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Can less be more?

Discussion in 'Serious Boat Building Questions' started by john the pom, Jun 4, 2008.

  1. john the pom

    john the pom Well-Known Member

    Here's a question that's probly been asked before in many forms. I'd like a bit of input from some of you more experienced (Ok thats all of you) :oops:
    Let's say a plan calls for six mm, or 1/4" ply with taped seams and epoxy. Would there be an equivalent in strength say with using 4mm ply and glassing/epoxying both sides?
    Ok probly a bit more flex,but that might well be overcome by a bit of extra bracing.
    Moving on, is there a formula, or guesstimate of what is substitutible?
    I know some of you guys use 3mm where thicker is called for, but I have in mind The 15' Dk dinghy. The individual panels are not that wide. This is a stand uppable inabble boat. It calls for 6mm and 9mm ply. Would there be any benefit with weight/strength either way?
  2. hairymick

    hairymick Well-Known Member

    G'day John,

    Best person to ask would be Matt but for what my opinion might be worth, I think it is do-able and have often thought about doing just that. My boat would be having the smallest of outboard motors or an electric.

    I haven't seen the drawings for a DK but have been looking at it very closely for a couple of years now. :oops: (yes, it too is on the to do list" :oops: :oops: however, I would be inclined to go for 6mm bottom of very good quality ply and perhaps 4mm for the sides.
  3. a Bald Cypress

    a Bald Cypress Well-Known Member


    Something to think about.

    In most applications, the designer is looking at load displacement when they list the materials to be used.

    What may be perfectly acceptable for the bottom of a boat with the load spread out and aided in support by crossing a portion of the internal bracing. May [may] not be strong enough for a "point" load Standing might just put to much of a load on to small an area.

    That is not to say that the boat will break, it just may exceed the design safety factor. Exceeding this load factor repeated times "may" over time, lead to failure.

    Also, adding the extra tape and ribs may push the weight over what you would have with the 6 oz cloth. [something to think about]You could come out lighter with the heaver cloth in and out ?

    NOTE: The above information is in no way ment to be the undisputed facts. All points are personal opinions only and most likely worth just what you paid for them.

    Additionally, no animals were harmed in any way during the typing of this message.

    P.S. I'm not sure if I approve of the content of this message or not. I reserve the right to flip flop at any time.

    Your milage may vary. Please insure your seat back tables are stowed.
    Always were a seat belt and drive defensivly.

    Take two asprin and call me in the morning.
  4. JEM

    JEM Well-Known Member

    My friend Morten Oleson designed the DK Dingy. He has his section on my forum page. That would be an excellent place to post your question because I bet many more folks are wondering the same thing.
  5. Kayak Jack

    Kayak Jack Well-Known Member

    Other considerations come into play here too. Namely, geometry.

    A piece of plywood, say 3' X 4', that is perfectly flat will flex much quicker than a curved surface, of four or six panels, made from the same piece of wood.

    Think of aircraft fuselages. Rounded, they present a strong structure. Unwrap that tubular shape, and make a single, flat piece and you will have a flimsy structure.

    If your boat is to have a flat bottom of fairly large proportions, you can easily add linear bracing. Strips of plywood that are, fer instance 1" wide X 1/8" thick X several feet long, running along the inside of the boat bottom, parallel to the centerline, will beef up a lightweight bottom substantially.

    Other things to consider are type of construction. Most of our wooden boats that have curved bottoms are semi-monocoque or full monocoque construction. A full monocoque shape gets all of its strength from the skin - no framework. Best example of that is an egg.

    Aircraft are of the semi-monocoque construction design. A very good compromise for weight and strength. Sin on stick kayaks are the opposite; all strength comes from the frame. (See Nobucks' demo of the skinless frame floating around with floatation bags installed.)

    Plywood, epoxy, and fiberglass allow us to build these strong-skinned, light weight boats. It presents us with choices that builders did not have 30-50 years ago.

    My personal choice tends towards lighter weight hulls, with as little bracing as needed. Other builders go in different directions. Nice to have choices.
  6. john the pom

    john the pom Well-Known Member

    Thanks guys. The beauty of this forum is the ease with which you can,- make that we can- access information and trains of thought that don't come readilly to the inexperienced ( I like to think of it as unsullied :lol: )
    And of course the mixture of opinions/thoughts/ideas gives a good basis for making informed decisions. I'm beginning to have my own thought processes but wouldn't rely too heavily on them till at least some of you had given them the once over.
    Matt, in hindsight it was really quite rude of me to ask others before asking Morten, but I didn't even think of it, and that is fixed now. Thanks for reminding me :oops: that Morten ought really have been my first contact re this question.
    Once again thanks to you'se all for sending me in all sorts of directions that I needed to be aware of. :)
  7. Kayak Jack

    Kayak Jack Well-Known Member

    And, we've a few more wild goose chases, snipe hunts, and red herrings to go.
  8. FrankAS

    FrankAS Active Member

    Shame on you YOU forgot the Scotch!!

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