I appologise if the following is already widely known (I'm always the last to discover such things).....
There is a marina that I occasionally like to use, but it is subject to fresh to strong westerly winds and the berths are positioned north/south and consequently the wind
is on the beam, and my boat experiences the most fearful amount of mast
pumping as the wind
accelerates/decreases through the 14-15knot mark. Apparently all masts will vibrate in a beam wind of a certain strength to a lesser or greater degree, but in my case it was extreme. I have tried endlessly tweaking the rig, but with no success, and after a particularly bad vibrating, sleepless night decided on a little research
Any long conical structure exposed to the wind will vibrate, this due to the occillation of the wind around the structure and the resultant vortexes thus formed. This is know as the Von Karman effect or the Von Karman vortex, named (if you're interested) after the Hungarian/American physicist and engineer
Theodore Von Karman.
Fortunately, he also devised a solution. I had alway assumed that those little spirals around tall metal chimneys were to add strength to the structure. They're not - they are there to disrupt the wind occillation and vortex, thereby eliminating any harmful vibrations that could result in the collapse of the chimney.
My inprovisation was to use the spinnaker halyard
and haul a couple of old mooring
lines to the masthead and wind the whole mess around the mast
a few times and make it fast at the gooseneck.
I had an expectation ranging from nothing to mild, hopefully moderate inprovement. What I got was a complete cure. The mast is absolutely vibration-free. All I need to do is one day make my rope
spiral look a little bit more pretty, but just now I'm revelling in my long awaited success. Aint science wonderful!