articulating rudder for sailboats

Discussion in 'Hydrodynamics and Aerodynamics' started by seandepagnier, Jul 9, 2021.

  1. seandepagnier
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    seandepagnier Senior Member

    I have found a lot of evidence of these in powerboats to improve turning radius, but I am more interested in on a sailboat to allow using a smaller surface area and therefore lower weight and drag. The idea is also the chord of the foil could be thinner which is also less drag without sacrificing maximum deflection angle before stalling.

    1) would the angle of deflection of the articulating part vary at all depending on speed?
    2) how can I model or simulate the hydrodynamics of this? Suggested free software?
  2. The Q
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    The Q Senior Member

    From what I've seen they have more drag due to the various joints that are required..
  3. seandepagnier
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    seandepagnier Senior Member

    What have you seen?

    Why does adding a joint need to add drag? How can I simulate it?
  4. kapnD
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    kapnD Senior Member

    Articulated rudders are most effective on powerboats, as they utilize to advantage the accelerated prop wash over the rudder.
    I’ve never seen one where the angle was linked to vessel speed, but like an airplanes flaps, could probably be “tuned “
    To some small speed gain at the expense of great complications.
  5. Milehog
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    Milehog Clever Quip

    Is the rudder loaded?
  6. Skyak
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    Skyak Senior Member

    You don't seem to be getting an answer. I think it is because anyone educated enough to know what it takes can see a flaw in your hypothesis. I suggest you look at the operating angle of attack of a racing sailboat rudder and compare an optimization of area and aspect ratio for a single element to anything you hope to achieve with multiple elements.
    1) yes the angle matters but if you don't care about efficiency at low speeds it matters little compared to other problems.
    2) Javafoil is a free program capable of modeling multi-element. There is better free software that is more difficult to understand and use. JF is not good at separation which is likely to be important to your designs viability.
    My approach would be to first find the two foil sections that add up to less drag than the single section at the low angle of attack that sailboat rudders mostly operate in (roughly 5 deg). That is the benefit you hope to carry through. Next, look at the aspect ratio and induced drag when you extrude the two options out. My thought is that at low aspect induced drag is large compared to your profile improvement and at high aspect your two smaller thinner sections are very challenging to make efficient and robust. I don't mean to discourage you (I don't know what you plan) but rather to explain why this path is not common.
  7. CT249
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    CT249 Senior Member

    It was tried on a 12 Metre; France 3, I think. I don't think it's been tried since.

    I struggle to see how some rather complex gearing, flaps and fairing can reduce weight. The flaps may save on area, but that foil area can just be foam and carbon and therefore provide positive buoyancy anyway. Nor does there seem to be much reason why the chord could be thinner, particularly once you allow for the controls and actuation devices.
  8. gonzo
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    gonzo Senior Member

    A thin foil can have more drag than a thicker one depending on the size and relative speed. Also, thin foils will stall earlier than thicker ones. There is a huge amount of data about it. NACA foils are still used; sometimes tweaked a bit.
  9. Earl Boebert
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    Earl Boebert Senior Member

    L. Francis Herreshoff put a flexible rudder on "Live Yankee," which bent in an arc like a fish fin.


  10. upchurchmr
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    upchurchmr Senior Member

    There was an article in the AYRS publication "Rudders".
    A 4 segment rudder was installed on a 30+ foot ocean racer.
    Claimed improvements were that the effort to steer was reduced significantly, allowing the helmsman to steer for 4 hours rather than 30 minutes - if I recall correctly.
    No claim was made for reduced drag that I recall.
    All of the vertical joints were exposed.

    I always wanted to start from there and put something similar on a small boat - like a Tornado catamaran. Sold the cat and never did anything.

    I think the suggestion was not to thin up the section, but to reduce the overall size of the rudder.

    If you compare a foil with no camber to one with camber you might get an idea of what could be done. You would primarily be trying to reduce drag when the rudder is changing course / maneuvering.
    This assumes you would not put extreme curvature into the foil.

    Good luck.
  11. comfisherman
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    comfisherman Senior Member

    West coast is getting a lot of the staumbaugh articulated rudders. Usually the proponents of them are more arduous in their love for the rudders than the bulb guys of a decade ago. It's the same level of zeal as the glazed eye proselytite, apparently they make the boat turn better, lower your taxes, give you a massage, and make you better in bed....

    Local guy got one, and I made the mistake of asking about it in casual conversation. He fired his boat up and began backing around the harbor yelling "look at it turn" as he almost clipped half the boats in the harbor.

    But my day job is chasing fish and my side hustle is finding better ways to do it. So gave it a go for a few days on a boat filling in. Never having run the boat prior to the rudder left me with no much to go off, but I've run enough boats in this class to have a pretty good idea.

    At the slow speeds we operate gear at it did a remarkable job at vectoring thrust. Even over a thruster on the net in some instances. After about 5 knots..... on every boat it didn't appear to do much over any other design. Granted in the industry it's being targeted at we will spend 16 hrs a day making slow turns, so credit where it's due.

    All that to say.... cannot see a practical application for a sailboat outside of super tight harbor maneuvers.
  12. seandepagnier
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    seandepagnier Senior Member

    I think the +- 5 degrees of deflection is optimistic. Sure in flat water it may be true, but the real issue is surfing in waves where you need significantly more rudder power and more deflection as well, and this forces you to have a rudder with several times more drag than what is required in flat water or slower speeds.

    Now, maybe a rudder that can move up and down (be less deep) could be more useful here as it could reduce drag in light wind as well as work in shallow water. I need a way to retract the rudder anyway for this reason. The reduction in drag when surfing is less important since you are already going (too) fast. I do not care about low speed maneuvering as I use a sculling oar which gives me essentially the same control as an outboard that can rotate 90 degrees on each side.

    I am also considering the potential increased drag where the two sections hinge. Not sure how to reduce this. Maybe a skin on frame rudder that can be flexible could work?

    As for the 4 section rudder reducing effort to steer, it sounds more like a trim tab arrangement which moves the section in the opposite direction (thus increasing drag) to greatly reduce effort. Or maybe it just had less area? The rudder will be somewhat balanced to minimize effort.

    As for thin foil stalling earlier than thick ones and giving more drag: This is true, and exactly what the articulating part hopes to improve by allowing larger angle of deflection without stalling with a thin foil.

    I will start with trying to simulate cambered vs non-cambered foils somehow as I think this would be the starting point, and as mentioned it may very well not be worthwhile.

    As for the extra complexity, sure it is there, but this is really trivial to so many other things. I dont have refrigerators, watermakers, engines, radar etc.. all far more complex so I really do not consider it to be the main issue.
  13. Doug Halsey
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    Doug Halsey Senior Member

    Are we really talking about multielement foil sections, or just single element ones with one or more hinges?

    I would like to see some photos or sketches.
  14. tspeer
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    tspeer Senior Member

    Xfoil can be used to model the section characteristics of an articulated rudder that has no slots between the elements. You will still need to calculate the induced drag associated with the planform shape and estimate the wave drag from surface effects.

    Reducing the rudder area will shift the center of lateral resistance forward. This will affect the directional balance of the boat and may require other adjustments to trim properly.

  15. Skyak
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    Skyak Senior Member

    I presumed 2 element because he said "articulating part" and not camber. Now he says camber. That can be studied with simple online tools
    NACA 4 digit airfoil generator (NACA 2412 AIRFOIL) (
    NACA 0015 (naca0015-il) (
    Cutting to the chase, looking at the effects of camber and thickness you will come to the intuition of "thin foil theory".
    Xfoil is good free software to analyze and generate profiles.
    Asymmetric foils can achieve the same lift at lower drag but in the context of a sailboat it is difficult to raise performance in the rudder and easy in the keel or daggerboard because it doesn't have to articulate/reverse and it generates more lift.

    A high aspect 'daggerboard' style rudder that can lift to adjust to power needs would be simple, effective, and very hard to beat on a sailboat.
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