Probably/possibly 'an induced harmonic vibration' .... a certain member of the mast or rigging system being excited at its 'natural frequency of oscilation', easy to develop and easy to remedy. Second possibility, as another poster listed is a loose leech (control line) on a sail or improperly adjusted jib fairlead car.
Just like a violin string, any part of the rig can start vibrating, sometimes violently, if the natural frequency of vibration for that 'part' is being 'induced' to vibrate at that certain resonant frequency. This can include rigging wire as well as the mast ... and they can at that natural frequency begin to 'pump' at a rate to match the natural frequency of the component. As stated above, sometimes a violin or other stringed musical instrument will sometimes 'play' (apparently all by itself) an untouched string if the string is in resonance with another being played .... resonant induced harmonic vibration.
The natural frequency is dependent of the 'stiffness' or tension of the part vibrating. Generally if a naturally occurring oscillation occurs in a shroud or wire ... simply change the tension in the shroud - shifts the natural frequency to a different 'pitch' and will take a totally different set of circumstances to make the wire vibrate. Remedy, change/increase the rig tension to shift the natural frequency to occur at a harmonic condition much higher than you normally sail.
Sometimes in very high wind conditions long distance passage sailors experience violent mast pumping (induced harmonic response) when the wind across the sails and mast is 'matching' the vibratory exitation node (natural frequency) of the mast ... on such boats the sailor then simply applies tension to a running backstay etc., or can simply pull on one of the shrouds perpendicularly with a length of line to a winch and the tension imparted changes the natural frequency to the system/part that is vibrating. If the mast is vibrating (pumping) you can easily 'tune-out' the pumping by setting up the rigging so that the mast has more (forward) 'bow' to it. Traditional masts (not B&R rigs) usually should be set up with 3/4" forward bow for a single spreader rig and 1/2" forward bow for each spreader set on a multiple spreader rig; this 'bowing' changes the natural frequency of the mast by changing the geometric 'moment of inertia'
Is such an excitation dangerous? Yes indeed, as the collective/additive impact forces of such a vibration can subtract from to the fatigue endurance life of the system/component, rapidly weakening the metal, etc. components and shortening the service life; such vibrations can rapidly lead to the metals involved becoming - 'tired out'.
When this 'vibration' occurs go forward and look to see actually/exactly WHICH component is vibrating. Changing of the natural harmonic frequency is done by changing the tension, changing the geometry (by increased 'bowing' of a mast), or changing the total mass (adding a small mass/weight to the middle of the vibrating part sometimes will sometimes do it).
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Sails can sometimes develop aerodynamic flutter ... a condition similar to the above. The fibers or membranes of sail material are elastic (almost all solid materials are elastic) and whole sections of a sail can develop the same harmonic vibratory effects. If the sail is not MAL-shaped or otherwise badly trimmed a slight change in edge tension (via halyard/sheet, etc.) which changes the shape of the sail will lessen or remedy such 'flutter'.
The leech is most prone to 'flutter', especially if the leech cord tension is ignored or there isnt a leech cord to add a bit of tension to the leech. Tightening the leech cord will change the natural (flutter) frequency here too, ... so will moving the jib fairlead car a wee bit forward if one doesnt have a leech cord.
(Very thin sectioned keels can also vibrate at a 'node speed' for the same reasons. for instance the bilge boards on the exceptionally fast Inland Lake Scows at super high speed will begin to loudly hummmmmmmm - induced harmonic vibration!!!)
Again in sails as in other 'solids', changing of the natural harmonic frequency is done by changing the tension, changing the geometry/shape, or changing the total mass (adding a mass/weight to the middle of the vibrating part sometimes will sometimes do it),
Hope this helps.
