Flat plate rudders.

Discussion in 'Boat Design' started by Pylasteki, Jan 15, 2010.

  1. Pylasteki
    Joined: Apr 2008
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    Location: North Carolina

    Pylasteki Junior Member

    Hi guys.

    I'm thinking about welding up a stainless steel rudder for my Sailboat. A 1962 Pearson Triton.

    How bad is a flat plate rudder with end plates on each end for stalling out at low speed?

    I want the boat to trim with the helm straight. Seeing as the rudder hangs on the aft end of the keel, I'm curious if it'll disappear in the wake of the keel in front of it while centered.

    Will I have extra pressure on the tiller due to weight of the steel plate instead of a wooden rudder? It looks like it'll weigh around 40 pounds if I use 1/4 inch plate, which is probably overkill...

    Thanks for the thoughts.

    I'm wanting a bomb proof rudder that will take to hard grounding without much complaint. Able to be fixed anywhere without needing epoxy resin, mahogany and bronze fasteners... Cake and eat it too.

    How bad is the performance hit?


  2. Pylasteki
    Joined: Apr 2008
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    Location: North Carolina

    Pylasteki Junior Member

    Oh, the rudder will have a full length shaft and sit in a bronze cup made by buck algonquin at the bottom of the keel.

    I am using an outboard, so the prop aperature is disappearing. The aft end of the keel is faired around the rudder shaft in a half moon.


  3. LyndonJ
    Joined: May 2008
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    LyndonJ Senior Member

    Depends on the aspect ratio, the longer and skinnier it gets the lower the speed it stalls.

    Lift drag characteristics are not as good for a flat plate but they make effective rudders on lots of boats, usually behind a skeg or keel but sometimes balanced rudders are flat plat projecting in fromt and behind the shaft.
  4. Frosty

    Frosty Previous Member

  5. Pylasteki
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    Pylasteki Junior Member

    Cool stuff Frosty, but geared toward faster boat speeds than mine.
  6. TeddyDiver
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    TeddyDiver Gollywobbler

    Better to have the keel side so accurate as possible and the trailing edge sharp.. so a triangular "foil"..
  7. CDK
    Joined: Aug 2007
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    CDK retired engineer

    A flat plate easily becomes a warped plate.
    Much stronger, lighter and cheaper are two thin sheets welded parallel to the rudder shaft first, then clamped at the rear and the trailing edge welded together.
  8. Frosty

    Frosty Previous Member

    Then fill it with oil.
  9. Steve W
    Joined: Jul 2004
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    Steve W Senior Member

    What CDK said is probably best,you will want a bit of internal structure to maintain the foil shape,put a threaded fitting in it so you can fill it with oil as frosty said,better still fish oil, then drain it out and screw in a plug,a pretty simple fab job on something that size and much stronger than a flat plate.We did one years ago on a 46ft motorsailor and the owner had us weld a horizontal hand hold out of rod on each side that he could use to hold on to if he needed to be down there cutting a rope off the prop etc.
  10. Frosty

    Frosty Previous Member

    You may need a hole or cut away in it to get the shaft out.
  11. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    Try looking up NACA folis on the web for the shape required
  12. Boston

    Boston Previous Member

    ok so Frosty
    looking at that diagram on the site you posted



    so which ends up being the more efficient

    the thicker one can actually be made the lighter as has been described
    it has a larger forward section which means it takes more energy to shove through the water
    looks like it has the same drag

    the thinner one should use less energy to push through the water but is weaker for its weight and gains turbulance of the leading and trailing edge if its doing anything but going in a straight line

    so which one ends up being most efficient for a rudder ?
  13. Frosty

    Frosty Previous Member

    Hmmm dunno what do you think?

    I use wedges for my speed, good rudder is a wedge.

    He needs a foil but he wants a flat he also wants the boat to trim with helm straight, (never with a flat rudder behind the keel) I suppose for his auto pilot.

    I gave him some info he can make his own mind up.

    Ive given up telling people what they need.

  14. naval ark
    Joined: Mar 2006
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    naval ark Member

    Neither of those images seem particularly appropriate - when designing a rudder a much more important characteristic will be stall angle, i.e. usually the higher the better. A properly shaped section will keep the flow attached on the low pressure side for as long as possible, to avoid separation and the onset of stall - and the corresponding loss of lift and control.

    There is no doubt that the 'two thin faces' versus the 'one thick slab' is the better solution for many reasons, most of which have been covered above.

    Btw, isn't that a Philip Rhodes design? If that's the one I'm thinking of, you have a very nice boat ;-)
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