Rudder shape

Discussion in 'Hydrodynamics and Aerodynamics' started by 23feet, Oct 9, 2015.

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

    Hi Folks,

    My rudders have to come off for maintenance. [​IMG] This could give me an opportunity to do something about the design compromises in the rudders. Given that I don't want to get into reshaping the skeg, could there be any benefit to giving the rudders a foil shape? Or is this a waste of time given the large gap between skeg and rudders, and that the skegs are too big and will mess up the water flow over the rudders?

    Thanks,
    Roger
     
  2. 23feet
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    23feet Junior Member

  3. PAR
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    PAR Yacht Designer/Builder

    New rudder blades could be shaped to close the gap and with some skeg shaping, you could get some foil shape incorporated as well. Typically the skeg and blade are both part of the foil sectional shape, with the skeg forming the leading edge and the blade forming the trailing edge. If the skeg isn't well shaped, some modest gains can be seen with a "fined" down trailing edge.

    Given their fairly low aspect ratio, I don't think much can be done for these puppies, in terms of big improvements, though some turbulence and drag could be eased with a tapered trailing edge. You could minimize the amount of skeg shaping with a "slab sided" foil section. This would have you only radius the leading edge, which is simple enough, then the new tapered blade. It'll perform nearly as well as a real NACA 00 section, but for a lot less effort.
     
  4. Oleboynow

    Oleboynow Previous Member

    There is one thing you could do
    Hinge a new high aspect rudder
    That is all you could do with your shallow draft
    A deep span with lesser chord would change the helm completely
    If you have time
     
  5. 23feet
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    23feet Junior Member

    By "slab sided" foil section, you mean increase the thickness of the skeg, and taper it back through the rudder?

    Thanks

     
  6. PAR
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    PAR Yacht Designer/Builder

    No, a slab sided foil is a flat sided foil section, with an rounded or ellipse shaped leading edge and a tapered trailing edge. I use this on some cruising designs, because they're much easier and much less precise than NACA sections, yet have similar attributes, compared to a 00 series of roughly the same planform and width (see attached). At displacement speeds, these slab sided foils are nearly as effective as the NACA counterparts.

    I'd agree with Ole inasmuch as it wouldn't be difficult to make a new, much more efficient set of rudders for that boat. I'd use a kickup or cassette style of assembly, to retain the shoal nature, yet dramatically increase their aspect ratio, planform and sectional dimensions to get some efficiency, more effective lateral area and helm balance/feel too.
     

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  7. 23feet
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    23feet Junior Member

    With a high aspect rudder blade, one would remove the skeg entirely - right?
     
  8. PAR
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    PAR Yacht Designer/Builder

    You could, though it's not entirely necessary. I'll suspect there's a fair bit of lateral area incorporated into the skeg, so if you don't want the helm to go lee on you, the skeg should remain. This said, it certainly could be reshaped for less drag and retain the lateral area, particularly if the new blades picks up area.

    When making these types of changes, it's important to make a good assessment of the steering qualities, so the new design elements can address them. Was the helm heavy in 10 knots on a close hauled course? How much rudder deflection (degrees) was necessary, to hold this course? Etc., etc., etc.

    Something like attached would be a big improvement, retaining the shoal nature and offering a touch of balance too. Proportionally, it's not right, but you get the idea with this 5 minute sketch.
     

    Attached Files:

  9. 23feet
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    23feet Junior Member

    PAR, Thank you for taking the time to do this. I also have your other image of a generic folding rudder design and it all looks quite doable.
    Roger
     

  10. 23feet
    Joined: Dec 2014
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    23feet Junior Member

    Another question about this design. The boat is a cat and is often sailing up to 2x hull speed when reaching. The existing skeg and rudder are very slim - about 5/8". If I have the "new" rudder case extending to the bottom of the current skeg it will be causing a lot of drag, no? Perhaps have the case ending at the waterline, and only the rudder blade folding down against the existing skeg?

    Thanks,
    Roger

     
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