Tortured Plywood - Design & Construction

Discussion in 'Boat Design' started by b_rodwell, Mar 10, 2006.

  1. mattotoole
    Joined: Nov 2004
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    mattotoole Senior Member

    There's quite a bit of friction (hysterisis losses) within belts too, which is why they're not used for bicycles (where power is relatively low and every erg counts). Chains are still more efficient, twisted or not. Modern bicycle chains are somewhat flexible to accomodate 9 and 10 speed gear clusters, so with enough length they'll twist OK too. Sure there's more friction but I bet it's still less than with a belt.

    Also, bicycle chains, sprockets, and other hardware are widely available and inexpensive, and a working unit can be assembled with minimal machining or other custom work.

    I do wonder how a twisted chain compares efficiency wise to a 90 deg gearbox though.

    I also wonder how close that model airplane propeller comes to a purpose-built one (which would be many times more expensive).
  2. BG_Geno
    Joined: May 2006
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    BG_Geno Senior Member

    If you dont mind my asking, how much was that? They don't list prices on their site. Also, weight?

  3. SolomonGrundy
    Joined: Feb 2005
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    SolomonGrundy I'm not crazy...

    I picked mine up surplus for $50, it weighs about 3.5 lbs.
  4. b_rodwell
    Joined: Apr 2002
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    Location: Sydney, Australia

    b_rodwell Junior Member

    On the drive unit:

    It was helpful getting feedback on the suppliers of angle drives. My latest thoughts are not to use an angle drive for the following reasons:

    For a suitable three way drive, one supplier quoted US$350. This seems
    a bit expensive.
    I would expect a bevel drive to be less efficient than a twisted chain drive.
    I would like to have a 4:1 amplification. The most I have seen with bevel
    drives is 2:1

    My latest preference is the twisted chain drive unit you can buy from China for around $250:

    I would be interested in critical comments about my thoughts and any experience in using this unit.

    On CAD:

    I have been able to use Rhino to derive the shape to cut the plywood. It uses some of the techniques previously described but with 2 additional steps. It would not unroll for me without the additional steps.
    1. Slice the hull into segments using an athwartship plane.
    2. Define the edges of each segment. (additional step 1)
    3. Contruct a surface from the edges. (additional step 2)
    4. Unroll the new surface.
    5. Layout all the surfaces aligning the corners.

    For most of the hull I was able to take slices 500mm apart. However near the bow and stern, it would not unroll untill I made it finer slices. The smallest slice was 20mm.

  5. BG_Geno
    Joined: May 2006
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    Location: South Texas

    BG_Geno Senior Member

    the angle drives can be had much cheaper, and you don't use the angle drive for your gearing. Get a 1:1 angle drive, and use the sprockets (chain ring in the crank and freewheel on the angle drive axel) for any ratio you like, including 4:1 you mention. Twisted chain is a some what problematic solution from everything I have read. They often have derailing issues etc...some of the best boats I have seen, including long distance endurance type boats use bevel gear drives.

    as for paying $250 plus shipping for that twisted chain unit...those are very easy to make for $50...plans can be found with materials list and instructions many places on the net.

  6. ThomD
    Joined: Mar 2009
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    ThomD Senior Member

    You can use CAD to unroll the surface, rolling the surface up is a different mater. The easiest way to predict that is with a model that takes an hour or so to build. It is hardly worth the trouble of designing such boats. The keel for a certain type is going to be a straight line with some curvature towards the for ward and aft (some examples are in the G book). Other than that the keel is frozen stitch and glue style and you get a deck jig. You can experiment with sellective surface stiffening, but it is hardly worth it. Do it on a model, build the model, get the hydro off the model, then continue your build. It is hardly worth the trouble of designing it formally.

    Possibly the most active user of this tech is Kurt Hughes, and no doubt he knows exactly where he is headed when he draws a design. He was also an early adopter of CAD, but for a long time the boats were designed in CAD,and the hulls were designed from experience, the overlap bordered on coincidence. Not that they didn't match, but they didn't match because one led to the other, you had to have Kurt in the middle. If anyone could have done it...
  7. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Well I have tried to write this twice already and messed it up. So this will be short.

    This thread has lots about pedal power: There is a well proven design for a right angle drive, flexible drive shaft and propeller. There is help with sizing the angle drive, and prop for your boat and weight.

    Think about strip planking. I made the boat pictured (for rowing, but the hull system will work with modification) for my wife. 50# but it could be 35# with 3/16" cedar and 4Oz cloth inside and outside. 11' long was sized for a total weight of 175# - a higher weight would be accomidated by lengthening the hulls. Very low wetted surface and drag (virtually none).
    The required mold was very cheap and simple. Using a system from Bjorn Thomasson would make it quick to make, review the "square edged" strips method.

  8. messabout
    Joined: Jan 2006
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    messabout Senior Member

    If any one is interested, I have a near new right angle gearbox almost exactly like the ones linked in Sol Grundy's post. PM me for dimensional details. It is a 1;1 box from a Small French printing press. Nicely done, plenty strong.

    Elsewhere there is a very long thread about pedal powered boats. It has an enormous amount of information contributed by some really clever guys.. The thread was very active a year or two back.

  9. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Ummmm ... its the one UPchurch put in the previous post :rolleyes:
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