front rudder

Discussion in 'Boat Design' started by rosbullterrier, Apr 13, 2013.

  1. rosbullterrier
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    rosbullterrier Junior Member

    I've looked in 'searches' for thoughts on this with no result.
    Why is boat steering not at the front?

    Now I realize the answer is probably blindingly obvious to all you knowledgeables but to me, just having rebuilt my twin outdrive 26' boat which steers like a jittery rear wheel drive dumper truck, I wonder why rudders are always at the back.

    Should there be a concern about vulnerability at the front - I'm sure the rudder shape could be sufficiently streamlined to shrug off the odd log.
    What about two front rudders?

    If this was feasible, rear propulsion and front steering (for a displacement vessel / catamaran) would surely allow more stable progression.

    Thinking, I now realize why an outdrive planing boat steers better on the plane!
     
    Last edited: Apr 13, 2013
  2. michael pierzga
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    michael pierzga Senior Member

    Its been done many times. The Americas cup boat was noteworthy

    Obviously its vulnerable. Docking and maneuvering in close would be improved.

    Since Ive never sailed a boat with a forward rudder , I dont know the defects.

    Their must be something .
     
  3. lucdekeyser
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    lucdekeyser Senior Member

    front and back rudder

    In this video of the Harryproa "Blind Date" sailing you see front rudder steering at 45" and 17' and back rudder steering from 2' on. As the proa is fore aft symmetrical it is easy to experiment without refitting for either manner of steering as you would need to in any other boat. I have the impression that the best sailors (and the best looking ones ;-) steer with the front rudder. The difference may be due to the left right asymmetry though and may not extrapolate to any other type of boat.
    Luc
     
  4. Manfred.pech
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    Manfred.pech Senior Member

  5. jehardiman
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    jehardiman Senior Member

    Why no front rudders? In general they are less efficient than stern rudders. In operation rudders do two things, Cause drag and generate a turning moment. A bow rudder in causing drag destablizes the hull (i.e. lowers the stability deratives), which makes steering more frequent therby causing more drag because the rudder has to be turned more, generaly against the induced turn which is wasteful. In generating turning moment a fore rudder becomes uncontrollable because as the bow swings into the turn the AoA increases proportional to the rate of turn (do the vector math) meaning more steering control is needed.

    An aft rudder not only stablizes the hull, but is far more consistant in steering input required.

    Edit to add. Are you having to correct because the head is wandering or is it that when you turn the wheel the control is immediate and forceful. The first is caused by other issues than the rudder, the second could be caused by several reasons related to the refit.
     
    Last edited: Apr 14, 2013
  6. Milehog
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    Milehog Clever Quip

    Do the drives an appropriate amount of toe-in?
     
  7. rosbullterrier
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    rosbullterrier Junior Member

    Thank you for your information, jehardiman. My boat does not have rudders it is a twin outdrive.
    My thoughts on the matter were brought about by the behaviour of my power boat which I have have just completed a long rebuild and have no experience; but I would like to build a different and competitive hull for our annual Seagull race! To fix twin Seagull outboards and steer separately would help the mechanical design - but it appears not the hull design!
    I have altered the toe in and out Milehog but without appreciable difference.
    After reading that these outdrives have natural fidgety steering which need continous correction at slow speeds I shall learn to live with it . . .
     
  8. gonzo
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    gonzo Senior Member

    If your boat steers so badly there are problems with the setup or the hull and not the basic system. There are several things to look into: Drive toe-in alignment, steering parts having too much wear and slack, drives trimmed unevenly, power steering valves sticking, etc.
     
  9. Milehog
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    Milehog Clever Quip

    Some deep V hulls are cranky at slow speed regardless of setup. Something to do with shedding alternating vortices.
     
  10. gonzo
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    gonzo Senior Member

    He didn't indicate at what speed, so I assumed it was wear&tear and maybe adjustment.
     
  11. Milehog
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    Milehog Clever Quip

    Hard to diagnose without more details so we just toss stuff out there.
     
  12. jehardiman
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    jehardiman Senior Member

    Sorry, missed the outdrive part, but realisticlly, the outdrive lower leg is effectively a rudder as far as the stabillity derivatives are concerned. While outdrives steer better at low speed (one of the reasons they were invented), a boat that is constantly needing correction, especially at low speed, may be a trim issue. Try moving weight aft to get the bow up and Center of Lateral Resistance aft. Depending on the hull type, some planing boats can never have the weight too far aft for good tracking.
     
  13. fredrosse
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    fredrosse USACE Steam

    The sidewheeler I built has twin stern rudders that fold up, and a portable bow rudder so I can go ashore stern-first, and beach the boat. Docking a sidewheeler is often difficult, but beaching this way allows exiting ashore without even getting your feet wet. See youtube "sidewheel steamboat"

    The bow rudder can also be used to steer the boat going forward from the bow seat, very popular for young passengers. It works OK either way, but is usually stowed during normal cruising.
     
  14. rosbullterrier
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    rosbullterrier Junior Member

    Thank you for the further information with steering which I shall experiment with.
    The steering has no play and I have changed the toe in in small increments. The steering isn't that bad - just needs more continuous correction than a novice would expect. As Milehog would concur with previous advice I have read - at low speed these can be other than hands free.
    It's a deep vee, 8 metre twin outdrive - the speed in question under 10 knots.
    ( and shudder to mention but it has the OMC hydraulic stringer engine/outdrive trim system)

    But this concern was only a by the way in the question on the usefulness of a bow rudder in the design of a small lightweight Seagull racer!
    And you don't think the ideas up to much . . .
     

  15. DennisRB
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    DennisRB Senior Member

    Also think of how a dart or arrow flies. The fins or feathers are at the back. Putting those surfaces at the front would lead to an unstable form requiring constant immediate adjustment (impossible) to fly straight. Same would happen with a boat. It would want to turn around constantly and broaching would be a huge issue due the the CLR being way forward.

    The high performance racing sailboats with forward rudders still have a balanced CLR when you include the keel and rear rudder they normally also have. There is no such thing as a sailing boat with the only foil being a forward mounted rudder. I haven't even seen one with a keel and single forward rudder although I am sure it has been tried. The keel would have to be way back to compensate. Usually the forward mounted rudder is there instead of the keel, as the keel has usually given away its duty as a leeway preventer by becoming super high aspect and canting to windward.

    Since powerboats don't usually have keels they can move way back to keep a balanced CLR, a forward mounted rudder would be a broaching disaster in anything but a flat sea.
     
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