Plywood canoe

Discussion in 'Wooden Boat Building and Restoration' started by latestarter, Oct 2, 2012.

  1. latestarter
    Joined: Jul 2010
    Posts: 362
    Likes: 36, Points: 28, Legacy Rep: 233
    Location: N.W. England

    latestarter Senior Member

    Based on what I have read you are both right.
    Filling the weave will increase the strength to some extent but at the expense of extra weight.
    Just using the minimum amount of epoxy to wet out the tape/cloth is supposed to give the maximum strength to weight ratio.

    The adhesive tape I am using does not seem to leave a residue, instead it pulls up the wood fibres when you remove the tape, fluffing up the surface, but a couple of passes with sandpaper tidies it up.

    Overcoating without sanding seems to be very dependant on the epoxy system used and temperature. The system I am using quotes 12 hours at 25C degrees.
     
  2. rwatson
    Joined: Aug 2007
    Posts: 5,869
    Likes: 301, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Fair enough - no need to generate acrimony


    You might consider that you are mixing terms here. 'Strength' and 'stiffness' are not the same thing - in fact 'strength' is quite a misleading term. The only thing that more layers contribute is incompressibility ( and only up to a point at that) - sheer strength and linear strength are not enhanced much by extra layers. Epoxy without re-inforcing is little more than extra weight from a structural point of view. The term 'resin rich' is not used as a desirable characteristic


    I was talking about between epoxy coats of course.


    The term 'same engineers' need to be quantified more than just hearsay. The 72 hour rule must have some basis in fact, because I have heard it quoted many times. As far as West System instructions - there is no mention of the 72 hour rule.

    of course - which is why I said 'in areas not subject to wear and water'. Also, there are many examples of enough epoxy to be dimpled, but still smooth enough to be easily cleaned.
     
  3. rwatson
    Joined: Aug 2007
    Posts: 5,869
    Likes: 301, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Yes, exactly. The manufacturers instructions take precedence over any 'general advice' that you may hear.
     
  4. txriverrat
    Joined: Sep 2010
    Posts: 165
    Likes: 12, Points: 0, Legacy Rep: 159
    Location: Texas

    txriverrat Senior Member

    We agree on a lot of this.
    I have a saying that most folks dont realize is important to small boat building. LOCATION SENSITIVE
    Here is a prime example ,I live it Texas and it is HOT so I cant put several coats on in one day,here is the reason ,outgassing, if I put epoxy on a wood boat first thing in the morning
    in a couple hours it will look like a bubble machine, I have to run my first coat late in the afternoon so there is no out gassing . After the first coat is dried then I have some more leeway. Helping on a lot of small boats and the ones I have built 72 hours has worked well for me. Oh in the last few years I have been using US Composites epoxy with excellent results.
    Using acetone to wipe of places tape has been , One boat I built I had white spots in the glass everywhere I had used tape , I had to sand a 16 ft boat down to bare wood removing the epoxy and cloth to fix it , yep I am going to wipe down anything that I feel might contaminate where I am fixing to cover.
    I still disagree with you on the fill coats of epoxy, stiffness comes from the sandwich construction, compression or tension strength comes from the epoxy and ,I understand that. .
    I have a friend that has a medical condition where he cant lift over 40 lbs , so he builds extremely light boats, he epoxy both sides of his ply ,no cloth , that has been tested and increases the strength and stiffness 6 times. Throw you a piece of plastic down get a brush and lay down two coats of epoxy 6 inches wide and a foot long ,when dry try to pull it into ,just plain epoxy has a lot more strength than you think
    I do some racing and most is on rivers where you have rocks logs steel concrete portages , every wood boat I have seen with a hole knocked in it has been scrimped on epoxy and glass on the interior to save weight,so this is one of my pet peeves, your outside glass and cloth is for wear for the most part, the inside glass and cloth is where you get the tensile strength when you run up on a rock log or whatever. A couple lbs of epoxy on a 14 ft boat doesnt amount to anything ,also this is a wide boat we are talking about and it gets more important the wider the panels are .
    Like I said location sensitive has a lot to do with a boat and if it floats and is safe it is a great.
    Ron
     
  5. rwatson
    Joined: Aug 2007
    Posts: 5,869
    Likes: 301, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    I presume that when you say Epoxy, you mean combined with Glass.

    A meaningless 'statistic'. The term strength has to encompass all the modulus - tension, shear as well as compression. You might get a good increase in the compression strength, but the other components wouldnt get to 6 times.

    If this 'friend' of yours wants lighter weight Fiberglass Specific gravity: 1.0 to 1.3, while specific gravity of epoxy is around 1.2. So leaving out the glass is not saving much weight, but is reducing the 'strength' by a huge amount.


    While the tensile strength of epoxy is fairly good, see (http://www.westsystem.com/ss/typical-physical-properties/)

    epoxy tensile strength is around 7000 psi or ~48 megapascals.

    Compare that to pure fiberglass at ~170 megapascals.
    http://www.mtu.edu/materials/k12/experiments/tensile/

    Dont forget that total tensile strength has a lot to do with the size of the material section. Two layers of epoxy is pretty thin.


    When you run onto a rock or something, the hull gets forced inwards. Its the tensile strength of the OUTER layers that holds the hull stiff. The inner layers go into compression, not tension.



    In summary, I still cant find any reason to believe that the fill coats of epoxyprovide any significant amount of extra strength.
     
  6. txriverrat
    Joined: Sep 2010
    Posts: 165
    Likes: 12, Points: 0, Legacy Rep: 159
    Location: Texas

    txriverrat Senior Member

    When you run onto a rock or something, the hull gets forced inwards. Its the tensile strength of the OUTER layers that holds the hull stiff. The inner layers go into compression, not tension.

    This is exactly backwards ,when the bottom of a boat is pushed upward as running on a rock will do the outside is under compression the inside is under tension , coat you a piece with glass and cloth and try it you will find out I am correct.
    We disagree on a lot but thats ok and like I said in the beginning these or my findings and I still stick with them.
    Ron
     
  7. txriverrat
    Joined: Sep 2010
    Posts: 165
    Likes: 12, Points: 0, Legacy Rep: 159
    Location: Texas

    txriverrat Senior Member

    One of my other hobbies is archery , I have built long bows Indian flat bows and regular fiberglass backed recurves.. Maybe this will be a good example of the compression and tension forces. Your bows dont have to be reinforced on the backside of a bow (the part to you ) they do need reinforcing on the face or front side because the tension of drawing will pull the fibers apart and you have a high percentage of failure in unbacked bows.
    Ok you grasp the bow in the center and push foreward ,same thing a rock does to the bottom of the boat , it compresses the back of the bow and stretches the front thats how you get your power out of a bow.
    Also something else to consider is you are making an arch the one that is on the side that pressure is applied from is shorter than the one on the opposite side from the pressure, shorten an arch you get compression stretch one you get tension.
    Ron
     
  8. LP
    Joined: Jul 2005
    Posts: 1,414
    Likes: 58, Points: 58, Legacy Rep: 584
    Location: 26 36.9 N, 82 07.3 W

    LP Flying Boatman

    You get both, but I'm only trying to start an argument. :p

    Sorry, Latestarter. The temptation was to great.

    I was thinking about your build the other day and at the risk of starting a controversy, I had a thought. I believe you said you are painting the exterior of your hull. If so, you can fill your weave with thickened epoxy and save some coats, some time, some work and maybe even a touch of weight over straight epoxy. I've used wood flour. Your could use talc or micro spheres or a mixture. The wood flour I used was nice as it made a hard shell. Remember though, any extra has to come back off. instead of the hard stuff you can use softer stuff to make a fairing compound. Mix the two and you get something in between. If you are going bright, then straight epoxy it is.
     
  9. latestarter
    Joined: Jul 2010
    Posts: 362
    Likes: 36, Points: 28, Legacy Rep: 233
    Location: N.W. England

    latestarter Senior Member

    Nice one LP. :D

    Just to put this point in context see thumbnail.
    Tension is on the inside at the rock but away from the rock it is on the outside.
    For a plate surface Ttop will be much greater than Tbottom and failure is going to occur above the rock.

    I can't see what is controversial about your last paragraph.
     

    Attached Files:

  10. rwatson
    Joined: Aug 2007
    Posts: 5,869
    Likes: 301, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Ah yes - my mistake. I was not thinking it through. I think my mind was picturing the core.

    The main point is - a fill coat of epoxy (not glass) on the inside or outside skin , that doesn't increase overall thickness ( just fills in the weave) , will not affect strength (tension compression/sheer) by any significant amount.
     
  11. latestarter
    Joined: Jul 2010
    Posts: 362
    Likes: 36, Points: 28, Legacy Rep: 233
    Location: N.W. England

    latestarter Senior Member

    Autumn is here and I decided that my garage could do with further insulation and draught proofing. In previous years I did not venture into the garage much in the winter months. I bought 2 sheets of foil backed foam insulation and filled in the access to the loft and under the steel beams. Also rigged up a tarpaulin so the electric blower heater had less volume of air to heat.


    Rather late in the proceedings I checked using a laser for twist using poles tied to the underside of the sheer although the middle one rested on the workmate adjusted to be the same distance from the sheer on both sides.

    I continued with filleting the chines to about a foot from each end. I wanted to see the shape of the ends the right way up before making the ends permanent.

    Yesterday I turned the boat over using ropes and pulleys. Some left over insulation board provided a cushion to rest the hull on.
     

    Attached Files:

  12. LP
    Joined: Jul 2005
    Posts: 1,414
    Likes: 58, Points: 58, Legacy Rep: 584
    Location: 26 36.9 N, 82 07.3 W

    LP Flying Boatman

    Very nice! Love the view out those big windows in your garage. :p Those window closures are interesting. Are they for keeping the weather out of overly curious onlookers?

    The canoe looks nice too. :D
     
  13. latestarter
    Joined: Jul 2010
    Posts: 362
    Likes: 36, Points: 28, Legacy Rep: 233
    Location: N.W. England

    latestarter Senior Member

    Thanks LP.

    I built the garage about 10 years ago with the idea that it could be converted easily to living accommodation, did not think curtains or blinds suited a garage, not easy to get oil off, so used plywood instead, which also gives added security and insulation.
     
  14. Dhutch
    Joined: Oct 2008
    Posts: 83
    Likes: 1, Points: 0, Legacy Rep: 22
    Location: Cheshire UK

    Dhutch Junior Member

    I like it. If I was building a garage I would build it in the same way. The marginal cost of decent windows, wall and ceiling inslualation, etc. Makes it far nicer to work in and heat during the winter, as as said, makes it such that for a change in decoration, can become living space. Shutters on the windows keep prying eyes out!


    Daniel
     

  15. latestarter
    Joined: Jul 2010
    Posts: 362
    Likes: 36, Points: 28, Legacy Rep: 233
    Location: N.W. England

    latestarter Senior Member

    The prying eyes are mainly cats, cattle and sheep.
    It is nearly 4 weeks since my last post and have made a bit of progress.
    Having turned the boat upright I spent some time setting it up loose on 3 stands but found I could not get the amount of rocker right and stay within the beam dimension.
    As expected the hull was floppy (trying to lift one end just spread the topsides at the middle) but I was surprised how stiff it was in torsion as a U-shape is not good at resisting torsion. Had the boat been twisted it would have been difficult to correct. At this stage about 85% of the chines were fully filleted on the outside so adjustment was not available.
    So I refitted the frames. The middle one made good contact all round and could be held with some wooden blocks to align it on the bottom and a screw at each top edge. I used a hot glue gun to attach the wooden blocks but the wood was so cold the glue set before it was positioned so I had to warm the blocks first.
    The other frames needed a slight push to get them in contact all round so I used 4 screws through the bottom to hold it in place in addition to the top 2 screws. The holes will be hidden by the butt strap and internal fillets.
    Having done that the boat came into shape with the rocker being within a couple of mm of the design and the centreline of the frames and ends aligning. There was a couple of mm twist on the winding poles but there is excess plywood on the top edges as I have still to decide on the shape of the sheer.
    If I was to build it again, I would have paid more attention to keeping the frames in the correct position and the planks in contact with the frames while filleting the chines, it seems to have turned out well but could have gone seriously wrong.
    Having got it in position I spent some time getting the ends lined up with a plumb line then ran a bead of West Six10 to fix the plank ends.
    I expect to start filleting and taping the inside of the chines in the New Year.
     

    Attached Files:

Loading...
Similar Threads
  1. Sunburned One
    Replies:
    38
    Views:
    1,602
  2. sdowney717
    Replies:
    30
    Views:
    1,284
  3. Travis Grauel
    Replies:
    26
    Views:
    2,805
  4. Paul D
    Replies:
    13
    Views:
    2,008
  5. Floatything
    Replies:
    0
    Views:
    1,375
  6. juan manuel luna
    Replies:
    21
    Views:
    2,289
  7. Old salty
    Replies:
    4
    Views:
    3,733
  8. Vesimakara
    Replies:
    2
    Views:
    2,073
  9. DSR
    Replies:
    13
    Views:
    1,607
  10. theoldwizard1
    Replies:
    4
    Views:
    3,383
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.