Forward mounted rudders????

Discussion in 'Boat Design' started by stupidbaker57, Feb 7, 2018.

  1. stupidbaker57
    Joined: Jan 2011
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    Location: Lakeville Ma

    stupidbaker57 Junior Member

    'Member the guy that put a motorcycle engine in a hydroplane about 5 years ago? That would be me. I now have a plan to built a runner style or canard type hydro with home built surface drive around 12 feet long. Question is,,,,would mounting 2 rudders, one on the back of each sponson work? I'm thinking of connecting the rudders together and controlled by a steering gear similar to a lawn tractor. My surface drive will be self trimming but not steerable. Power will be a Honda Civic.
     
  2. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Yes, of course it would work.
     
  3. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    Can you say "dynamic instability"... I knew you could.....

    With the rudder in the back of the boat, the more it yaws, the less steering effect it has and the boat is therefore stable.

    The problem with a rudder in the front is that as yaw angle builds the amount of steering angle effectively increases and the system is unstable. It's the same thing as a tail dragger airplane when you're landing. It's basically unstable so you can do it for a bit, but as the aerodynamic steering effect of the rudder goes away, the airplane WILL swap ends and ground loop if you aren't constantly correcting for it.

    You don't want to go there unless you had much much bigger fins at the transom and then those front fins won't be very effective, and if they were you'd be looking from whence you came in a few milliseconds if you were going very quickly.
     
  4. Mild Bill
    Joined: Nov 2007
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    Mild Bill Well, not entirely mild.

    Rudders on the back of each sponson would be too close longitudinally to the center of lateral resistance to generate effective steering force. Try this thought experiment: A rudder is mounted exactly at the CLR. Deflecting the rudder will generate a sideways force, but since the force vector goes through the CLR there is no turning moment to force the hull to change the direction it is pointing.
     
  5. jehardiman
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    jehardiman Senior Member

    Yes, rudders mounted ahead of the props (but still aft) will work, but only at speed (look at modern submarines). And at speed, the asymmetric inflow to the props will be problem and may cause significant vibration/cavitation/structural issues.
     
  6. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    With the motor on the back and the driver at or behind the rudders you've got the longitudinal CG behind the rudders. The definition of instability is that any slight perturbation toward a turn, results in increasing the turn. This is exactly what you have with a rudder ahead of the CG, since the lateral force generated is ahead of the cg as the boat starts to turn the centrifugal force pulls the aft end of the boat outward, increasing the yaw and increasing the rudder angle relative to the flow, increasing the turning effect. If you are at any reasonable speed the boat would just snap spin. The only way you can overcome that is to have a much larger fin at the back of the boat so you generate a restoring force that is greater than the force generated by the front rudders.
     

  7. stupidbaker57
    Joined: Jan 2011
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    Location: Lakeville Ma

    stupidbaker57 Junior Member

    Well that explains that! Thanks for that info. I have since been able to redesign the drive to allow it to steer. Since I'm working with a small craft of 9 or so feet, would steering it with cable and pulley be safe, or would push pull rack steering be better. Hydrolic is out due to the extra weight.
     
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