front rudder

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

  1. parkland
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    parkland Senior Member

    I'm not an expert, but I think the major reason the rudder wouldn't work in the front, is because the water in the front is being pushed out of the way, and won't flow past the rudder the right way for it to work.

    I'd think if you had a rudder on the front, but several feet away from the boats hull somehow, I think it might work.
    And by work, I mean you could turn, using it, but still wouldn't work as well as the usual rear rudder.
     
  2. fredrosse
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    fredrosse USACE Steam

    There have been many airplanes that use rear pusher propellers (like boats), and have forward mounted control surfaces. It can work ok either way.

    The forward rudder on my boat has none of the problems mentioned in this thread, works OK going forward or astern.
     
  3. johneck
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    johneck Senior Member

    I have never seen an airplane with a forward mounted rudder. A forward rudder is certainly better for steering, but it creates a destabilizing moment. The vessel will have to be continuously steered to maintain a straight course as jehardiman stated. Vessels are generally designed to have a positive lateral stability, meaning they will continue on a straight line if unpreturbed.
     
  4. parkland
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    parkland Senior Member

    I think it works on airplanes, because the wings go out and up at an angle, so if you push the front of the plane in one direction, the whole plane pushes to the opposite side, and the angle of the wings cause it to start banking in the turning direction.

    In essence, the wings angle causes the entire plane to be sort of self leveling, or self compensating...
     
  5. Petros
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    Petros Senior Member

    front rudders on aircraft are virtually non-existant. Famed designer Burt Rutan tried it once, on the model below. It was to simplify the control runs for simplicity. It had powered controls and to compensate for the destabilizing effect the winglets had to be larger, adding weight and drag. He never designed a front rudder again, and the one that flew (below) is hanging in a museum.

    [​IMG]
     
  6. DennisRB
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    DennisRB Senior Member

    Comparing this to a sailboat you could swap "winglets" for "keel", in that they both add lateral surface behind the rudder. The canards on that plane are balanced by large wings at the rear of the plane, and the front rudder is balanced by large wingtips.

    A power boat does not have appendages like that so its not even comparable at all. If you put a front rudder on a power boat you would then need to add a huge fin at the back of the boat. Or you could just put the rudder at the back ;)
     
  7. JGF
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    JGF New Member

    Johnny come lately, but...

    Three surface aircraft exist. It's likely that the OP and others won't read this post since theirs are from a while back. That's fine.

    However, if you are wandering around and stumble upon this post, check out the (small) variety of three surface aircraft out there, some of which are production and respected aircraft. It seems that for sailboat balance
    Foreplane~Fore-rudder
    Wing~Keel
    Horizontal Stabilizer~Rear Rudder

    You could do a lot worse than to fly around in a Piaggio Avanti, or in a sailboat of similar flavor. A planing hull would be hard to deal with, of course.
     
  8. capt vimes
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    capt vimes Senior Member

    you cannot compare a plane with a boat... two completely different breeds...
    btw: a "horizontal stabalizer" is called elevator, the "vertical stabilizer" would be called rudder on a plane...
    front mounted elevators are out there and they are called "canards"...
    but they only bring the nose up or down and once you doing a steep turn work as rudders and the rudder is becoming the elevator...

    back to boats:
    what happens when you turn a front rudder to say the left (to make a left turn) at some speed?

    the bow will go off to the left which in turn brings the boat by its momentum bow down and make it heel to the right... exactly what you do not want to have!
    it is a little bit like bicycle riding: you want to go left, you have to turn right first! ;)

    rear mounted rudders do not have this behavior because they bring the stern to the right (if you are doing a left turn), the boat start heeling left... exactly what you want to have!
     
  9. parkland
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    parkland Senior Member

    All I can say, is that backing up in a boat with a single engine and rudder, it is very hard compared to driving forward.
    Obviously because the propeller pushes water past the rudder even at low speed.

    So I say, once underway, a front rudder is pointless.
    At slow speeds, it would work great if there is a propeller there, which there isn't, lol.
     
  10. JGF
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    JGF New Member

    Comparing a plane to a boat doesn't have to be grumpy

    All written with good will toward the gracious Captain Vimes...

    In near silence, pulling the release of the tow line of my sailplane at 500' and a rapid climb of the mountain wave up to 7000' at Sugarbush, just me and the air, it didn't matter if I called it an elevator, a horizontal stabilizer, a canard, or a toucan.

    Rather than throw in some holdover french to impress boat builders with my understanding of aeronautics, it seems helpful to use terminology that more easily translates to the topic at hand.

    You can tell a patient that their A1C levels are abnormal, or tell them that their diabetes isn't well managed. Which do you suppose most obscures the information? Which makes me sound impressive? There's no need to impress, particularly when it obscures understanding.

    Back to boats...

    Bow rudders are impossible. They won't work. The physics is all wrong because my free body diagram says so.

    It's unfortunate that nobody told the shipbuilders of the Skane that bow rudders don't work. Someone should call the yard and alert them to their grave error. With their ship operated forward and backward, those folks are in for a rude awakening.

    I just reached the captain. He said that he is too busy making tight maneuvers with his massive ship to discuss the physics, but that he'll forward my concerns. How dismissive!

    The OP asked an interesting engineering question and the first few posts were "its been done, and here are some challenges."

    Here's humble me, just trying to contend that "bow rudders" exist on aircraft and not just one-off Rutan rejects.

    Now on to more pleasant activities like sailing. You'll find me on the water, making wildly uncontrolled turns with my rudders mounted fore and aft, desperately attempting to avoid the out-of-control Skane.
     
  11. tspeer
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    tspeer Senior Member

    Ahem. Actually the fixed horizontal tail is properly called the stabilizer. An elevator is a flap hinged from the stabilizer. An all-moving horizontal tail is also called a stabilizer or a "stabilator" because it combines the functions of the stabilizer and elevator.

    The term "canard" is the opposite of "tail". A fixed canard surface is not called a stabilizer because it actually destabilizes the plane.

    And I think planes and boats are very comparable. A plane doesn't turn because it is rolled. It rolls to point the lift vector into the turn. Additional elevator control is required in the turn because of the increased angle of attack needed to create the increased lift, which goes as 1/cosine(bank angle). The rudder on an airplane is only needed to trim out unbalanced yawing moments caused by the ailerons or engine out, etc. The rudder deflection required in an airplane's turn is quite small.

    If you use top rudder to raise the nose in a turn, you will be generating sideslip and setting yourself up for a spin. Good fun in an acrobatic airplane, but not a good idea if you are just trying to navigate.

    There is a rolling moment due to the rudder, but it is small compared to the rolling moment due to leeway on the keel. It's actually the side force generated by leeway that alters the course of the boat. If there were no side force, the boat's heading would change, but it would continue skidding along the same course it was following before. You might say that the heeling in a turn is due to the boat tripping over the keel, much more than the moment generated by the rudder itself. The heeling will be pretty similar for bow rudders vs stern rudders.

    The bow rudder actually starts altering the boat's course before a stern rudder because the side force from the rudder is in the direction of the turn. A stern rudder initially produces an acceleration to leeward, and it's not until the boat turns enough for the leeway to overcome this force that the boat starts turning in the desired direction.

    The same thing happens with airplanes. The vertical response to an aft tail is initially negative and then the airplane responds upwards once the pitch attitude has changed the angle of attack. This creates a zero in the pitch response dynamics whose time constant is called Ttheta2 in MIL-HDBK-1797.
     
  12. Squidly-Diddly
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    Squidly-Diddly Senior Member

    IIRC stern wheelers had front rudders, and they also 'bow beached' so I guess they swung the rudders up out of the way.

    I'd also imagine they had spare rudders always ready, and the rudders would be 'break away', since a river boat would be forever hitting logs, dead ex-gamblers, etc.
     
  13. parkland
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    parkland Senior Member

    Front rudders in front of the paddle, but still on the rear of the boat, I thought. :confused:
     

  14. Poida
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    Poida Senior Member

    My inboard does not steer in reverse, it goes to starboard and I think that has got something to do with the prop's rotation.

    It may work at higher speeds, but you generally only go in reverse for manoeuvring.

    I was messing about with a surf cat and changed the arrangement of rudders and a fixed outboard and managed to make the boat go sideways.

    So I can envisage this happening with a front rudder, the front will go around and the rear will drift sideways.

    I won't try and impress everyone with my knowledge of the correct terminology of the front and rear of a boat.:D

    Poida
     
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