Balanced rudder with full skeg?

Discussion in 'Boat Design' started by richp10, Oct 10, 2018.

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  1. richp10
    Joined: Apr 2018
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    richp10 Junior Member

    Hi, I need to rebuild my rudder and skeg and was considering changes. Currently it is a half skeg with a horn balanced rudder. I *might* just rebuild as is, but I would prefer a full skeg that is much harder to foul on lobster pots. (The hinge between skeg and balancing portion of rudder creates a gap that any rope that the boat runs over will slide into).

    Does anyone have any pictures - or experience of or comments on - a full skeg *with* a balanced rudder? So as the rudder turns it creates a slot between the rudder and skeg because the rudder chord is astern of the skeg.
     
  2. Blueknarr
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    Blueknarr Senior Member


    Welcome to the forum.

    Unfortunately full skegs can't hold balancing rudder. For a rudder to be balanced, a portion is it must project forward of its pivot.. Where the max cord is doesn't affect the balance. Balance is the ratio of area for and aft or pivot.

    Sorry to disappoint.
     
  3. JSL
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    JSL Senior Member

    Rich
    perhaps a photo of what you have would help everyone better understand what you have.
     
  4. BlueBell
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    BlueBell Ahhhhh...

    I would think both possible if a rebuild is in store.
    A full keel with a no-snag, balanced rudder.
    However, I have no knowledge or experience with either.
    Good luck... do the rebuild.
     
  5. richp10
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    richp10 Junior Member

    Here is what I have right now. At very least I could make the gap between the skeg and balance horn smaller - but I wonder if there was a way of making a 'nose' or slight overlap so that hitting a rope would tend to slide down and off rather than into the cap. Thoughts?

    upload_2018-10-10_19-2-25.png
     
  6. tspeer
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    tspeer Senior Member

    What you need is a shielded horn rudder. Like the horizontal tail in this illustration:
    [​IMG]
    You could add to the leading edge of your skeg so it extends past the gap in the horn. That would make it harder for ropes to get into the gap.
     
  7. richp10
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    richp10 Junior Member

    Yes, that's the kind of thing I was thinking of.. thanks!

    So the balancing section will still work, yes - even though it is creating a 'slot' when turned?

    Has anyone seen pictures of this on a yacht - it seems an obvious thing to do but I have not come across examples when googling around.
     
  8. Blueknarr
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    Blueknarr Senior Member

    The balancing section still works. It will still catch enough water to relieve helm pressure. The horn will still stick out enough to catch lines etc as well. Only now it can't return to neutral position.when lines snag between it and the skeg extension.
     
    Last edited: Oct 10, 2018
  9. richp10
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    richp10 Junior Member

    Not on a boat, but this is an example of a full skeg with balanced rudder:

    [​IMG]I would have thought this would behave quite differently to a standard horn balance as I have now.. the balancing section would be hidden behind the skeg until maybe 10% at which point there would be flow through the slot. Maybe it would work but I can't find any evidence or recommendations for this approach. Current design with maybe a nose on skeg to shield the hinge gap would be safer bet.
     
  10. tspeer
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    tspeer Senior Member

    Yes, it will. There's a suction peak that forms on the leading edge of the horn as it emerges from behind the skeg, and this creates a moment that opposes the hinge moment from the rest of the rudder. It may not provide as much of a balance as the exposed horn, but there will be some balancing provided.
     
  11. tspeer
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    tspeer Senior Member

    The area shown is probably for a mass balance, because the area is too small to provide an appreciable aerodynamic balance. It's important for control surfaces on aircraft to have their centers of gravity on or ahead of the hinge line to prevent flutter.

    I think what you're looking for is an overhang balance:
    [​IMG]
    When the rudder is deflected, there's a pressure peak formed on the leading edge that balances the load aft of the hinge line. There are a couple of ways you can go with this. One way is to have a comparatively blunt leading edge with rounded corners. The rounded corners stick out into the flow when the rudder is deflected. The other way you can go is with a more elliptical leading edge that mostly stays behind the trailing edge of the skeg.

    Hinge moments are notoriously difficult to predict accurately, so you will need to do some experimentation to get the right balance for both small and large deflections. You may want to design your rudder and skeg so you have a choice of hinge locations so as to be able to vary the amount of overhang.

    Here are some relevant NACA reports: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092895.pdf, http://www.dtic.mil/dtic/tr/fulltext/u2/a801276.pdf
     
  12. JSL
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    JSL Senior Member

    That is for an aircraft and air is a lot less dense than water. It could work but a lobster trap rope could still foul it.
    This is one reason (the other being hydrodynamics) why have done non balanced rudder/skeg as shown . This 44' steel boat has sailed Washington, BC , New England states, and around the world and as far as I know there has never been a serious fouling problem.
     

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    Last edited: Oct 11, 2018
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  13. richp10
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    richp10 Junior Member

    >> I think what you're looking for is an overhang balance:

    Yes, thanks for that.. you are right. I was aware the area needed to be bigger but your answer fleshes that out.

    Clearly a full skeg, fully balanced rudder can be achieved, though as it will be hard to predict how it will behave I remain uncertain as to whether it would be wise! I will dig into those reports, though they are certainly challenging!
     
  14. richp10
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    richp10 Junior Member

    Yes, this seems to be the safest design - *as long as* you can do without balanced rudder. Am curious, do you have tiller or wheel steering as unbalanced rudder seems more realistic on wheel.
     

  15. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    I thought you wanted a full keel with the rudder built into it.
    This would have been a major design change and would have needed a professional involved.
    That would have kept any tangles out of the rudder.
     
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