Retractable Rudders

Discussion in 'Boat Design' started by Don H, May 19, 2012.

  1. Don H
    Joined: Apr 2012
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    Location: Queensland Australia

    Don H Junior Member

    About 30 years ago I went on a reef trip in a 20 meter Aluminium Cat with retractable rudders. Each rudder was the shape of a quarter of a circular disk. The point which would have been the centre of the circle was mounted to the lower edge of the transom and each rudder was vertical but angled at around 45 degrees to the transom.
    When travelling in a straight line both rudders would be up. To turn to port, the port rudder would be pushed into the water by a hydraulic ram. And when turning to starboard the starboard rudder would be pushed in. Only 1 rudder would be in the water at any time
    The structure holding the rudders was reasonably substantial and the rear of the rudder was supported by a heavy roller that handles the pressure against the rudder but still allowed the rudder to be lifted or lowered.
    When lowered they would throw quite a large amount of spray. How much each rubber was lowered depended on how hard the turn needs to be.
    Recently I have been thinking of costs and efficiencies on a smaller cat (15m power cat) and this design has me wondering.
    On the plus side it seems there would be little rudder drag, much of the structure stays out of the water when not used or at rest so less cleaning or maintenance, No through hull shaft or seals, the main steering load is on a heavy roller rather than linkages or shafts.
    Downside is a big structure at the back of the boat, spray when turning. It would cost more than a normal setup but I don’t think by an enormous amount

    It doesn’t seem to be a popular steering system so I’m hoping someone with more knowledge than me on steering systems can tell me why?
    Is the drag caused by rudders not that significant?, Is it just people want steps at the rear of a cat not a structure?

    Thoughts please.

    Thanks Don
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Kickup rudders aren't anything new and don't need a hydraulic ram to deploy them. The setup you've described sounds unreasonably complex for the benefits it offers, not to mention the weight penalty.
  3. Don H
    Joined: Apr 2012
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    Location: Queensland Australia

    Don H Junior Member

    Hi Par, my description may not be the best. They are not kick up rudders.
    Each side is set at about 45 degrees in opposite directions. This never alters.There is no turning the rudder , its up or down. They were pushed down into the water to turn and then brought back up when the desired course was correct. Being a cat once the rudders were up it would travel relatively straight.
    The ram is needed as once the pressure builds against the roller the rudder isn't going up or down too easily.
    Unfortunately i don't have a photo of the setup.

    Thanks Don
  4. Jeremy Harris
    Joined: Jun 2009
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    Location: Salisbury, UK

    Jeremy Harris Senior Member

    What you're describing sound like dipping rudders. The one on the starboard hull would be angled out to starboard when dipped, the one on the port hull would be angled out to port when dipped. They are a relatively low drag rudder system best suited to a boat that doesn't need constant rudder input.

    The idea is that the boat runs straight ahead with both rudders out of the water (so no rudder drag), and dips either the starboard or port rudder as required to turn. The deeper the rudder is dipped, the greater the turning force applied.

    Rick Willoughby used them on his pedal powered boat, as they reduce drag much of the time when compared to a conventional rudder. I'd guess they are only really suitable for a power boat with a lot of directional stability, as otherwise one or other of the rudders would be in the water too much of the time, causing more drag than a conventional set up.

    See also here:

    and here:
  5. Don H
    Joined: Apr 2012
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    Location: Queensland Australia

    Don H Junior Member

    Hi Jeremy, you are correct. That is the exact concept i saw but on a much larger scale. I read the early post from 2007 abut it didnt go into much detail. I googled "dipping rudders" but again the info was limited. Pedal powered craft looked to be the only application. I wonder if there is another name for the design. Certainly doesnt seem to be used very often and i wonder why. I was on this boat several times and it looked to work well.

    Thanks Don

  6. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    INCAT have been using them on their cats for a long time, even on their very first 40ft ferry crossing the Derwent river.

    They are only good if you have twin props and/or bow thrusters for close up maneuvering, where rudders don't have to finely adjustable.
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