Skin on frame stiffness.

Discussion in 'Wooden Boat Building and Restoration' started by NoEyeDeer, Jun 1, 2011.

  1. NoEyeDeer
    Joined: Jun 2010
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    NoEyeDeer Senior Member

    Want some feedback from people who have a bit of experience with timber-framed SOF boats. Two main questions here:

    1/ When reading about these boats it is quite common to fnd the author waxing lyrical about how wonderfully wobbly they are. "Flexibility is a virtue" and all that.

    So, what about a situation where someone specifically wants to build an SOF boat that has minimal longitudinal deflection? Why? For mimimum resistance for fast rowing in calm water. Under these conditions, even a comparatively small sag in the hull will increase resistance, so minimising deflection would be desirable.

    What's "minimising" mean in practice? Think along the lines of a maximum of 1/10" sag over a 21' boat with one person sitting in the middle of it.

    Is this techinically feasible at a reasonable weight? If so, what sort of scantlings would be likely to achieve this result?

    2/ Surface finish: what's the smoothest finish you can realistically get, and how do you get it?

    Assume boat is skinned with 8oz Dacron.
     
  2. gonzo
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    gonzo Senior Member

    I don't think that is feasible. If you want a rigid hull SOF is not the right system.
     
  3. NoEyeDeer
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    NoEyeDeer Senior Member

    Yes I am a bit concerned about it, but I'm also curious as to whether it can be done. Doesn't matter if the boat flexes a bit in waves (most boats will, to some degree). I'm just wondering if its possible to minimise resistance under optimum conditions.

    ETA: Of course the other way of tackling it would be to work out how much hog the boat needed to be built with so it would float straight in the water with one person aboard.
     
  4. rwatson
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    rwatson Senior Member

    It is impossble to get no flex on an SOF hull. I remember a physics lesson about trying to take the sag out of a cord under tension - cant be done. However, getting the fabric as tight as possible is V important, and means the fabric must be strong.

    I think you are misunderstanding the sag principle though - there is NO sag fore to aft.
    Its all between the stringers.

    This is why your HOG idea wont work - because you have to have stringers to a certain depth, and they control the minimum hog, not the fabric.
    The amount of flex is always determined by the distance between stringers and the tension of the fabric. There is no sag longitudinally.
     
  5. NoEyeDeer
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    NoEyeDeer Senior Member

    I'm not talking about deflection of the cloth between the stringers. I get that part. I'm concerned about deflection of the entire framework over the length of the boat.

    As an analogy, I'm aiming for something more like a truss (dont bend much) than like a cane basket (all squishy all over the place).

    There is no question that the framework will bend longitudinally to some degree. All structures deflect to some degree to carry a given load. The question is: how much?
     
  6. rwatson
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    rwatson Senior Member

    The question you asked was "how wonderfully wobbly they are"

    This can only refer to the fabric. The frame is extremely rigid, as it has to be to prevent distortion by the tightly stretched fabric.

    The longitudional bend, and the lateral bend will be to all intents and purposes - zero! You can put the bow and the stern of an SOG frame on two packing cases - and jump up and down in the middle, and the flex will be ever so slight. It has to be - to withstand the stresses of pounding waves and perhaps being filled up with water and rolled around a bit.

    "Think along the lines of a maximum of 1/10" sag over a 21' boat with one person sitting in the middle of it."

    The other concept you need to visualise, is that sitting in a hull in the water does not produce sag. It would if you were on dry land, and only had the bow and stern suspended, but in water, the bouyancy under your bum prevents the middle of the keel trying to 'sag'.

    The only sag you get is if a wave tries to lift the bow or stern, and bend the hull from the weight in the centre.
     
  7. NoEyeDeer
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    NoEyeDeer Senior Member

    Nope. I was asking about the framework.

    Ok, cool. That's what I wanted to know. Thanks. :)

    So what about getting the smoothest possible surface finish? Any tips on that?
     
  8. Submarine Tom

    Submarine Tom Previous Member

    Ya, use a lot of frames...

    -Tom
     
  9. rwatson
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    rwatson Senior Member

    If you have to use fabric, use the smoothest fabric :)

    But that might not be the lightest, or strong enough.

    Thick nylon might be the best compromise.
     

  10. NoEyeDeer
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    NoEyeDeer Senior Member

    Polyester seems a better option, judging by all the swotting up I've done. As I understand it, nylon tends to lose tension easily.
     
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