Shilling rudder geometry - pivot %

Discussion in 'Hydrodynamics and Aerodynamics' started by dpaws, Dec 31, 2016.

  1. dpaws
    Joined: Aug 2014
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    Location: Trieste, Italy

    dpaws Junior Member

    Good evening all - & season's greetings.

    I noted that a 40% chord pivot point is often quoted for a Schilling profiled rudder blade, and this seems further aft than for a more traditional NACA 00xx section.

    The Schilling will be turned manually by tiller arm, so human arm strength limitations apply!

    Does anyone have hands on experience of a manually operated Schilling rudder with this pivot; how does it feel? Does 40% give the light feedback and gentle self-centring action that I'd prefer? My instinct is to move the pivot forward to around 20%, more typical for a NACA rudder section on a sailing yacht for example or am I missing the point of the Schilling entirely?

    I effect I'm asking for a recommended Schilling pivot point for my boat - a 65'x7'x 2'9" 23 tonne barge (UK narrowboat)

    Much obliged
     
  2. patzefran
    Joined: Feb 2011
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    Location: france

    patzefran patzefran

    Shilling rudder seems primarily designed to provide very low turning radius at low velocity, by nearly complete dflection of the propeller stream. So 40 % is not designed for balancing rudder, but to intercept completely the propeller stream at 70° rudder angle. Should be very effective to turn sharply at very low velocity, but will need more force to the helm at high velocity to avoid control lost (as the center of lateral loads is ahead the pivot axis !). Not designed for fast boats !
     
  3. dpaws
    Joined: Aug 2014
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    Location: Trieste, Italy

    dpaws Junior Member

    Many thanks for your thoughts - all understood. So I can move the pivot point forward but only at the expense of full deflection performance.

    Our maximum canal cruise speed is less than 4 knots, and typically around 2 knots so hopefully my arms shouldn't ache to much day to day. Occasionally we venture out onto some strong rivers but we don't have the power on tap to exceed 6 knots I suppose.

    I'd still appreciate feedback from people with hands-on experience, is there a pivot "sweet spot" for aching arms?
     
  4. patzefran
    Joined: Feb 2011
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    Location: france

    patzefran patzefran

    I have used 20 % and I think it is a good figure for low helm force and rudder stability. Perhaps using coupled rudders at 20% pivot, on both side of the propeller will give you low load helm and near the same turning capability as a single 40% Shilling ?
     
  5. dpaws
    Joined: Aug 2014
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    Location: Trieste, Italy

    dpaws Junior Member

    Thanks - but unfortunately the dual rudders are not feasible for the hull design, also a problem for debris in the shallow canals. 20% was my initial guess for sketching, but I understand that the presence of a flared trailing edge moves the centre of pressure (lift) aft along the section.

    "Normally, a balance is comprised between 16%-20% for a rudder with the center of pressure at 20%-25% of the chord (depending on AR). In the case of the friend's rudder (NACA 00XX + V TE), I believe it should have been increased to somewhere between 25% and 30%, to compensate for the new shape (if I remember well, D. Gerr wrote about up to 35%-40% compensation necessary for a Thistle rudder, in a PB magazine article)."

    (daiquiri : 06-18-2010)
     

  6. patzefran
    Joined: Feb 2011
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    Location: france

    patzefran patzefran

    The aft wedge should effectively move the center of pressure aft a little !
     
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