Rudder vibration

Discussion in 'Hydrodynamics and Aerodynamics' started by motorbike, Aug 22, 2017.

  1. motorbike
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    motorbike Senior Member

    I built a high aspect spade rudder sometime ago for a 10 m yacht, it came from the drawing board of a well known local naval architect. I had a CNC cut female mold made up and made the shells in them. The foil section is quite fat similar to an NACA 0012 but with a slightly hollow run aft.

    It was reasonably well built but not perfect due to the glue going off during the process of joining the halves together, perhaps a slight 2 or 3mm mismatch at the bottom of a 2m board, not noticeable and faired back in very carefully.

    The issue is that the rudder has a very slight vibration, it never goes away and is noticeable by me anyway. It was suggested that it could be the vortexes coming off the aft end of the foil being slightly miss matched and that the solution would be to angle the trailing age 45° Does anybody have any suggestions? Is it slowing the boat down I guess is my first concern, and am I likely to make it worse by altering the foil shape if I do chamfer the trailing edge which at the moment is square and about 4 mm wide.
     
  2. TANSL
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    TANSL Senior Member

    That 4 mm wide edge can induce the formation of vortices that generate the vibration you have noticed. Try to make this edge of the smallest possible width. Although that may not be the definitive solution, it will certainly help.
    Does the frequency of the vibration vary with the speed?
     
  3. HJS
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    HJS Member

    The trailing edge of the rudder must be completely flat with two sharp corners. In this way, two standing vortex are created instead of two alternating vortex that create vibration. The trailing edge can be wider than 4 mm.
    JS
     
  4. TANSL
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    TANSL Senior Member

    Where do you measure the thickness of the trailing edge? I still believe, although I may be wrong, that a straight trailing edge of 4 mm thick is not convenient. Snap14.jpg
     
  5. HJS
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    HJS Member

    I've done it on all my rudders and centerboards, and I know it's working.
    The boatbuilder has sometimes not observed the detail in my plan, and therefore in his ignorance rounded the trailing edge. When we then shaped it as intended, the vibration disappeared.
    JS
     
  6. TANSL
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    TANSL Senior Member

    Perhaps there is some explanation for what, at first glance, to a non-expert in this matter like me, seems contradictory.
     
  7. motorbike
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    motorbike Senior Member

    Fine trailing edges are problematic, in an ideal world they would be infinitely small but as I understand it they can be up to say 10mm with no ill effect if the edge is sharp. That is the problem, sharp edges dont hold paint and get damaged easily. The vibration doesnt worsen with speed, its just a constant very light rumble all the time once youre over about 4 to 5 kts
     
  8. gonzo
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    gonzo Senior Member

    High speed rudders are usually a wedge with the wide side towards the stern. Also, the aft face is flat. This prevents vibration.
     
  9. jarmo.hakkinen
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    jarmo.hakkinen Junior Member

    In Principles of Yacht Design, Larsson suggests following trailing edge shapes, and their relative vibration levels are shown here.
    Nimetön.png
     
  10. Joakim
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    Joakim Senior Member

    The vibration motorbike describes doesn't sound like the normal hum coming from foils at higher speeds (say 8+ knots for a 10 m boat). I have a 11 m boat and I have made the trailing edges quite thin. 2-3 mm for the keel and 1-2 mm for the rudder. The original was 14 mm and 4 mm. I extended the profiles until thin enough with epoxy filler. The keel got 50-60 mm extension, which I reinforced with glasfiber. The rudder needed just less then 10 mm and is just filler. After five seasons no problems with the paint and once slight damage to the keel from a chain. That was easily repaired with just the filler. No humming or vibration at any speed now. There was some at higher speeeds before, but the reason for this job was drag reduction.

    Then I have a 6 m boat with about 10 mm very sharpe edged flat trailing edge. Hums like crazy at 8+ knots. Class rules do not allow to make it thin.
     
  11. HJS
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    HJS Member

    What i 6 meter boat are you refering to? What does the profile look like? The reason for the vibration can be something completely different than the trailing edge.

    JS
     
  12. Joakim
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    Joakim Senior Member

    It's a 606: Om båten http://606-forbundet.se/baten/
    Here are profiles and some optimation within the rule (page 7, in Swedish): http://www.606.dk/uploads/1/5/7/7/15772636/606-racing_2_2004.pdf
    In the latter the author says that the profile doesn't seem to be any commonly known one and may be drawn with free hand by Pelle Petterson who designed the boat in late 60's.
     
  13. UpOnStands
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    UpOnStands Senior Member

    Any value in forming a concave surface on the aft edge?
     
  14. PAR
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    PAR Yacht Designer/Builder

    Two approaches work, for cutting down noise, a single or a double edge chisel shape, with a 2:1 aspect ratio. If desired, you can "roll" (ease) this transition zone, though not much gain is offered over the 2:1 bevel. Maintenance issues with these edges are well understood, which is why you see all sorts of trailing edges. I tend to make them about 1/4" (6mm) on none sparkling performance blades and chisel them (2:1) on higher speed craft. To improve maintenance issues I often insert a length of metal (aluminum usually), so it can be beaten back into shape and/or sharpened. I did run into an odd board that was chiseled and it developed a sympathetic (harmonic) vibration, at a particular speed. The solution was an asymmetric chisel shape.
     

  15. SamSam
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    SamSam Senior Member

    Just a suggestion, but has anyone ever put a flexible trailing edge on a rudder to try and stop vibrations? Say a slot where a replaceable strip of some flexible material could slide into and then the vortexes would dissipate on the strip instead of on the rudder.
    I think even two strips of duct tape might work for an experiment. 2" wide duct tapes with 0.5 " stuck to the trailing edge of the rudder and the trailing 1.5 " of tapes stuck to each other.
     
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