Propulsion steering issues

Discussion in 'DIY Marinizing' started by licensedtochill, Apr 13, 2021.

  1. licensedtochill
    Joined: Apr 2009
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    Location: georgia

    licensedtochill Junior Member

    Any help would be appreciated.
    I had a lot of free time this past winter due to COVID and decided to get started on an idea that’s been floating in my head for awhile.

    I needed a boat for a river near my house that stays shallow enough during summer that a regular outboard motor is not feasible

    Took a 10’x36” aluminum Jon boat and converted to a air cooled inboard with a pivoting shaft(similar to a disappearing propeller)Shaft housing and propeller are from a Thai longtail kit that was shortened down.

    Everything functions as designed except the steering/ rudder.

    The boat will turn but not a sharp/responsive as I would like.
    Steers more like a 18 wheeler than an car

    Included are pictures of boat and rudder setup.Any ideas on what I can do to increase steering response.
    Current rudder size is 5”x9”
    Current prop is 6.5”
     

    Attached Files:

  2. fastwave
    Joined: Jan 2007
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    Location: europe

    fastwave Senior Member

    My guess is that the rudder is sitting in the wake of the transom and will only work effectively if you are on throttle and the prop wash helps it.
    This is something that can be learned by the operator.
    Similar to a jet ski
     
  3. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Ya gotta move the rudder.
    I have no idea where or how much but if it ain't working there, try moving it.
    Or change the rudder shape, that may be an easier place to start.
    Sorry, not very helpful.
     
  4. Will Gilmore
    Joined: Aug 2017
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    Location: Littleton, nh

    Will Gilmore Senior Member

    how about getting rid of the rudder and putting a universal joint just ahead of the prop? Direct drive steering should make for better control.
     
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  5. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    I think the corners of the Jon boat are providing more resistance to turning than the rudder is generating moment to turn. Like others have suggested, you need to vector the thrust like a typical OB motor.
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I think a possible problem is your rudder is hinged at the leading extremity of it, only half the prop wash is affecting it, when put into a turn, if it pivoted back further toward the middle of it, the more you put the helm over the more propwash involved.
     
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  7. Barry
    Joined: Mar 2002
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    Barry Senior Member

    and the part of the prop whose flow is not directed to the rudder is continuing to push the boat in a straight line.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    And I guess when the drive is set high, there isn't a lot of rudder in the water anyway.
     
  9. Barry
    Joined: Mar 2002
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    Barry Senior Member

    If you move the pivot point to close to the mid point of the rudder, then when the rudder is turned, the entire rudder will see the prop stream. Obviously that means that the fixed pivot point on would have to be
    moved rearward. Additionally, there will be less feed back to the steering wheel as the forces will be more balanced as compared to the way that you have it now
     

    Attached Files:

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  10. licensedtochill
    Joined: Apr 2009
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    Location: georgia

    licensedtochill Junior Member

    Yeah, I was unsure of rudder design when I fabricated it.

    Does the pivot point of the rudder make that big of a difference?
    I just assumed the same amount of water would flow over rudder regardless of pivot position
     
  11. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Mr E identified that only 1/2 of the rudder was impacted by the prop stream which reduces the effectiveness of the rudder. With the rudder turned hard one way, the open half of the prop thrust is trying to push the boat
    straight ahead.

    A rudder will work if it is not in the prop stream but it needs forward motion of the boat for the rudder to provide some lateral force to push the stern of the boat around. So the prop stream impinging on the rudder significantly helps push the stern of the boat around especially at low speeds when the boat does not have forward movement.

    With an end pivot point any prop stream load will be trying to twist/turn the rudder in a specific rotation requiring an equivalent but opposite steering push rod input. A rudder with the pivot in middle will be "balanced" in that there is an almost equal force on both sides of the pivot point. Plus with the pivot in the middle, the entire prop stream will assist in turning and the current open half will help push the stern around
     
    Last edited: Apr 17, 2021
  12. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    The way water flows around a rudder, or other foil in a fluid stream, forces are greater toward the leading edge. You could move about 20% of the area forward of the pivot point without too much risk. More than that and the rudder might get out of control. Approximately. Very dependent on surface conditions, foil section etc. But 20% is pretty safe.
     
  13. licensedtochill
    Joined: Apr 2009
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    Location: georgia

    licensedtochill Junior Member

    Thanks for the help fellas.
    I now understand what you mean about pivot point on the rudder


    So rudder size seems to be alright?

    If I add material on front of rudder to move pivot point rearward is there an issue with rudder being too close to propeller?
     
  14. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Looking at your pictures, the propeller is very close to the rudder already, it looks like you will have to extend the bracket the rudder is pivoting on. You want probably in the range of 30-40% of the rudder ahead of the pivot line. And as mentioned, that will eliminate a lot of steering feedback.
     

  15. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    An easier possible solution would be to fit slender wedges either side of your rudder, at the trailing end, if you think your steering can stand the strain, that will certainly increase the steering effect, albeit creating a little drag, not much though.
     
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