Q: What can be done about mast pumping, the irritating mast oscillation that seems to occur in 12-18 kts wind while the boat is docked or anchored? I've played with the lower shrouds and backstay tension to no avail.
Any member of roughly circular cross section in a fluid flow will generate vortices in the lee of the flow at some minimum critical speed. I won't dwell on why, but these vortices ocsillate first from one side and then from the other. They are called Von Karmen vortices and much work has been done in the aerodynamic world on their initiation and prevention. They can be very destructive over time. As the oscillations increase in strength and frequency the member (the mast in our case) experiences an exciting side force. The effect of this force will peak as the exciting frequency of the vortices nears or matches the natural frequency of the mast. The mast's natural frequency is primarily a function of (a) its "unsupported" length; (b) its stiffness (moment of inertia/radius of gyration of the section); (c) the class of restraint/support (ie cantilever or simply pinned as in our case), and (d) the stress in the mast due to rig tension, although this is a secondary factor here. The final technical factor is that of harmonics or the number of harmonic waves induced in the member. What some of you have observed is a 1st harmonic (one wave) vibration of a near circular section occuring when the frequency of the Von Karman vortices came close to or matched the natural frequency of your mast and rig. What you also likely observed is the cyclical nature of the vibration, lessening and worsening with the gusts of the wind. Pumping will also change with the swing of the boat at anchor as the wind hits the mast from slightly different angles. The reasons are as follows: From an engineering perspective pumping CAN be stopped or reduced by: (a) changing the "unsupported" length of the mast by, for example, inserting wedges at the partners in a boat with a keel stepped mast, (b) moving to a stiffer section (not to practical except its something to watch on composite masts), (c) dramatically changing the tension in the rig, (also not too practical), and finally (d) turning the boat 90 degrees to present a different stiffness and cross section to the particular wind conditions. Let's get to the practical: The best and proven way of stopping pumping is to stop the formation of the Von Karman votices. This is done by interupting the formation of turbulent flow around the mast section. All this is a long winded way of saying: you must wrap several halyards bunched together around the mast in a spiral and haul them tight!
I've found that if you use the main halyard and the spinnaker halyard and "marry" them just below the spreaders you can end up with one long "split" wrap above and three "married" wraps below which seems to be enough. I also have an extra jib halyard too so I wrap the works. The final configuration is not important as long as it disturbs the flow around the mast as much as possible. As I said I've never experienced any pumping after using this method on my last three boats. Not too pretty but it works.
The spiral does not prevent the formation of all vortices but does stop the formation of neat ocsillating ones which are the problem. Haul them tight to prevent the perenial problem of slapping halyards which we have historically moved AWAY from the mast to sleep! We were just making things worse. The best commercial example of this type of solution is to look at an oil refinery next time and notice that all vertical towers of any consequence have something spiralling around them; often stairs but something if not stairs. Similarly any of you in San Francisco will notice pilings in a heavy tidal flow often have cables wrapped in a spiral around them so the "pumping" doesn't work them loose. Finally, the "humming" of the rig is a related problem caused by somewhat the same phenomena. The same solution can be used but in this case the tension on the rig will have more effect as we are dealing with higher harmonics and less rigid members (the wire). Anyway, try it, you'll like it.
Gary Wiseman, Up Spirits #894
Your answer has explained why it's eliminated during the winter when the boat is on land. To accommodate the winter cover, I've taken the lazy jacks and wrapped them around the mast so your solution does work.
Ron Hill, Apache #788