rudder shaft help

Discussion in 'Boat Design' started by phum, May 20, 2013.

  1. phum
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    phum Junior Member

    Hello all,
    I am doing a refit including a new rudder.
    The old shaft is 60mm by 3mm wall ss tube.
    I have had no trouble with this setup but I am told its to light.
    Boat is 8Mts and 1300 Kgs and rudder size is .3sq. Mt approx.
    What size should it be?
    Thanks,
    Peter
     
  2. PAR
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    PAR Yacht Designer/Builder

    Without a fair bit more information about your boat and it's intended operational area/conditions, any suggestions would be just a guess. Can you post some pictures of the boat? A 2 3/8" shaft with an 1/8" wall is pretty stout on a 1.5 ton 26' boat, that I'll assume it's a sailboat, so additional shaft diameter probably isn't an option. This means if you want stouter, step up to a 1/4" (6 mm) wall stainless tube (not pipe). 304 in fresh, 316 or 316L (preferably) in salt or brackish.
     
  3. TANSL
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    TANSL Senior Member

  4. daiquiri
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    daiquiri Engineering and Design

    And also:
    - the boat type (power, sail?)
    - the rudder type (spade, skeg, transom-hung?)
    - the rudder profile (flat plate, foil section, schilling or else?).
    Cheers
     
  5. PAR
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    PAR Yacht Designer/Builder

    Agreed, I'm making assumptions based of the general size of this rudder, it's shaft and the boat's length. I'm reasonably confidant it's a sailboat, as even a displacement powerboat (of this length) wouldn't have a rudder this big. A transom hung rudder wouldn't need a shaft, so I'm guessing it's under belly and through a port and probably a spade, though could be on a skeg. Considering the shaft diameter, it's probably not a plate, though it could be a schilling.
     
  6. phum
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    phum Junior Member

    sail
    spade
    foil

    Quarter tonner

    I would actually like to decrease the shaft diameter to get a better rudder profile

    Peter
     
  7. TANSL
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    TANSL Senior Member

    Sorry for the, probably, stupid question but could you do the contrary: change the blade profile (NACA or similar) and keep the shaft diameter?
     
  8. phum
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    phum Junior Member

    Blade is already an approx naca00 profile I would like to bring the percentage down to 10 or 12 %

    Peter
     
  9. TANSL
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    TANSL Senior Member

    I see. But reducing the shaft diameter can be tricky.
    Just curious, what advantages do you get the finest blade?.
    If the blade is designed well, there should not be advantages to change.
     
  10. PAR
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    PAR Yacht Designer/Builder

    Post a photo of your setup or tell us which 1/4 tonner you have and if it's the stock rudder.

    You can decrease the shaft diameter, but you can run into issues. Is there an armature attached to the shaft (within the blade)?
     
  11. phum
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    phum Junior Member

    Its a Mull, don't know the history of the rudder, Its over 30 years old.
    I will build a new blade and shaft.
     
  12. daiquiri
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    daiquiri Engineering and Design

    Your shaft dimension (60x3 mm) sound about right to me, based on general data you have provided. The diameter could be decreased by using a round solid bar instead of a hollow tube, but the weight of the rudder stock will significantly increase (by 3-4 times the weight of a hollow tube).
     
  13. sean9c
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    sean9c Senior Member

    Shouldn't be a problem getting the required strength and going to a smaller diameter. You will have to go thicker wall or possibly even a solid bar.
    Here are the formulas required to calc the loads and strengths. Start on about page 80. The formulas are pretty conservative but it's a place to start.
    Dave Gerr also has the required formulas in one of his books and they were reprinted in Professional Boatbuilder
     

    Attached Files:

  14. PAR
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    PAR Yacht Designer/Builder

    Is this the early "Ranger 26" version (1968 - 1975):

    [​IMG]


    or is the fairly rare "Ranger 26-2" version (1980 - 1982)

    [​IMG]

    The early version was known to have issues with compression under the mast, over the forward bulkhead.
     

  15. phum
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    phum Junior Member

    neither of the above although similar to 2. Its a Sonata produced in Australia
     
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