Mast pumping is "a frequency response' of the mast to energy (wind)
pressure. Think of a violin string that vibrates - same thing. Every
structure on earth has a frequency that will excite it to vibrate at a
maximum .... called the natural frequency. How to stop a mast, etc.
from vibrating: change the natural frequency by tightening or loosening
the rigging and/or changing the 'vibrational' shape of the mast by
adding a little fore/aft "pre-bend" bend when you tune.

Harmful - yes in some cases. The metals of the mast, shrouds, and
shroud terminal connections have a service life based on 'load cycles'
(in the millions) and if these load cycles are appliied beyond a
certain strength limit (called 'endurance limit') will rapidly result
in *fatigue* of the metal(s). Stainless steel is especially vulnerable
to fatigue. Most of these oscilations will be be under the 'endurance
limit' and are harmless .... but you can never tell when the endurance
limit is exceeded (due to the dynamics of the oscilations .... and the
'bad' cycles start 'deducting' from the number of maximum cycles before
the metal fatigues (service limit).

At sea, one uses running backstays, etc. to dampen such oscilations.
There are other simple methods / tricks on a sailboat mast that will
render a mast less vulnerable to vibrate: Most single spreader masts
will GREATLY benefit from establishing a 1" fore/aft "prebend" - which
bends the mast in the middle towards the stern of the boat.; and which,
**radically** changes the natural frequency ... double spreader rigs
usuallly need 2" of pre-bend, etc. To set pre-bend, tune the mast
dead straight, etc. then tighten the forward lowers and loosen the aft
lowers until the pre-bend is set .... that's it. Simply sight up the
mast at the sail track, pull a halyard tight and adjacent to the sail
track and measure/estimate a space of 1" forward bow/bend.
A good sailmaker usually will include such normal pre-bend into the
luff cut of mainsail anyway .... so you probably should take advantage
and tune your mast with the pre-bend to start with ... and get a better
shaped mainsail as a side benefit.





In article , MLapla4120
wrote:

Hi,
I have a sailboat and when it gets windy at the slip, sometimes there is
a vibration and the mast and rigging seems to resonate and vibrate. Someone
told me this is 'mast pumping'. I was also on another sailboat visiting and
it
did it also. Nobody seemed concerned.
My question is; should I be concerned? We get regular wind here at twenty
knots and sometimes more. The pumping seems to last about 3-5 seconds.
Then, it stops and resumes a little later.

Thanks for any info.