Waterline length includes the rudder? Displacement speed.

Discussion in 'Hydrodynamics and Aerodynamics' started by 23feet, Oct 19, 2020.

  1. 23feet
    Joined: Dec 2014
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    23feet Junior Member

    In a canoe-sterned boat with the rudder extending from the stern. Calculating displacement speed, does the waterline length include the rudder, or is it only the part of the hull that is displacing a chunk of water?
     
  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    In the physical world a thin rudder will have very little to know impact on wave making resistance. An exception would be a sailboat with a considerable side force on the rudder. Keep in mind that the "displacement speed" is just an estimate (based on physics) of the speed at which the wavemaking resistance increases rapidly with speed. Nothing dramatic or even noticeable happens at a particular speed.
     
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  3. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Many traditional craft did exploit the transom-hung rudder to steal a bit of extra length. These included pinkys and cat boats from a century or two ago. But to do so effectively, you have to have a nice smooth curve of displacement that blends in the rudder volume. So to make this work decently, you need a keel or skeg extending to the transom to carry the displacement curve out to the rudder.

    Here's a pinky that's working it.
    [​IMG]
     
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  4. fredrosse
    Joined: Jan 2005
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    fredrosse USACE Steam

    "In a canoe-sterned boat with the rudder extending from the stern. Calculating displacement speed, does the waterline length include the rudder, or is it only the part of the hull that is displacing a chunk of water?"

    In summary of what is presented in the last couple of posts, if the rudder is contributing only a very small amount of buoyancy displacement, then it does not add any effective length to the hull, nor to the displacement speed calculation. A thin fin rudder extending beyond the stern does nothing with respect to displacement hull speed.
     
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