So the real question is how is the vibration felt... is it in the range of the 0-6 beats per second... is it felt as "thumps" or is there some other characteristic? I would thing something that low could be seen as a flicking in the mast. (and probably somewhat scary... I can imagine the mast and the shrouds all "whipping" a bit at that low frequency)

Can you change the pitch of the prop to make any difference?