Holes in rudder?

Discussion in 'Sailboats' started by michigangeorge, Sep 24, 2012.

  1. michigangeorge
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    michigangeorge Junior Member

    Thanks for the interesting link, daiquiri :)
     
  2. michigangeorge
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    michigangeorge Junior Member

    Harry, are you defining the modern era as post 1900? All the drawings I've seen show junks with reasonably streamlined hulls. They may not have been faired to the degree we've come to accept today - but they were not scows nor do I remember seeing the old "cods head, mackerel tail" shape either.
    Are you referring to the crudeness of construction and fit of planking?
    Thanks
     
  3. Petros
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    Petros Senior Member

    it is possible that the diamond shaped holes will energize the flow on the rudder and make it more effective, kind of like putting vortex generators on the rudder of a an airplane that has a rudder that is too small and ineffective.

    It is not efficient, the energy to create the vortexes comes from the forward motion, but it will make the rudder more effective. The vortexes keep the flow attached to the surface in an otherwise rough and separated flow field.

    Likely in the case of very large junks, they are willing to trade off a little drag so they can maintain steering control, especially at lower hull speeds.

    It is always better though to have a streamlined and more efficient rudder than to have to use vortex generators to make it more effective.
     
  4. daiquiri
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    daiquiri Engineering and Design

    Sorry, but I disagree. Holes would just decrease lift and increase drag by allowing the flow to bleed from the pressure to the suction side of an airfoil, and by creating localized vorticity. Are your claims based on some test report or research I might not be aware of?
    Think this way: why doesn't Boeing or Airbus use diamond or whatever-shaped holes on their airliners' wings or rudders? If it could save just one glass of fuel during each trip, be sure they would embrace the concept immediately. If you are certain of this beneficial effect of holes on lifting surfaces, it could be your good opportunity to become a very rich man. ;)
    Cheers
     
  5. yipster
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    yipster designer

    [​IMG] here's some more on those trepanated chinese rudders, dont look like they did it for steps tho


    and sorry daiquiri, but i find its increase lift and increase drag
    here a foil simulator to compare Cl and Cd on various foils and with cord thickness down on a flat plate but unfortunatly there is no way we can introduce holes
    but planty online on slotted wings although normally we dont see holes in wings yet fixed slots do direct a flow to the lower presure foil surface and delay flow separation at higher angles of attack
    and allow a higher lift because the stall is delayed. Downside is that a foil with a higher angle of attack will obviously gain more drag why it is only used in special conditions
     
  6. daiquiri
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    daiquiri Engineering and Design

    I don't think you can compare airfoils with slot-flap to an airfoil with holes. They have different types of airflows. Slotted flap have areas of 2D and 3D flows, whilst the flow over a rudder like the one in the previous picture is fully 3D. Hence their effects will necessarily be different too.

    Imo, if we want to search for the analogies, a porous flat-plate could be the closest match to this type of lifting body. Perhaps there are some research papers in internet about the aerodynamics of porous flat plates, but right now I don't have time to search for them. Perhaps you could... ;)

    Cheers
     
  7. Harry Josey
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    Harry Josey Junior Member

    1900 is a good date for the modern era. The automobile had just hit the streets, flight was just around the corner and the I.C. engine was ready to take over from steam. W.W.1 is abetter date. The basic components were in place and the world took off at a run and hasn't slowed since.
    I probably used the word streamlined injudiciously. The chinese are very pragmatic. While the underwater lines are quite clean, they are designed for the speed the craft can be expected to maintain. Not what it might hopefully achieve with full sail in a class5 hurricane.
    The number of holes in the rudder shown by Yipster seems a bit extreme
    but when you look at the shape of that stern maybe extreme measures were called for.
     
  8. rwatson
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    rwatson Senior Member

    "A sizable junk can have a rudder that needs up to three members of the crew to control in strong weather helm. ...... From sometime in the 13th to 15th centuries, many junks began incorporating "fenestrated" rudders (rudders with large diamond-shaped holes in them), probably adopted to lessen the force needed to direct the steering of the rudder. "

    http://en.wikipedia.org/wiki/Junk_(ship)

    Any thought that the square edged, diamond shaped holes contribute any useful flow patterns has to be the height of engineering fantasy.
     
  9. michigangeorge
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    michigangeorge Junior Member

     
  10. yipster
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    yipster designer

  11. rwatson
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    rwatson Senior Member

    The concept that the junk rudder fenestration is related to any aerodynamic principle is not at all apparent in any of those links.

    Its not possible that sharp angular holes and geometric shapes contribute in any hydrodynamic principle like the examples you present here.
     
  12. sharpii2
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    sharpii2 Senior Member

    I still think the holes are to bleed off shock loads.

    Yes, some of the high pressure leaks through them to the low pressure side, but maybe not as much as one may think. Particularly, if the rudder has some thickness and the holes are cut straight through.

    In order for the water to flow through the hole, when it is traveling across the rudder in an oblique angle, it has to make a sudden turn.

    I imagine some of it does, but most of it doesn't.

    Now, imagine the rudder turned at a 90 degree angle to the flow. Now almost all the water encountering a hole, flows right through it.

    Another way to imagine it is to visualize the rudder being used as a table and trying to mound as much dry sand on top of it as possible.

    With the hole less rudder, a sizable mound will be possible, with it's highest point somewhere near the rudder's Center of Area (CA).

    Now, take the rudder with holes in it and do the same. Even though it has maybe 85% as much area as the hole less one, I bet you won't be able to get nearly as big of a mound on it.

    This is because, as my thinking goes, the holes have created many smaller unbroken areas in the rudder. Each will hold a mound with a height that is proportionate to that area's smallest dimension. A whole lot less sand.

    It is my personal theory that a shock load, such as the boat getting hit by a confused sea, is similar in its proportions to the mound of sand.
     
    Last edited: Sep 30, 2012
  13. michigangeorge
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    michigangeorge Junior Member

    Interesting thought and one I am in agreement with sharpii2:idea:
     
  14. rwatson
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    rwatson Senior Member

    Thats essentially what the article in Wikipedia is saying.

    The square cut holes would not make the rudder innefective at normal operating angles, as the flow would not attempt to pass through the gaps. Who knows, the turbulence inside the holes might even act as a solid surface to some extent.

    But a big sea coming up behind, especially from one side or the other, would create a potential safety hazard, trying to drive the tiller through the helmsman.

    The big holes would let the deluge through, and ease the pressure dramatically.
     

  15. upchurchmr
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    upchurchmr Senior Member

    To get back to the original question.
    How about attaching a hinged step to the transom, hinges as far down as possible. Hinged in order to allow it to be pulled down when you need to reboard. The step would be below the hull when deployed, allowing you to get an easier first step, similar to the original question. Once you reboard, a lanyard would allow it to be pulled back up to the stowed position. A manual latch would be required to keep it up of course.

    Now you don't need to penetrate, fenestrate (is that a word?), or otherwise change (worsen) the poor hydrodynamics of a barn door rudder.

    Just a separate question. Why not make the barn door with a NACA foil shape to give better control? I am assuming the old shape is being kept just in honor of ancient experts?
     
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