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Thread: Rudder Question

  1. #1
    Join Date
    Apr 2005
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    Default Rudder Question

    I've just finished gluing(epoxy) up several layers of Meranti marine ply for the rudder of my Tomcat build. I know that the edges should have a tapered shape,but not sure how much. Been searching for closeup photos to no avail. Can someone point me to some (preferably closeup) pix of some rudders? Thanks

  2. #2
    Join Date
    Apr 2010
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    South Australia and Tasmania
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    Default Re: Rudder Question

    Others who know this stuff will be along shortly but I think you want to Google NACA foil

  3. #3
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    Oct 2013
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    Default Re: Rudder Question

    generally considered a very good shape

    38936119_10156506080978290_2341760557989756928_n.jpg

    any combination of this general shape would be considered better than a slab w/ squared off edges

    kinda like looking down on a fish from the top

    a rather blount rounded nose w/ a tapered trailing edge

    i've usually seen rounded off leading edges w/ flat sides and a tapered trailing edge on larger home made plywood rudders and they will point the boat as the skipper wishes

    here's what i am making for my Swampscott Dory

    IMG_1436.jpg

    marked center line boldly on trailing edge and sanded the taper almost to it

    keeping the glue lines parallel and hand block sanding to a smooth fair surface

    IMG_1448.jpg

    i almost said "it aint rocket surgery" butt it can be if you are seeking that extra bit of speed

    aero/hydro-dynamics... ;-)

    it is what we did to our old stock mahogany Sunfish foils back in the 60's to gain an edge on the stock units

    btw, the Tite-Bond lll reduced by 10% is what i coat fir plywood with to reduce checking before priming and painting it for use

    fir even marine fir plywood is a stinker in outdoor/marine conditions and the TB lll helps helps hold the fibers together

    there WILL BE others who join in w/ their own 2 worth

    this is just mine

    sw
    "we are the people, our parents warned us about" (jb)

    steve

  4. #4
    Join Date
    Apr 2013
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    Pittwater, NSW, Oz
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    Default Re: Rudder Question

    Here's a link to (probably more than you'll ever want to know about) rudder (and keel) design:
    https://www.ericwsponberg.com/wp-con...der-design.pdf
    Bottom line get the NACA 0012 profile, make it as deep and eliptical as you can and and you won't go far wrong.

  5. #5
    Join Date
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    Default Re: Rudder Question

    But don't taper the trailing edge to a pointed edge or the rudder will 'flutter' (vibrate) at speed. The water flowing along the surface of the rudder as laminar flow needs a sharp edge to cleave from the rudder surface, so sand the trailing edge square to the centreline of the rudder to make a flat surface between 1/8" and 1/4" wide.
    Hope for the best, but plan for the worst.

  6. #6
    Join Date
    Apr 2005
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    Default Re: Rudder Question

    Thanks guys for the input. Being a Catboat, I don't think speed is a major concern, but the leading edge portion just aft of the prop should, I think, be somewhat "streamlined" - no??? BTW, as I mentioned it is made of Meranti marine ply- what finish is appropriate- just bottom paint, like the hull, or should it also be covered with FG cloth & epoxy?

  7. #7
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    Default Re: Rudder Question

    mmd is correct about not tapering to a feather edge

    see the top pic in my earier reply which agrees w/ mmd's reply

    treat the surface of the rudder like you treat the outside of the hull

    however IMHO thoroughly coating bare Meranti(specially the edge grain) w/ pox should be seriously considered as a minimum treatment for either a bright or painted surface

    sw
    "we are the people, our parents warned us about" (jb)

    steve

  8. #8
    Join Date
    Mar 2010
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    central cal
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    Default Re: Rudder Question

    5EBA910E-3888-4E17-9E25-1BA7E4D2B3F3.jpg
    The plies give good info on where you are, shape wise. I made a little template and just kept checking as I went. I applied glass because I wanted to have a hard, smooth surface.
    The aft edge was built up with some thickened gloop then faired i to the foil, such as it is, and then filed flat as MMD recommends.
    Peace,
    Robert

  9. #9
    Join Date
    May 2009
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    Gauteng, South Africa
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    432

    Default Re: Rudder Question

    The rudder on a Devlin Winter Wren also approximates a NACA foil like swoody posted. The leading edge has been more rounded that what it appears on this picture. Here it is being covered in glass and epoxy.

    DSCF0003.jpg

  10. #10
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    Nov 2004
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    Default Re: Rudder Question

    Quote Originally Posted by mmd View Post
    But don't taper the trailing edge to a pointed edge or the rudder will 'flutter' (vibrate) at speed. The water flowing along the surface of the rudder as laminar flow needs a sharp edge to cleave from the rudder surface, so sand the trailing edge square to the centreline of the rudder to make a flat surface between 1/8" and 1/4" wide.
    Rudder vibration can also be a signal to the helmsman that the boat is sailing in the slot. We always knew when the schooner "Wanderlure II" was sailing sweet by the telegraphing to the tiller of laminar flow vibration. Pacific Catamarans have aluminum dagger boards that whistle at speed.
    Jay

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