Call me thick but why does the hull sing?

Discussion in 'Laser Talk' started by sirena, May 27, 2017.

  1. sirena

    sirena New Member

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    Call me thick but why does the hull sing?
     
  2. Wavedancer

    Wavedancer Upside down? Staff Member

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    Actually, that's a pretty good question that involves vibration, fluid flow etc. Too difficult for me!
    In the meantime, one of my favorite sailing songs:
     
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  3. torrid

    torrid Just sailing

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  4. cskudder

    cskudder Active Member

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    Speaking in physics terms, it's a harmonic oscillation. The water creates a force on the foil which bends the foil, which then returns or swings back (like a pendulum) at a speed (frequency, cycles per second or minute) which is close to the speed at which the water deflects and returns to create the force again. So the foil bends back + forth. When the timing of the cycles of the water force and the foil's bend+return are close enuf, they reinforce the back+forth, which creates the noise.

    <<Southern Cross >> ... excellent choice!

    FWIW - in airplanes, aerodynamics- a similar harmonic oscillation becomes what is called "flutter" in control surfaces like ailerons, elevators, trim tabs, etc. Flutter can grow strong enough to damage the control surface or tear it off its hinges, which often enough results in the loss of the aircraft. It took down many aircraft before it was understood well enough to guard against it in design.
     
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  5. sirena

    sirena New Member

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    Thanks...i kinda like it though...its nice to think she sings for me when shes happy...
     
  6. Duncan_vdH

    Duncan_vdH Member

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    I thought it was the centerboard's way of saying "Please raise me a little bit":)
     
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  7. cskudder

    cskudder Active Member

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    ... and another FWIW, it does increase drag, altho generally only slightly. Hardly enuf to care about, except possibly in an extremely tight race. And +1 to what Duncan said - raising the daggerboard usually reduces or eliminates it. That raises the resonant frequency of the daggerboard's vibration higher, so that it no longer matches the resonant frequency of the water's movement.

    Sorry that all sounds so dull compared to "my boat singing for me when she's happy" ...
     
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  8. sirena

    sirena New Member

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    Hi Duncan...you say raise it a bit..she sings when im on a beam reach and i already have it probaly higher than i should...but ill try raising a bit more
     
  9. cskudder

    cskudder Active Member

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    Also try putting it further down. Usually any change to the board's position will reduce or eliminate it.
     
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  10. Rob B

    Rob B Active Member

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    Take a sanding block to one of the corners of the trailing edge of the dagger board. Use 220 grit sand paper to cut either the right or left edge. Finish it off with 400 grit wet sanding paper and the vibration will go away. It's also faster, (if that's any concern).
     
  11. sirena

    sirena New Member

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    Cheers ...I'll try that...Also the main sheet catches on the back corner. I try to whip the slack out of it when i go about or Jibe but it still catches...Any advice....
     
  12. sirena

    sirena New Member

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    Cheers....nothing's dull about sailing...unless there's no wind...
     
  13. Rob B

    Rob B Active Member

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    yes. Take some emory cloth and sand the underside of the aft corners. It rounds them off a little and helps the sheet slide across. Also try grabbing the mainsheet mid-boom as the boom comes across during a jibe. If you whip the slack out at that point it helps too.
     
  14. sirena

    sirena New Member

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    You like yr sand paper..Is this all legal for racing as well..
     
  15. inlandfreddy

    inlandfreddy Member

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    If You go about from close hauled it doesn't happen, so you can immediatly avoid it while tacking from any angle by sheeting in to closehauled when You head up. If you do that as You turn, You also stay powered up a longer time. While jibing You can also sheet in a bit and then time a pull in the sheet as the boom crosses over, this might take some practice, but after a while it will feel natural.

    There is lots of instruction videos for laser sailing on Youtube also.
     
    Last edited: Jun 6, 2017
  16. sirena

    sirena New Member

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    I've been watching....at the moment I'm fine tuning everything...trying to get everything set right...At the moment where i sail there is only myself.. and the wind is either below 10 knots or as like today 20 knots..And being a large expanse of inland water..I tend to go out in the stronger conditions...I appreciate all advice...
     
  17. LaLi

    LaLi Active Member

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    Sanding the trailing edges of the foils is legal.
    Sanding the gunwale is not legal.
    Heel the boat more to windward when starting the jibe.
    When the boom comes across, grab the sheet just below the forward boom block. Don't move your hand (with the sheet in it) in the direction that the boom moves. Don't try to "whip" it.
    In very light wind, pull the boom over by grabbing both parts of the sheet between the traveller and the boom.
    Tacking: see inlandfreddy's post.
     
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  18. Rob B

    Rob B Active Member

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    Not sure I understand this one. If you're doing the correct light air roll jibe you won't be able to grab the sheet there because you're sitting near the dagger board. If you jibe correctly it won't catch. However, it does on occasion. Then I just grab it off the weather corner with my back hand.
     
  19. LaLi

    LaLi Active Member

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    You mean grabbing the sheet by the traveller block? Well, you have to move back and to leeward for a second or two to reach it (admittedly bad), but once you have it in your hand, you put your weight right back where it belongs... and you have total control of the boom (very good). Works best when the air is so light that friction within the sheeting system exceeds the force with which the wind pushes the boom out.
     
  20. Emilio Castelli

    Emilio Castelli Member

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    You use your hiking stick to get to the mainsheet between the boom and the traveler. Don't have to move much at all.
    E
     

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