New rudder for Zebedee

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  • 02 Dec 2018 02:17
    Reply # 6941597 on 6899643

    Excellent! I'll just copy them.   Many thanks! 

  • 01 Dec 2018 22:20
    Reply # 6941436 on 6899643
    Anonymous member (Administrator)

    Here is how the endplate ended up looking like on a Freedom 30. The owner reported about big improvement. This is very close to how Phil Bolger designed them. I would just copy this, both at the lower end and for the 'cavitation plate', sitting about 10cm below the waterline.

    This isn't rocket science, it's just a question of forcing the the water flow aft in stead of letting it escape under the tip (and let air be sucked down at the surface).


  • 01 Dec 2018 20:18
    Reply # 6941272 on 6899643

    What shape is best for the bottom and top plates?  I notice in the article,  that the bottom plate was in a delta shape. Should they extend aft beyond the rudder? 

  • 21 Nov 2018 01:26
    Reply # 6918948 on 6899643

    Excellent!  I've found the article. Many thanks! 

  • 20 Nov 2018 10:27
    Reply # 6915459 on 6899643
    Anonymous member (Administrator)


    I suggest you look up JRA magazine 74, p.26. There, Karlis Kalnins describes how re rebuilt his rudder, almost identical to yours.


    Last modified: 21 Nov 2018 08:46 | Anonymous member (Administrator)
  • 20 Nov 2018 03:15
    Reply # 6915141 on 6899643

    The chord at the top is 33 ins and at the bottom 31 ins. With 15% camber the difference between the two is 0.3 ins . 12 % of 33 is 3.96 and 15 % of 31 is 4.65 difference of. 7 ins. It might be easier to do the same shaping for both from the leading edge and just extend the flattish part just before the trailing edge at  the top. As you point out the difference is small . Would that still be as effective? 

  • 20 Nov 2018 02:01
    Reply # 6915089 on 6899643

    Hi Alan,

    after seeing the rudder last Thursday I would suggest making the leading and trailing edges of the underwater section of the rudder vertical. This would add some area ahead of the pintle at the top and some extra area on the aft edge at the bottom. This could easily be done by adding a sheet of 18mm ply each side of the rudder with the overlaps trimmed on the center line to fit. Any additional shaping on the aft end could be done on the outside after it is all glued together. Half of a one and a half inch heavy wall PVC pipe would make a good leading edge to the rudder. It would need to be well roughed up before gluing in place and fiber glassing.

    All the best with the project, David.

  • 19 Nov 2018 20:18
    Reply # 6914649 on 6899643
    Anonymous member (Administrator)


    what if you just keep the rudder's profile as it is and just give it a constant thickness? Then the thickness/chord will increase somewhat as you move downwards, but that may not do much harm. If you start with 12% thickness/chord at the upper end, how will it be at the lower end?


    Last modified: 22 Nov 2018 09:38 | Anonymous member (Administrator)
  • 19 Nov 2018 19:45
    Reply # 6914574 on 6899643

    It would be much easier to shape the rudder if it was a parallelogram.  One way to do this is by trimming 2 ins off the top of the leading edge tapering to zero at the bottom.  This would reduce the balance from 18% to 15.6%. Would this significantly affect the rudder performance?  PS the wetted area of the rudder is not exactly as shown because Zebedee is 7 ins lower in the water. 

  • 12 Nov 2018 20:02
    Reply # 6901266 on 6900517
    Anonymous member (Administrator)
    David Tyler wrote:

    The standard textbook Principles of Yacht Design reckons on the NACA 0012 foil for a rudder to be robust against stalling. An end plate will probably help, but adding one has made my rudder hum at high speed (it didn't before).

     A humming rudder is not the end of the world. My Frøken Sørensen's rudder sometimes did that when the speed exceeded 6.3kts. That rudder was a deep foilshaped thing, attached as a swing-up dinghy rudder (no endplate). My armchair guess is that the rudder on Zebedee is less likely to start humming, as it is supported at the lower end.
    No guarantees, though.


    Last modified: 13 Nov 2018 22:29 | Anonymous member (Administrator)
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