My C-Dory had the most annoying vibration and resonance in the cabin at certain engine speeds.  I loved the boat but didn't like seeing my bow pulpit vibrating while listening to the stove rattling and hearing this droning vibration throughout the interior.  I tried a prop change and it did nothing.  I installed the complete Vibra-Stop kit with pads and bushings.  It made such a difference I decided to become the U.S. Distributor!  My oldest daughter said it all when I asked her if she noticed a difference: "Dad, the boat is so much quieter, what did you do to it?"