Rudder alignment

Discussion in 'Boat Design' started by TeddyDiver, Jun 26, 2011.

  1. TeddyDiver
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    TeddyDiver Gollywobbler

    I'm planning to set the rudder a bit of centerline. The reason for this is that it makes easier to install and remove the shaft. The question is if there's a point (like how it balances propwalk etc) why it should be positioned on the left or on the right side of the centerline. The propeller is right handed.
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    It would actually be more correct for you to put the rudder on centerline and place the propeller shaft off-centerline. And for a single-screw set-up, when standing behind the boat looking forward, and the propeller spinning right (clockwise), then set the propeller shaft to the port side of rudder (i.e. the rudder is slightly to starboard of the prop centerline).

    Here is a way I always remember this: You are standing behind the boat looking forward, and determine which way the propeller turns. Imagine that the propeller will "walk" in the direction that it turns (or "roll" if you will). That will pull the stern in that direction, and so point the bow in the opposite direction. So you want to position the propeller to counteract the bow and turn it back toward centerline. This will work the same if you put the prop on centerline and position the rudder accordingly, i.e. slightly to starboard of the propeller. So, for clockwise turning props, position the rudder to starboard of the prop centerline, and for counterclockwise turning props, position the rudder to port of the prop centerline.

    For twin screw powerboats, the same is true. The propellers should turn outboard--clockwise on the starboard side and counter-clockwise on the port side. The position the rudders just outboard of the propeller centerlines.

    Eric
     
  3. TeddyDiver
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    TeddyDiver Gollywobbler

    Thanks Eric, starboard it will be.
     
  4. Easy Rider
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    Easy Rider Senior Member

    How-a-bout a rudder on both sides of the prop of a single screw. The propwash will miss both rudders while going strait ahead and this may greatly reduce rudder drag on the hull. And little or no propwalk going either direction. One could increase total rudder area without increasing drag. A T shaped shoe may reduce or eliminate drag reduction though. Interplane drag of the two rudders may be a negative but flat plate rudders may solve that. Eric S what do you think of that.

    Easy Rider
     
  5. Adler
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    Adler Senior Member

    Wood Shipbuilders Practice

    Last 50 years the wood ship-builders following a practice, when they installed a single propulsion unit.
    They usually installed a piece of wood with reglet shape at the aft keel-end on the same side where the propeller turns - (when we are looking from aft).
    This method solves the half problem -"dynamic".
    If the boat was moved forward the correction was automatically made.
    In case that the boat was in manoeuvring, where the hull momentum was near to zero, then the rudder should be used to "open" the bow from the dock line - for example - when the propeller pulls astern.
    On that case the eccentric installation of the rudder shaft line is recommended.
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    A large part of the turning ability of the rudder is because it directs the flow of the prop wash to the side to which you want to turn, so if you take the rudder well away from the prop wash, you are going to lose turning ability. OK, you may get some of that back by adding another rudder, but then you are doubling the rudder installation cost and making the steering system more complicated. You will not eliminate any drag, but you will increase drag by having the second rudder. So this is a case of keeping things as simple as possible--the KISS principle. I see no benefit to having two rudders spaced outside the prop wash over one rudder still within the prop wash.

    Eric
     
  7. Saildude
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    Saildude Junior Member

    If you take the rudder out of the prop wash then the slow speed turning will be much worse. I use the prop wash to turn my boat at zero speed every time I leave the slip. Take the prop wash away and then the boat needs to be moving to turn.

    Sailboats have big rudders which seems to make the prop wash work better than on a powerboat with a smaller rudder
     
  8. LP
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    LP Flying Boatman

    Eric,

    Are we actually talking about the prop "walking" or is this of an accepted terminology for a completely different physical property of propellers?

    In aviation, there is P-factor that is produced when the propeller is operating at angles to the relative flow. The decending blade (typically) is operating at a higher angle of attack than the ascending blade and thus produces more lift, which in turn creates a turning tendency that needs to be counteracted through the use of rudder pressure.

    I can see the same properties applied to a propeller in water that is also angled to the relative flow. In our right handed scenario, this shifts the center of lift/thrust to the right of centerline, creating a left turning tendency. I'm not sure that shifting the rudder would have any desired effect regarding these properties, IMHO. However, for TD's situation, it is certainly as good of choice as any. And . . . I can certainly be overlooking other aspects completely that would validate the shift of the rudder to the right. Perhaps, there are drag effects associated with the offset rudder that creates a correcting moment. (?)

    One more thought regarding shifting the rudder. Shifting to the right/stbd in a clockwise turning prop, would place the rudder in the greater thrust side of the prop and contribute to greater effectiveness of the rudder.
     
  9. Saildude
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    Saildude Junior Member

    I think we have two terms in the same thread - the original post had a question about prop walk, and my post and at least one other addresses the use of prop wash to maneuver.
     
  10. LP
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    LP Flying Boatman

    Emphasis added.

    I believe this is a comprehensive question to cover all aspects of the intended design.
     
  11. TeddyDiver
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    TeddyDiver Gollywobbler

    Yes.. feel free to broaden the subject.. it's all very intersting.
    I'm actually planning for two rudders. The other one mentioned here is the main rudder steered with a wheel from the pilothouse. The second one will be "auxiliary" kickup rudder for wind vane and occasional steering from the cocpit.
     
  12. Eric Sponberg
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    Eric Sponberg Senior Member

    Being a pilot, I am familiar with P-factor, and it is felt the most when climbing, such as during take-off. In boating, it is a similar torque effect, although the propeller is not angled to the flow as it is on an airplane in a climb. It can be detected by setting the boat's engine in gear at idle so that the propeller is turning as slowly as possible, and keeping the rudder as straight on centerline as possible. You'll see a tendency of the boat to turn. It is used to effect in docking, for example, if you are trying to move up against a dock sideways--you put the engine in gear at idle to let the propeller "walk" you into the dock, letting it turn the appropriate way, of course.

    As for drag effects, these are tiny when the rudder is still within the prop wash. However, if you imagine a single rudder boat with the rudder set well off to one side and not in the prop wash, the drag of the rudder would have greater turning effect the more offset it is. But you lose other effects from being outside the prop wash, plus with a far-offset rudder, you should probably have a mate to it on the other side. This is done in sailboat ocean racing, for example. Because the boat heels, a rudder on centerline is not as effective as a rudder off centerline--so the boats are equipped with twin rudders. The low rudder is used on the tack, and the high rudder is retracted. This is reversed tack to tack.

    The flow across the rudder is complex while it is in the prop wash. The faster flow tends to reduce vapor pressure and so increase lift, but there is also an impingement component that tends to increase pressure, cancelling the effect. Certainly, the best angle of attack of the rudder to the prop wash flow would be different, and to get that, you have to turn the rudder toward the propeller, which is the way you want to correct anyway.

    The mind begins to hurt with all this analysis, so it is best to just follow the guidelines as I stipulated above.

    Eric
     
  13. LP
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    LP Flying Boatman

    Where's the aspirin?

    Thanks, Eric.
     

  14. Eric Sponberg
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    Eric Sponberg Senior Member

    I find Aleve works REALLY WELL.

    Eric
     
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