My Tieara 3100 with twin 454 Crusaders and velvet drive transmissions has developed a bad vibration at the rear of the boat that starts at approx 1800 rpm seems to peak at approx 3000 rpm and goes away at 4000 rpm and above. The vibration has slowly been getting worse over the past year. I plan on checking the engine alignmentthis week but a cursory review of the engine mounts indicates that they are in bad shape and so I suspect that realigment will not be easy. I have also read on this forum that vibration like this can be caused by the rudders being out of alignment. Also it has been approx four years since the props were last tuned. My question is that with so many possible causes of vibration, does the rpms at which the vibration is worse give any clue as to the possible cause?