.... more on harmonic vibrations.
No, not necessarily ...... if this case of possible mast vibration is the case, anything that has the same (or fractional) vibrational characteristics on the boat will all vibrate at the same time (some more, some less). Its like the strings on a guitar or other stringed instrument .... if you strike a high A note, the lower A (on another string) will also vibrate because its a harmonic and 'resonates' at the same or fractional frequencies. If you strike an A on one guitar, the A on a close-by guitar will also start vibrating - harmonic. Youve experienced this in automobiles and machinery .... all of a sudden one particualr part or section starts to magically vibrate (natural frequency) all by itself and if you change the 'excitation frequency' (changing speed, rpm, tension, mass of the part ... the natural frequency will change and the part will no longer vibrate. On a boat, every mast will oscilate at a certain frequency (sometimes very noticeablly - called mast pumping); usually it is encountered during high wind situations but if the 'tuning' matches the frequency, it can occur at quite low wind velocities (rare). Changing the tension on the rigging will usually shift the natural frequency and stop the vibration ..... only to have the vibration appear at different situation that matches the 'next' excitation frequency. Mast pumping at low wind velocities although rare is a fact; but, not as rare as a section of a sail that is oscilating to a certain frequency and causing an adjacent structural member (such a a jibstay, etc.) to vibrate as a 'harmonic'. Next time out, take a good look at the leech of the genoa and see if the vibrations (even very teeny vibrations) are coming from the genoa leech .... probably the most common cause/source of such vibrations. Another cause of 'mysterious' vibrations on some boats is caused by thin flat centerboards. At certains angles to the flow of water going past them they start to vibrate (or acutally 'hum') and if this hum/frequency is at the same natural frequency (or fraction of) of any other part or section of the boat that part will vibrate (harmonic resonance) also. Induced harmonic oscilations are a nuisance and can cause rapid rigging failure (fatigue); the solution is usually to change the frequency by changing tension or mass.