similar size canoes made of composite materials that weigh around 40-45lbs. Wenonah canoe company's Prospector 15 is made from kevlar sandwiching a structural foam core and their boat weighs 48 lbs. What type of composite panel do they use? (I wrote them an email asking about the particular lay-up, but I didn't get a reply. Guess they don't like sharing trade secrets.) I'm sure they have the ability to use vacuum bagging and some other high-tech processes, but a lightweight hand lay-up should still be possible.

    Maybe it's difficult to beat plywood in a composite panel that thin, but the commercial guys do it somehow.

    Again, thanks for the reply. I'd love to hear some more opinions about this subject and I hope this thread can grow and develop.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum Goose.

    What you're asking is a common request and the "testing" has been done, so you can save the bother and cost. The reason you see thin plywood in craft of this scale is because there's more to the hull shell panel requirements than just impact and stiffness. Simply put, when you factor in all the usual requirements, strength, stiffness, impact, abrasion and penetration resistance, plus materials cost, plywood bests most everything.

    You can't fairly compare production boat building to a back yard build. Scantling and material choices are based on a price point and market share, not building ease, weight or the best materials for the project. This is why you see molded materials and a predominance of thermoset resin systems.

    In small craft, you can beat plywood scantling physical attributes using goo and fabrics, but at the expense of labor and cost. This is the typical butt kicker with these materials.

    So, if you want to try a "new" composite build, go for it, but I'd research the costs and have a real close look at how much effort it takes to get the boat completed, before ordering dozens of yards of fabric and gallons of goo.
     
  5. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Bluegoose,

    Since you are not looking for the absolute lowest cost - make a test part.
    Use 3/16" ply and glass both sides with 6 oz. Then 1/4" foam with glass both sides again 6oz.
    Bend it until it breaks, use some weights to see when each breaks.
    Drop a 1# weight on another sample until it permanently dents the test piece.

    You will learn more from a test than opinions, since you will understand exactly what happened.

    Of course you could try Dave Gentry's Rushton IGO Canoe built in Skin on Frame (SOF). http://gentrycustomboats.com/IGO page.html the pictures are really neat in clear vinyl, but as he says make it in much tougher dacron skin. 40#.
    Dacron is quite tough. I am replacing a dacron skin on a kayak and it is difficult to take off. (The cause is a very poor first effort, made as cheaply as possible)

    Other SOF canoe designs are available.

    Just saw PAR's comment - there certainly are more requirements than I talked about for complete tests
     
  6. redreuben
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    redreuben redreuben

    Build a table big enough to produce a full length panel and infuse panels of Paulownia strips and carbon or Sglass.
    Or.
    By a strip plank design and build in same. Materials choice = available $$
    Unless you have an autoclave in your garage you won't be able to build a foam/honeycomb panel with the same bashable properties as ply for the same weight. (bashable = impact /abrasion/damage resistance)
     
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  7. upchurchmr
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    upchurchmr Senior Member

    They build homebuilt aircraft with out infusion or autoclaves out of foam and glass.
    You need to look around.

    Impact resistance is not helped by infusion or autoclave. Weight can be improved using those techniques if you really know your laminate design.
    Impact is improved by thickness of the outer laminate (Hitting a sharp object that punctures) and can be improved by a thinner core (allowing movement of the sandwich panel when it hits something if there is momentum to be concerned with). Eventually you have to be concerned with the inside laminate if there is lots of deflection.
    Good goo is also important - epoxy instead of polyester.

    This is just a canoe.
     
  8. redreuben
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    redreuben redreuben

    Upchurch,
    thanks for the insult, I do look around and have been for some time now.
    Homebuilt aircraft don't go fishing in creeks.
    Impact resistance is dramatically improved by increased fibre ratios. Resin is brittle.
    epoxy yes.
     
  9. upchurchmr
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    upchurchmr Senior Member

    redruben,

    Take it any way you want.
    To suggest a canoe needs infusion or an autoclave misses the entire point
    That is just technology for bragging rights, not anything useful.
    IMO
     
  10. redreuben
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    redreuben redreuben

    Upchurch,
    the op wants to build lighter than thin ply, to achieve that takes a bit of tech and money to achieve the same properties, lots of backyarders infuse panels now, no big deal. The mention of an autoclave was simple to emphasise the difficulty of the task at hand. I find it hard to believe you took that as a serious proposition !
     
  11. upchurchmr
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    upchurchmr Senior Member

    I might well have over reacted.
    Still it should be easy to beat "typical" methods.
    Bill Hamm over on the Guillemot kayak forum has a method which gives a fiberglass skin based on SOF methods, if a fiberglass skin is the OP's desire.
     
  12. Tungsten
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    Tungsten Senior Member

    Some #'s from previous testing,1/4" M80 with 2 layers 12oz +/-45 one side and one layer on the other.Vac bagged was about 280g ish per 2ft.1/4 meranti ply with 6 oz both side comes in about 450g per 2ft.

    Hammer impact tests where good on the foam,once you get over 20oz you have something.

    IMO vac bagging would be the only way as hand laminating one can easily use too much resin throwing the weight savings out the door.
     
  13. messabout
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    messabout Senior Member

    What will you be doing with the canoe? If tripping, the 15-6 canoe is probably what you might need. If for day outings,fishing, or short trips the fifteen and a half footer is too big. If you go into tight places or up small creeks a smaller canoe is a better deal. Smaller will be lighter. Not only lighter to lift but easier to transport and store.

    I suggest Thomas J. Hills Book; Ultralight Boatbuilding. His 15-11 canoe comes in at 49 pounds, the 13' -10" at 44 pounds, and the 11'-6" one at 25 pounds. Lots of good advice in the book about building your boat light but sufficiently substantial.
     
  14. Bluegoose
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    Bluegoose Junior Member

    Everyone,
    Thank you for your input and taking the time to reply to my question.

    PAR – I agree with your statement that this is a common request and the “testing” has been done. As commonly as ¼” plywood is specified for all kinds of small boats, I’d think alternate material testing should have already been done. I’m not trying to reinvent the wheel or recreate well established testing that’s already been done. Unfortunately I’m still unable to find any of those tests or comparisons. Do you have any specific examples of this type of testing?

    Please don’t think I’m trying to be stubborn or hard headed. If in fact, plywood is the “best” and lightest material to make a “recreational 15’ canoe” out of, then so it is. I was hoping to find a foam core/fiberglass sandwich panel combination that’s a little lighter than plywood but also has similar strength/abrasion resistance/impact resistance as plywood. Maybe that’s not possible without resorting to very expensive material (Kevlar or carbon fibers) and using more complicated assembly processes (vacuum bagging, resin infusion, etc…). The general consensus is that it’s tough to beat plywood, but it’s hard to find an alternate panel lay-up with enough specifics to “run the numbers” and get an actual weight comparison between the two materials.

    TUNGSTEN – thanks for giving some real specifics. You mention using ¼” M80 foam with 2 layers of 12oz +/-45 fiberglass on the outside and 1 layer of the same fiberglass on the inside. This lay-up weighs about 280g/sq ft (or .62 lbs/sq ft) when vacuum bagged. According to a couple of sources I found, a 4’x8’x1/4” piece of okoume plywood weighs about 22 lbs. This works out to 0.69 lbs/sq ft. Unfortunately that’s about the same as the sandwich core lay-up you’re proposing. Your lay-up definitely sounds like it would make a good, stout panel, but I don’t have enough experience dealing with these materials to know. If that’s what it takes to match plywood, then everyone is right. Just stick with plywood. Is this proposed panel lay-up overkill? I don’t know.
    A couple of people have mentioned going the skin-on-frame route. I’m not opposed to trying that, but I think it gets right back to the original question. Just how much “skin”, do you need to make a skin-on-frame boat comparable to a ¼” plywood boat? Everyone agrees that ¼” plywood is good, but how do we make a SOF boat that’s roughly “equivalent” (in strength, toughness, impact resistance, etc…) to a plywood boat?

    MESSABOUT – I don’t have the book, Ultralight Boatbuilding. I did check out his website though. He has a couple of “small” kayak-paddled style canoes, but I’m looking for something with a little more beam to it (and I don’t like sitting flat on my butt). How much beam is there to the 15’11” canoe that weighs 49 lbs? There’s a 16’ canoe with a 36” beam on his website, but it weighs 65 lbs. That’s about where I’m at with regular stitch-and-glue right now. I will admit; the lapstrake style boats are very sweet looking.

    I’m still curious as to how the commercial boat builders make 40-45lb canoes using a foam core sandwich design. Can they ONLY achieve this weight by using expensive materials (Kevlar/carbon) and more complicated lay-up processes (vacuum bagging)? Does anyone know the specifics of their lay-ups? If I had a spare $2000 sitting around, I guess that’s the easiest way to go.

    Again, thanks for all the insight and comments. I really do appreciate it. I look forward to others replies and additional input.
     

  15. Steve W
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    Steve W Senior Member

    Im surprised that 1/4 plywood is specified for your canoe, I would expect 4mm and personally would use 3mm with a full sheathing of dynel outside in lieu of taped seams and just tape the seams inside and epoxy sealed of course. Use a lighter weight wood, maybe paulownia, for gunwales, thwarts, seats etc and you will be in the 50-55lbs range without complicating things. My own kayak and the other 3 we built for my son and his friends at the same time about 10 years ago is close to 18 ft long with 2 extra bulkheads to allow for unbolting into 3 pieces, 4mm was speced,3mm and dynel was used, it weighs 42lbs and would be under 40 if not for the extra bulkheads.

    Steve.
     
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