trailing edge

Discussion in 'Hydrodynamics and Aerodynamics' started by Chaos, Sep 27, 2020.

  1. Chaos
    Joined: Sep 2014
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Arkansas

    Chaos Junior Member

    I have been searching for more information of the effects of the trailing edge (TE) of keels and rudders in these forums and others. I have not been able to find a full picture of the problem, just minor references here and there. The best I can ascertain is that the least drag TE would be a fine point. But such a TE would obviously be fragile and subject to damage and distortions in the real world under a sailboat. The solution is a blunt end TE. This edge might also be beveled, say 30 deg. perpendicular to chord to allow vortex shedding without creating oscillations and vibrations. I can see how oscillations of this type in a forced vibration mode could create drag. The rounded TE is not good on drag, period. References have been made by some organizations and posts that the TE should be 0.25 inch width. But, it would seem one should take into account the length and width of the NACA shape as to how wide the TE should be. Is there a rough rule as to what percentage of length or width?
    So, my conclusion is that to accommodate a TE that is not too fragile, a blunt end TE is the preferred compromise, but keep the width as thin and narrow as possible in order to minimize the increase in drag. And, what number is that width?
    Thanks for everyone's advise and comments.
    Ron
     
  2. DCockey
    Joined: Oct 2009
    Posts: 4,818
    Likes: 370, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

  3. jehardiman
    Joined: Aug 2004
    Posts: 2,823
    Likes: 377, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    For a standard NACA 00xx foil at boat speeds in water, the trailing edges should be cut off square at 2% thickness. See Heorner, both Fluid Dynamic Drag and Fluid Dynamic Lift because locking in the trailing vortex is important to prevent flutter.
     
  4. Will Gilmore
    Joined: Aug 2017
    Posts: 320
    Likes: 119, Points: 43
    Location: Littleton, nh

    Will Gilmore Senior Member

    To improve the flow of blunt tailing edge forms, I understand it has been the practice to texture the surface to encourage fluid adhesion as it flows into the curve. This prevents cavitation and eddying.

    Will (Dragonfly)
     
  5. DCockey
    Joined: Oct 2009
    Posts: 4,818
    Likes: 370, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    A rough surface only delays or prevents separation in a specific Reynold's number range, and then the roughness needs to be applied upstream. Roughness has no effect on cavitation.
     
    Jimboat and TANSL like this.
  6. CocoonCruisers
    Joined: Dec 2015
    Posts: 72
    Likes: 21, Points: 8, Legacy Rep: 10
    Location: Marseille & BuenosAires

    CocoonCruisers Junior Member

    (No direct experience with this and may not transfer well to keels):
    It seems fashionable lately among the french ocean racers to round only the upper (suction) side of their foil's TE, pressure side edge remaining a right angle.
    This is done to avoid micro-oscillations aka annoying 'singing' that was robbing some racers' sleep.
    Here is a presentation featuring the same trick for a tidal turbine, where the aim is to avoid disturbing marine life: Cavitation and Hydrodynamic Evaluation of a Uniquely Designed Hydrofoil for Application on Marine Hydrokinetic Turbines R. Phillips, W. Straka, A. Fontaine. - ppt video online download https://slideplayer.com/slide/8394439/

    EDIT: I'm still wondering about the applicability to keels, which are symmetric foils that will produce lift towards one or the other side depending on tack.
    Half the time the shape above would be 'the wrong way around', according to documented usage. But:
    - The scales of the macro flowfield around the whole foil section and the micro flowfield at the trailing edge are two orders of magnitude apart.
    - Most foil sections' trailing edges will roughly match the two flow speeds for smooth reconnection. So there isn't any wild flow going from one side towards the other at macro scale.
    - The detail is so tiny that it's effect will probably remain within the boundary layer.
    So i'd think i would be worth a try.
    What there is to gain:
    - Hopefully no singing/turbulence on either tack.
    - Twice the rounding diameter when compared to a less efficient 180° arc and a right angle seem easy to fabricate and less fragile than other thin shapes.
    Or am i missing something there ?
     
    Last edited: Sep 28, 2020
  7. Chaos
    Joined: Sep 2014
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Arkansas

    Chaos Junior Member

    Thanks for all the responses. Interesting where these discussions can go. I have spent just a short time reading Heorner. Great one can get it downloaded in PDF. The equations I have found so far use the change in chord, which makes sense. I will keep working on this. Of note, where did the 1/4 inch TE concept among a lot of sailors come from? Just a bit misleading!
    Thanks again. Ron
     
  8. Blueknarr
    Joined: Aug 2017
    Posts: 1,018
    Likes: 215, Points: 63
    Location: Colorado

    Blueknarr Senior Member

    1/4 inch is where most pointy ends crack up at.
     
  9. Chaos
    Joined: Sep 2014
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Arkansas

    Chaos Junior Member

    Yes, that would make sense, depending on the material and method of fabrication. But, how much drag are we giving up? My keel is 5.0 inches at the widest section of the NACA 00xx (appears to be), which at 2% results in a TE width of only 0.10 inch. The rudder ends up very narrow. (My rudder started out at about 0.375!!!) I can easily see a metal trailing edge laminated into the rudder, if one was starting fabrication from scratch. Carbon fiber reinforcement should also help in keeping the TE as narrow as possible. I need to convert the TE width to chord length change and compare the numbers to Heorner's text to get an idea of drag changes.
    Thanks for all input. Ron
     
  10. BlueBell
    Joined: May 2017
    Posts: 1,408
    Likes: 302, Points: 83
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Scalloping?
    Hello, look at nature.
    And put some barnacles on while you're at it. <joking>

    I'm surprised Jehardiman didn't mention it.
     
  11. Chaos
    Joined: Sep 2014
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Arkansas

    Chaos Junior Member

    Who used the word "scalloping"?
     
  12. jehardiman
    Joined: Aug 2004
    Posts: 2,823
    Likes: 377, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Actually, I have found that whale snot and ctenophore feces has more effect on the TBL than cetacean proto-forming.
     
  13. Chaos
    Joined: Sep 2014
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Arkansas

    Chaos Junior Member

    All I need is whale snot?? Good grief, why have I been working so hard on this?
     
  14. jehardiman
    Joined: Aug 2004
    Posts: 2,823
    Likes: 377, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Pretty much; everyone has known this since the early 1980's....now keeping it attached.....and distributed to the correct place....that's still open for work.
     

  15. philSweet
    Joined: May 2008
    Posts: 2,375
    Likes: 216, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Aha. So that's really what the riblets are for.
     
    jehardiman likes this.
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.