Balanced rudder with full skeg?

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

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  1. richp10
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    richp10 Junior Member

    No intention of changing the keel - question was about the skeg / rudder because these need rebuilding anyway due to corrosion - so the opportunity presents to tweak and maybe improve the design.
     
  2. Blueknarr
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    Blueknarr Senior Member

    I noticed there was quite the gap between the bottom of skeg and top of horn. You might try to close the gap so that nothing fit.
     
  3. JSL
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    JSL Senior Member

    Closing the gap might work if the rudder was kept 'straight' .
    I saw one fouling case where the skipper turned last minute to avoid the trap and the 'balance' portion was 'open' and the rudder 'hooked' the trap line.
     
  4. MikeJohns
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    MikeJohns Senior Member


    Yes. With a full skeg for a balanced rudder the skeg is a foil in it's own right and supports the heel bearing. Same as adding a balanced rudder behind a full keel, the gap needs to be around 30% or greater of the foil chord length ( rudder width) . There are a myriad of ways of designing the heel bearing support strut. I've designed a few that way, but more commonly the changes are to actually change an attached rudder to the design you currently have which is the best compromise of strength and balance.

    You can add a bit of rod as a line skid that attaches to the skeg, overlapping the hinge gap. But you really need a significant sweep in the LE of the whole assembly to shed lines.
    The better solution commonly applied is fitted from the keel to an articulating fitting on the hinge line at the bottom of the rudder. Small diameter steel rod, chain and wire rope are usually used .
     
    Last edited: Oct 13, 2018
  5. richp10
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    richp10 Junior Member

    Thanks Mike. This really seems to cover the ideal solution.

    I don't suppose you have any photographs of examples of a balanced rudder behind full skeg or full keel? I think I understand what you are describing but a picture would help me be sure.
     
  6. SamSam
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    SamSam Senior Member

  7. MikeJohns
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    MikeJohns Senior Member

    Line guard general arrangement shown on a semi-skeg balanced rudder.

    As for balanced sailboat rudder supported by a full skeg: it's the same as any fully skeg supported rudder except the heel support is extended to create a gap of at least 30% of the rudders width between the leading edge of the rudder and the trailing edge of the skeg or full keel.
     

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  8. Blueknarr
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    Blueknarr Senior Member

    Just for my education. How does adding a line prevent snags on the forward projecting balancing portion of the rudder?
    How do you keep snags from wrapping around the new line?
    Kind of defeats the shedding angle on the leading edge of skeg.
     
  9. MikeJohns
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    MikeJohns Senior Member



    It definitely works when overrunning a floating pot line or a set net and keeps ropes and lines out of the prop and rudder.

    A pot with a buoyed line has an anchor of sorts, a float and from the hull/keel enough friction to straighten the line up before it starts to drag past the hull. The float usually passes in contact with the vessel side or even partially submerged. It's the float that has the buoyancy to pull the line up quite quickly especially if partially submerged and can then lodge in the prop or the rudder. A line fitted apparently works well although I don't use it on my own boats.

    As for the semi balanced partial skeg rudder picking up lines, it might at full articulation of 35 degrees, but at 10 degrees or under with a NACA 0012 type section it's unlikely as there's too little portion ahead.

    The partial skeg in my drawing isn't sloped to shed anything, just to give a better planform.
     
  10. richp10
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    richp10 Junior Member

    Very useful again, thanks. It had nor occurred to me I could do the keel cable from a balanced / horn rudder - but yes, if you take it from the line of the rudder stock it should work.
     
  11. BigCat1950
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    BigCat1950 Junior Member

    I sailed many offshore miles with a balanced rudder hung from a full keel, with a bronze heel fitting to support the rudder shaft at the bottom of the rudder from the long keel, which was about 40% of the DWL length on the keel bottom. The rudder shaft was raked, and the prop was between the keel and the rudder. You could separate a full skeg from your rudder, make a good sized gap between them, and get a similar effect. 40 years and many, many offshore miles later, the arrangement is still going strong.
     
  12. Magnus W
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    Magnus W Junior Member

    Not that I know anything about hydrodynamics but consider the function of the MD80 stabiliser. It's not actuated by a rod, wire or hydraulics but rather by the trim tab. As in move the tab in one direction and the stabiliser will go the other way.

    The same way you could possibly design a flaps rudder but having the flap going the opposite way of the rudder. This could be done mechanically and when calibrated it should balance the rudder nicely without risking the horn snagging in a line. Perhaps not the solution for a high speed shaft drive boat but in your case, judging by the pics which leads me to assume you're not precisely that, it could be an interesting solution to you problem.
     

  13. JSL
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    JSL Senior Member

    The 'servo' arrangement you describe have been used on wind vane steering (sail boats) for many (40+) years . A 'flapped' rudder (articulated rudder) is used on many commercial boats (tugs, etc) and is similar to, and does the same job, as flaps on an aircraft wing... provide more lift.
    An alternative high-lift rudder is the Schilling Rudder.... not quite as much lift perhaps, but much simpler as there are no linkages or moving parts, other than the rudder itself.
     
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