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Re: a question about rudder angle

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  • graeme
    http://www.jimsboats.com/1may11.htm I was reading where Jim Michalak said he had trouble not rounding up when running DDW with a shoal low aspect rudder with
    Message 1 of 36 , Mar 6, 2013
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      http://www.jimsboats.com/1may11.htm
      I was reading where Jim Michalak said he had trouble not rounding up when running DDW with a shoal low aspect rudder with endplates. He fitted larger and larger endplates but without effect - the issue remained. He scrapped the rudder, fitted a deeper high aspect one, and fixed the trouble that way. He wrote he never went back to shoal rudders / endplates, and he has stuck with deep rudders since.

      Now surely past a certain point there is nothing to be gained by increasing the size of endplates - except an increase likely in wetted area drag... Endplates is endplates - to reduce losses at the foil tip. Why not test increasing rudder area before condemning/rubbishing low aspect rudders and endplates?

      Also, the issue was on one boat only. Short, wide, and deeply V'ed. The rudder may have been in surface layer water flowing with the boat on a run - dragged behind the V. The issue with any size endplate tried went away with broad reaching - perhaps leeway introduced some cleaner water flow to the rudder?

      Graeme
    • Nels A
      As you probably know Phil Bolger has used end plates very successfully on several of his designs. He writes about them in his book Boats With An Open Mind. I
      Message 36 of 36 , Mar 8, 2013
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        As you probably know Phil Bolger has used end plates very successfully on several of his designs. He writes about them in his book "Boats With An Open Mind." I think he first used one on his Tiny Cat. It improved rudder control a lot on the old traditional catboat "barn door" rudder design. And the chapter on it explains why he felt it worked well. Catboats were notorious for bad steering control downwind.


        http://www.instantboats.com/images/tinycatst600.gif



        Then he tried them on his Micro series. The Micros have inboard rudders so a deep rudder that can be raised doesn't work with an inboard rudder. This also allows the OB to be mounted on the centreline of the transom and in-line with the rudder. And the rudder (and motor) are protected by the shallow keel forward.  An inboard rudder is also an advantage on his  Micro Navigator allowing one to steer the boat from inside the cabin with a shorter tiller.


        His original Micro plans did not have an end plate but now has been added and it did improve performance. But all these are flat-bottomed hulls so I think you are correct  Graeme - probably not as effective on a vee bottom. And I think also work better in a design with more displacement . The inboard rudder is further forward and in deeper water than one hung off the transom, so is more effective even going downwind.

        Jim did design one hull called Eisbox that has the exact same rudder system as Micro - at the insistence of the person who commissioned it. But it had no shallow keel forward so needed a skeg ahead of it for protection. This meant the draft would be more and Jim doesn't like a hull with more than 4-5 inches of draft at most, so withdrew the plans for sale. I really liked the design and managed to convince Jim to haul out the mylars and send us some.  

        Nels



        --- In boatdesign@yahoogroups.com, "graeme" <graeme19121984@...> wrote:
        >
        >
        >
        > http://www.jimsboats.com/1may11.htm
        > I was reading where Jim Michalak said he had trouble not rounding up when running DDW with a shoal low aspect rudder with endplates. He fitted larger and larger endplates but without effect - the issue remained. He scrapped the rudder, fitted a deeper high aspect one, and fixed the trouble that way. He wrote he never went back to shoal rudders / endplates, and he has stuck with deep rudders since.
        >
        > Now surely past a certain point there is nothing to be gained by increasing the size of endplates - except an increase likely in wetted area drag... Endplates is endplates - to reduce losses at the foil tip. Why not test increasing rudder area before condemning/rubbishing low aspect rudders and endplates?
        >
        > Also, the issue was on one boat only. Short, wide, and deeply V'ed. The rudder may have been in surface layer water flowing with the boat on a run - dragged behind the V. The issue with any size endplate tried went away with broad reaching - perhaps leeway introduced some cleaner water flow to the rudder?
        >
        > Graeme
        >
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