Mast Pumping While Anchored

  • March 26, 2013 6:11 PM
    Message # 1252503
    Deleted user
    Hey Guys and Gals,

    It's blowin' pretty good here in Key West this week, a pretty steady 25kts with gusts sometimes holding at 30. 
    My anchorage is totally exposed to the wind direction, the next land mass in that direction is the Panhandle. So we're bouncing around a bit.
    I'm getting some mast pumping somewhere near 30kts.
    I'm typing in my v-berthkhjggdgh, see.....I told you so.  ;)
    My rigging is out of tune because I'm tinkering with some things lately.
    Many sailboats pump even when properly tuned (so I've been reading on the web) and some sailors make adjustments using running backstays to alieviate.
    I currently have no running backstys, the tangs are up there smiling at me awaiting my ambition.
    So my question is;
    Have you noticed your W32 pumping whilst at anchor?
    Is your boat totally pump free at anchor?
    Am I simply out of tune and if properly tuned I will experience no pumping?
    If pumping is common, what's the bandaid?

    Thanks,
    Carl
    Last modified: March 26, 2013 6:20 PM | Deleted user
  • March 26, 2013 6:47 PM
    Reply # 1252517 on 1252503
    Deleted user
    I found this just now.



    Mast Vibration

    Sleepless Nights!

    Anyone who has a vibrating mast knows how much of a nuisance this can be when trying to sleep or otherwise spend time down below. Mast vibration causes a great deal of noise below decks since the hull acts like a sounding board and amplifies the vibration noise.

    Vibration is more likely to occur on poorly stayed deck stepped masts, usually with single lower stays which do not provide fore/aft support. Technical Explanation

    The technical explanation of mast vibration is something called Karman vortex shedding. Basically, air flowing over the mast does not re-attach as it would over an airfoil and this causes regions of low pressure on the leeward side. These regions are unstable and tend to move up and down as the vortex shedding occurs. The rate at which the low pressure regions move is a function of windspeed, mast diameter and shape. When the natural frequency of the mast section coincides with the frequency of the vortex shedding, a resonance can occur which results in an marked increase in amplitude and severe vibration can occur. This phenomenon often occurs at quite low windspeeds (4-12) knots.

    Vortex shedding is common on tall slender chimneys - If you look at any industrial plant, you will likely see a chimney with spirals welded on. These spirals are intended to move the low pressure regions from the leeward side to the windward side and thus overcome the vertical movement of the eddies. Quite a lot is known about Karman vortices, but this has not always been useful in yacht design. Solutions?

    At the design stage, the designer should consider the following:
    Mast Support - Keel vs Deck or at least ensure the mast step is well supported.
    Mast staying - Use fore/aft lowers or swept back spreaders to better provide fore/aft staying
    Mast Section - Analyse the mast natural frequency vs Karman vortex shedding.
    Mast Shape - It could be that some mast shapes are better than others.
    Once you have this problem, there are a few things that could be tried:
    Mast Step 1 - Ensure that the mast base is fully supported on the mast step and not bearing on a point.
    Mast Step 2 - Ensure that there is adequate support under the mast step - there is sometimes a gap.
    Rig Tension 1 - If you have fore/aft lowers or a baby stay, try varying the tension.
    Rig Tension 2 - Try adjusting upper shroud tension - This affects the natural frequency of the mast.
    Temporary support - Tie a line from the Jib tack to the spinnaker eye or simply around the mast.
    New Stays - add fore/aft support by adding baby stay or additional lowers.
    Add a Vortex Fence - A narrow stiff (2 to 4") strip raised to say 2/3 of the mast height in the mast slot.
    Try using your mainsheet to tension the topping lift and provide some forward boom force to stiffen the lower mast.
    Try inducing some mast bend with the backstay adjuster - this will stiffen the mast.
    Summary

    This an annoying and difficult problem to solve - There is no simple answer. The only way is to keep trying ideas until something works for you. Good Luck!


    I have no idea who the author is, I thought it might be good to share here.
    I'm still interested for specifics solutions for our Westsails.
    Thanks,
    Carl
  • March 27, 2013 5:04 PM
    Reply # 1253335 on 1252503
    Carl,

    Read section A-13 of my Westsail Service Manual on running backstays.
  • March 27, 2013 6:18 PM
    Reply # 1253385 on 1252503
    Deleted user
    Bud,
    Thank you for taking the time to read my above post. I have read Page A-13 many times as I intend to install your running backstays. I understand mast pumping while under sail and the importance for running backstays. I assumed it could also be helpfull at times while at anchor but wasn't sure.
    Thanks,
    Carl

    Does anyone have zero mast pumping at anchor without the use of running backstays and if so whats your secret?
  • March 28, 2013 10:01 AM
    Reply # 1253838 on 1252503
    Anonymous
    I've never had this problem but I remember reading a long and interesting discussion on it.  Some people suggested two things that people agreed worked.  One is to wrap your halyards around the mast.  The other was to hoist something like a fender or two up the mast.  They'll interrupt the airflow and disrupt the harmonic vortex effects.