Rudder Size

Discussion in 'Boat Design' started by hillmaster, Feb 20, 2009.

  1. hillmaster
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    hillmaster Junior Member

    Hi

    i would like some help regarding the size of the rudder! what is the size that a rudder should have?
    i assume that the size of the ruuder should be proportional to lift that should be generated in order to turn the boat! but how much lift should be generated?

    thank you
    Hillmaster
     
  2. daiquiri
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    daiquiri Engineering and Design

  3. Ad Hoc
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    Ad Hoc Naval Architect

    Calculate the lateral under water area, and take between 1~2% of this, and that is rough guide for a rudder size.
     
  4. SViau
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    SViau Naval engineer / Designer

    Another question, maybe stupid.... Sailing boat, or motor boat ?
     
  5. Tcubed
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    Tcubed Boat Designer

    A number i use (i do not know if anyone else does this) is the coefficient of rudder volume ;

    Rudder surface * distance between center of lateral resistance to center of area of rudder / Boat displacement.

    This gives a number to compare boats by. Two things- the coeff for motorboats is always much smaller than for sailboats and the coefficient decreases for boats size. Look at a twenty foot sailboat rudder and then look at a tea clipper's rudder for example. The clipper has a very small rudder compared to its size. So you must compare coeffs between boats of similar size.
     
  6. SViau
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    SViau Naval engineer / Designer

    Good system to evaluate the power of steering developped for a rudder.
    This is more or less the same idea behind balancing a sailing boat with sail surface vs anti-drift surface and center of actions.
     
  7. mydauphin
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    mydauphin Senior Member

    Help with Rudder Identification

    I need to understand this rudder. The surface area numbers are confusing because some are in Inches and other is in feet. What percentage of Balance vs main area. What is this style of rudder called. I am trying to sell it and would like to know what size vessel it should fit. I have two and therefore a twin rudder vessel.
     

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    Last edited: Feb 26, 2009
  8. daiquiri
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    daiquiri Engineering and Design

    Kudos for this, but I hope you're using volume units for displacement. In that case it becomes a pure number and can be used without corrective coefficients when switching between various unit systems. ;)

    It is very similar to the Tail Volume Coefficient used in aircraft design, defined as:
    Vh = (Sh x Lh) / (Sw x MAC)
    where:
    Sh = horizontal tail area
    Lh = distance from tail's aerodynamic center to the airplane's CG
    Sw = wing area
    MAC = the mean aerodynamic chord of the wing.​
    Tail Volume Coefficient is a quick and efficient way to express the airplane tail effectiveness, because it relates the factors which cause the need for a tail (wing forces and moments) to the factors which influence the tail action (tail surface and position relative to the CG).

    Something similar could probably be concieved for boat design, bearing in mind the difference between sailboats' and powerboats' rudder usage.
     
  9. Tcubed
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    Tcubed Boat Designer

    Of course! that is the whole point , a dimensionless number.

    I don't know if you've noticed but i always use metric or S.I. Feets and inches belong in the middle ages..... So displacement in liters is the same thing (almost, slight difference for salt water) as boat mass in kg. And then i would use decimeters for the rudder term as well, obviously.

    And you guessed exactly right what made me create that figure. (i design light airplanes as well)
     
  10. dimitarp
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    dimitarp Junior Member

  11. daiquiri
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    daiquiri Engineering and Design

    Ehmm... For more info on what? Nothing about rudder design over there.
     
  12. Çemberci
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    Çemberci Senior Member

    An example is finding rudder size.

    oktay çemberci
    istanbul/turkey
     

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  13. mydauphin
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    mydauphin Senior Member

    Guys, I know you geniuses can figure out this simple question. See the rudder in my previous post in this thread. What is rudder surface area? What is percentage of area in front of axis for balance and behind for main area? Also for what size boat would this rudder work? It is for planning boat like a sportfisherman but is it for 30 footer, 40 footer, 50 footer, etc...?

    Thanks again, and you geniuses dont let me down.
     
  14. Tcubed
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    Tcubed Boat Designer

    Your rudder surface area is simply (part of the surface in sq inches)/144 + (part of surface in sq feet)

    somin like 3 1/2 sq ft..
     

  15. daiquiri
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    daiquiri Engineering and Design

    I don't really get the meaning of that "you genuises", but since I don't like thinking negatively I'll take it in a most affable sense... We're probably short of geniuses here, they've been all taken by the Microsoft and are designing blue screens now.

    As about your rudder over there, this is what I can tell you:
    - the total area is 3.92 sq. feet
    - the compensation area (area in front of the stock) is 0.98 sq. feet.
    - the compensation area is therefore 17% of the total rudder area, which makes it quite suitable for a fast cruisers, semidisplacement or planing type. It's probably less suitable for slow cruisers, which may require as much as 25% of compensation area.
    - an old rule of thumb says that, for twin rudders, the total rudder area should be 3-4% of boat's lateral plane area (which is basicaly the lateral projection of the underwater body). So the rudder you have is suitable for a boat with a lateral plane area of about 100-130 sq. feet (in a single rudder configuration) or about double that value (200-260 sq.feet) for twin rudder installation.
    Hope that will help.

    A (kitchen) Genius
     
    Last edited: Mar 8, 2009
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