Interesting.
Although from my experience if theres problems with teh exhaust it will cause high frequency vibration on the vessel, for something like that to travel a distance, through the earth, and crack things it would have to be low frequency. To me its more likely as its being transmitted from teh Hull of the vessel which is getting the vibrations from the engines.. which seem to be more and more hardmounted to the hulls on a solid bedplate, then having rubber mounts or chockfast resilient mounts. Could be from the propeller, but usually if theres an off pitch on one of the blades you would hear a 'singing' sound as well as feel a slight vibration.
