preventing wake eddies by spiral rope?

Discussion in 'Hydrodynamics and Aerodynamics' started by UpOnStands, Jan 3, 2016.

  1. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    for a circular unstayed mast of 120 mm diameter would wrapping a halyard around the mast, say 6 turns in total from top to bottom, prevent the formation of wake eddies (Karman vortices) at anchor?
    Would the net result be to lower the drag coefficient or increase it?

    oops should be mm not cm
    thanks to Mr Efficiency
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,035, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    120 cm diameter ? thats 4 feet !
     
  3. Squidly-Diddly
    Joined: Sep 2007
    Posts: 1,837
    Likes: 145, Points: 63, Legacy Rep: 304
    Location: SF bay

    Squidly-Diddly Senior Member

    I'm guessing: Yes, should prevent harmonic related action, same as varying tread spacing on tires. Quite sure.

    And, would decrease drag since a non-harmonic wont be converting as much energy. But not sure. Might be related to dipples on golf balls. 70% confidence level.
     
  4. philSweet
    Joined: May 2008
    Posts: 2,550
    Likes: 319, Points: 83, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Depends what you are trying to do. There will be resonance frequencies where the mast strums and pumps. You can use spiral wraps to disrupt the coherence of the vortex street. Drag is increased substantially. On a sailboat at anchor, this could be a problem. A drag increase in front of the LCR would encourage tacking about the anchor. The fix can be engineered, but you need a lot of information. It helps to know if you are trying to deal with a specific vibration mode. For suppressing the primary vibration mode of an unstayed mast, you need only apply the spiral to the top 1/3 of the mast. The wrap should have a diameter ratio of between 1/16 and 1/8 of the mast.

    to help with your googling, try the following search terms:

    spiral strakes
    helical strakes
    Vortex induced vibration (VIV)
    vortex prevention on stacks
    VIV suppression efficiency factor.
     
  5. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,035, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Plenty of chimney stacks with the corkscrew strip.
     
  6. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    lots of chimneys with the strakes but no mention of yacht systems.
    my interest is in having two identical side by side masts and wanting to detune them so no harmonics develop.
    reducing drag would be a plus.
     
  7. philSweet
    Joined: May 2008
    Posts: 2,550
    Likes: 319, Points: 83, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

  8. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    thanks for the paper but it appears to be focused on the direct aerodynamic interaction of cylinders which are closely spaced.
     
  9. TeddyDiver
    Joined: Dec 2007
    Posts: 2,589
    Likes: 125, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    Sure it works, thou drag depends of the windspeed and halyard diameter, propably goes both ways, lower in circumstances where oscillating vortices would occur. I'd reckon more rounds is much better than few.
     
  10. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    thanks for the input.
    sounds as if it might work them, which is promising.
    the other tactic is to build the internals of the masts slightly different so they vibrate on different planes.
     
  11. Squidly-Diddly
    Joined: Sep 2007
    Posts: 1,837
    Likes: 145, Points: 63, Legacy Rep: 304
    Location: SF bay

    Squidly-Diddly Senior Member

    What is LCR?
     
  12. TeddyDiver
    Joined: Dec 2007
    Posts: 2,589
    Likes: 125, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    Longitudinal center of resistance.
     
  13. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    thought it might be
    the masts are aft of LCR
    which I suppose makes running bare poles DDW rather tricky without a drogue or forward storm sail
     
  14. rob denney
    Joined: Feb 2005
    Posts: 840
    Likes: 222, Points: 43, Legacy Rep: 436
    Location: Australia

    rob denney Senior Member

    I'm curious about a rig with unstayed masts aft of LCR. Got any drawings?
    ta
     

  15. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

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.