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Thread: Removing propeller shaft casings

  1. #21
    Join Date
    Jan 2014
    Location
    Stockholm, Sweden
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    26

    Default Re: Removing propeller shaft casings

    The under water section of my rudder is 70 cm deep and 42 cm wide, so about 1.7:1.
    There is no post. The rudder blade is 3 cm solid plywood and hung on hinges on the transom. It is not completely flat but rounded in front, thickest in the middle and a bit thinner aft. Not much of a profile, though.

  2. #22
    Join Date
    Jul 2009
    Location
    currently Columbia/Panama
    Posts
    323

    Default Re: Removing propeller shaft casings

    Doh! I just went back and looked at the picture posted above by Puck and see these are not balanced spade rudders. So never mind my comments - a 12% section will be fine. I'd make it to match and work with the skeg profile.

    Mark
    Mark Cole
    Manta 40 "Reach"
    www.svreach.com

  3. #23
    Join Date
    Jan 2014
    Location
    Stockholm, Sweden
    Posts
    26

    Default Re: Removing propeller shaft casings

    The Catalac comes with different rudder versions. Mine has no skeg and a rudder balanced with some 5 cm in front of the pivot point. I guess that could classify as a balaced spade rudder. From Jans description "No skegs, and the pivot of the rudder should be around 50 mm of the front of the profile." it seams he has a similar construction.
    Please explain more: Why would a 12% section be suitable for a rudder with skeg but not for a balanced spade rudder?

  4. #24
    Join Date
    Jul 2009
    Location
    currently Columbia/Panama
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    323

    Default Re: Removing propeller shaft casings

    The foil section decision isn't related to whether it is a skeg or spade rudder, but with a skeg, you already have a fixed and constant leading edge, which typically defines the rest of the rudder decision. So there isn't as much to be done with the foil section on a skeg rudder in terms of gaining more lift or increasing stall angle like there is with a spade rudder.

    Mark
    Mark Cole
    Manta 40 "Reach"
    www.svreach.com

  5. #25
    Join Date
    Jan 2014
    Location
    Stockholm, Sweden
    Posts
    26

    Default Re: Removing propeller shaft casings

    As I belive I have something similar to a balanced spade rudder, and not a skeg, I wonder if you could share some more of your knowledge about suitable profile for low aspect rudders:

    Quote Originally Posted by colemj View Post
    12% is a common foil section for many higher aspect rudders, but it sounds like yours is a low aspect planform. Probably even 1:1 or less, given what Jan has written. For this aspect, a larger section will give more lift and stall much later than a thin one.

    Mark
    What would be suitable for a 1,6:1 aspect rudder, and why?

  6. #26
    Join Date
    Jul 2009
    Location
    currently Columbia/Panama
    Posts
    323

    Default Re: Removing propeller shaft casings

    I would think probably something in the 15-20% section range. We just made new rudders with a 1.6:1 AR and I went with a NACA0020 section. They work well, particularly at slower speeds and provide more lift when we have some weather helm upwind due to their increased angle of attack then.

    12% will work, but they will stall earlier and not provide as much lift at larger angles of attack.

    Attached is a plot of calculated lift and drag coefficients between NACA0012 and 0020 sections on a 1.6:1 AR rudder at 6kts over 1-20* angles of attack. The 12% section starts to stall at ~12*, while the 20% section continues to add lift up to 20*. At 20* of rudder (during a tack), the 20% section provides twice the lift of the 12% section, with less drag.

    Mark
    Attached Images Attached Images
    Mark Cole
    Manta 40 "Reach"
    www.svreach.com

  7. #27
    Join Date
    Jan 2014
    Location
    Stockholm, Sweden
    Posts
    26

    Default Re: Removing propeller shaft casings

    Thank you, Mark, for this valuable information. As my primary concern* was to reduce drag, but you convinced me that a fatter profile may improve tacking ability, I will go for a compromise, NACA 0015. Hope it works.

  8. #28
    Join Date
    Jul 2009
    Location
    currently Columbia/Panama
    Posts
    323

    Default Re: Removing propeller shaft casings

    Here are the calculation plots for 12, 15, 20 sections.

    Mark
    Attached Images Attached Images
    Mark Cole
    Manta 40 "Reach"
    www.svreach.com

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