curious rudder balance problem

Discussion in 'Hydrodynamics and Aerodynamics' started by Kim Klaka, Feb 20, 2017.

  1. Joakim
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    Joakim Senior Member

    Wouldn't the rudder blade float (at least without the stock) and thus its CoG should not induce a moment. Instead CoB could cause a moment, but that would be quite small due.

    I would be very surprised if that kind of boat would not have stern down trim while motoring.

    Does the problem occur at all motoring speeds or only when you are pushing close to hull speed?
     
  2. daiquiri
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    It would if it was buoyant. Is it?
     
  3. Joakim
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    Joakim Senior Member

    Hard to say. Most rudders float easily without the stock. Stock is in the turning axis thus it can be ignored while calculating the moment.

    The rudder seems to be quite unusual. Typically the blade is angled backwards, when installed under the hull. E.g. like this Jefa rudder: http://www.jefa.com/blade/RUD34.htm

    Note also it has balancing area in front of the stock only in the upper part of the rudder. Propeller wash would hit only the lower part, which has no area in front of the stock.
     
  4. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Re-reading the post #14 by Kim Klaka, he says that the rudder is not buoyant. In fact, a rudder of that size can generate some 35-40 kg of buoyancy force, so there is approximately 30 kg of net downwards-pushing force.

    As for the stock, that's why I have asked to show a drawing of the rudder, to better understand the position and angle of the stock centreline. Consider also that waterflow lines are not horizontal but are curved upwards in the stern area, so a vertical rudder is actually raked forward in the waterflow-aligned coordinate system.
     
  5. Joakim
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    Joakim Senior Member

    The stock will weigh a lot, especially if it is steel. Even the aluminium stock of the Jefa rudder I linked was 24 kg while the blade is only 13 kg. The rest of the rudder can still be buoyant.
     
  6. Mikko Brummer
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    Mikko Brummer Senior Member

    Looking at Kim's photo, could the propeller wake act as a narrow, discrete jet, hitting only (or more) the front part of the slightly turned rudder?
     
  7. Remmlinger
    Joined: Jan 2011
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    Remmlinger engineer

    Good point Mikko! Measuring the position of the rudderstock in the picture, the axis seems to be at 23%c - not far from 25%. If only the front part of the foil is loaded, a negative turning moment could be created.
    Uli
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Simple, carve away some 50% of the balance area in the propeller wake region....?
     
  9. CT249
    Joined: May 2003
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    CT249 Senior Member

    I had the same issue when I installed a spade rudder instead of the skeg hung one. The prop was only a bit more than half the diameter away from the leading edge.

    I threw the ageing diesel out and put in a stern hung outboard to save weight, cost and drag. Now the problem of "self steering" under power has vanished, although nothing else has changed. However, this rudder is arguably over-balanced since it sometimes develops lee helm when close reaching in light winds.

    Since the VDS 34 is a very popular boat, this issue seemed to be odd until I looked at Kim's pic. As with my boat, the balanced portion and the prop are too close IMHO. Unfortunately it's probably not a cheap fix. To me, the prop appears to be much closer than in the study plan of the 34. The tip of the folded blades are only about one "blade's length" from the leading edge of the rudder in the pic, whereas on the study plan the distance is about 2.5 times that. Kim's prop appears to be further away from the hull than in the plan, indicating perhaps that the boat was built with an over-length prop shaft.

    Perhaps a work-around is one I was considering on my boat; some heavy-duty bungee cords and some lines connected to the tiller to act as a limiter to the amount of travel and to counteract the turning moment of the rudder. Like Kim, I felt this problem posed a significant danger.
     
  10. Joakim
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    Joakim Senior Member

    The view angle of the picture Kim attached makes it very hard to "measure" the distance from the propeller to the hull and rudder leading edge. I wouldn't make any conclusion based on that picture.

    The propeller stream is quite small diameter at 0.5-1 m behind the propeller. Also due to shaft angle it has different angle than the flow while sailing or using an OB behind the rudder.

    As I pointed out earlier the balance could be different in the area of propeller stream. Also it could be that a turning the rudder slightly makes the rudder stall in the propeller stream region, since the high velocity stream would slow down when following the surface of the blade due to the stream spreading wider in all directions. That would leave stagnant pressure on the balancing part and much lover forces on the rest of the rudder.

    So adding more area at the trailing edge may do very little in the propeller stream region.

    Also Jefa rudder I linked shows that. The balancing part is only in the upper part of the rudder where the propeller stream would not hit.
     

  11. PeteK
    Joined: Oct 2020
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    PeteK Junior Member

    Kim, PeteK here from Busselton WA. Would you please see my thread on this site called Flat bottom planing hull sailing boat theory sought? I've rad a number of your articles in CH and wonder if you might be ineterested in advising/collaborating with me on this project. (I may have met you in the past as I used to be a yacht broker at Martin Box Marine about 15 years ago)
     
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