Stupid little rudder !!!

Discussion in 'Boat Design' started by Little Iris, Jul 7, 2014.

  1. Little Iris
    Joined: Sep 2007
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    Little Iris Junior Member

    Hi everybody,

    I need help to redesign the rudder of my boat.

    It is a full displacement boat, 10 meters in length and hard to control at low speeds (No surprise there).

    I have attached a sketch of the profile of the underwater body + rudder. Check the picture for more information. There is a swimming platform extending 0.6 meters at the stern "protecting" the rudder.

    Thanks,

    Mattias
     

    Attached Files:

  2. daiquiri
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Hi Mattias,
    the area of your rudder is in the ballpark of commonly accepted rule-of-thumb values. Check this old post which treats the same issue: http://www.boatdesign.net/forums/boat-design/rudder-area-dual-rudder-34242.html#post390282 .
    The sluggish performance of your boat at low speeds can also depend on things like boat mass and the mass distribution, but also on your ability to coordinate the rudder and the throttle during maneuvers.
    Cheers
     
  3. gonzo
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    gonzo Senior Member

    The area seems proportional. However, it may be ventilating when the boat pitches which can cause poor steering.
     
  4. PAR
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    PAR Yacht Designer/Builder

    There are some things you can do to the plan form of that rudder to help, but I'll bet the hull form is causing most of the low speed handling issues, more so than the rudder.

    Lift the rudder's top edge to seal the gap between the hull/fair body and blade. Make this gap as small as practical (1/4" or less). Change the angle of the bottom of the blade to match that of the shaft angle, so it's not angled up aft as shown. Also lower this edge much like to top (as much as practical), to gain some additional area.
     
  5. daiquiri
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    daiquiri Engineering and Design

    I am afraid this won't help much, PAR. If the hull form is the one shown in the OP picture then the gap will be filled only marginally, as the whole rear top part of the blade will remain subject to the flow leakage from pressure to suction side. See the attached drawing and the pointing arrows.

    I agree with you about the possibility to extend the rudder downwards as much as practically possible. But it will IMO improve things only marginally, because the area which mostly works at low speeds is the one shaded in light blue.

    Cheers
     

    Attached Files:

  6. PAR
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    PAR Yacht Designer/Builder

    Agreed Slavi, but any area is good area with gaps like that. My first glance suggests a 20% or more area increase is possible, which isn't something that should be ignored, if available, even though it may be in portions of the blade where it's effectiveness is less notable. It's probable the hull is well built down and shadowing the prop quite a bit. Some photos of the hull would be helpful.
     
  7. TANSL
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    TANSL Senior Member

    I think also the rudder is mounted upside down. Is not it more logical arrangement of the figure below?
     

    Attached Files:

  8. daiquiri
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    daiquiri Engineering and Design

    As PAR said, a photo or two of stern and rudder details would be very welcome. :)
     
  9. AndySGray
    Joined: Jun 2014
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    AndySGray Senior Member

    What angle does it swing through?

    i.e. if hydraulic what is the stroke of the cylinder and distance along the arm from the rudder shaft.

    There are many different options for cylinders and too short a stroke or too far from the fulcrum will compromise an otherwise well designed setup. Yes, the extra leverage may make it 'feel' lighter on the wheel but if the rudder only swings 30 degrees as a result...
     
  10. Little Iris
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    Little Iris Junior Member

    Pictures

    Thank you for all the replies!

    Here are 2 pictures.
     

    Attached Files:

  11. daiquiri
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    daiquiri Engineering and Design

    Some folks on this forum have tried to change the rudder profile from flat plate (which you seem to have now) to NACA section, with good results. Check this thread: http://www.boatdesign.net/forums/boat-design/rudder-design-31745.html#post348965

    Others have modified their rudder by adding a fish-tail to the trailing edge of their rudder, thus creating a sort of Schilling rudder (http://en.wikipedia.org/wiki/Schilling_rudder). And they seem to be happy with the final results, too. Check this thread: http://www.boatdesign.net/forums/boat-design/schilling-rudder-question-38440.html

    Those are another two possibilities.

    Cheers
     
  12. MikeJohns
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    MikeJohns Senior Member

    Mattias

    Yes it's a bit inadequate for that type of displacement hullform . You want to almost double the area, or at least to 5% of the immersed lateral area going up to 0.3m^2 wouldn't be over doing it.

    Id do as PAR suggested and fill in the top up to the hull, it's well enough below the waterline you could also square off the bottom but structurally on the lower support it's less onerous to fill in the top. Also the closer the rudder blade gets to the hull at any angle of articulation the more you gain in increase of effective size.

    Flat plates are as good as a NACA foil in this sort of application, you will even get less drag straight ahead and more lift with a flat plate with a more delayed stall and a constant center of pressure for a lot less work. But check the hull can take the extra weight aft. Otherwise use a NACA section or a hollow section metal, or of course wood or hollow GRP.

    An alternative approach is to square the smaller rudder off and add end plates and a V 'tail' which helps rudder characteristics a lot. Another good technique used on shallow draft displacement power boats is to add an articulating flap to the trailing edge.

    However when you change a rudder for greater effectiveness you need to ascertain that the supports, shaft and all of the steering gear are within spec for the increased loads.
     
    Last edited: Jul 9, 2014
  13. Ad Hoc
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    Ad Hoc Naval Architect

    I wouldn't have the stock so close to the lead edge either. Move it to circa 20-25% of the chord.
     
  14. MikeJohns
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    MikeJohns Senior Member

    Slavi

    Flat plates work very well in a marine environment and L/D for a balanced rudder behind a prop in these sorts of applications can be indiscernible from NACA section providing they have an 'effective' continuation of the shaft present that carries on through the plate and I think a small Vee'd TE. The effective flow lines determining the effective foil shape. But simple flat plates or simple thinner sections have better characteristics at small angles.

    If you are interested John Hardiman had a bit of information on tank tests regarding this, but I think based on proprietary defense data .

    Aside:
    Another forgotten function of the rudder is to recover some of the energy from the resultant prop rotational flow field. :)
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed they do.

    Flat Plate Rudder on Swath.JPG

    Here is a view from the stern of Swath we designed many years ago. This too has a flat plate rudder...simple ;)
     
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