above 30 knots my propellers begin to vibrate. can this problem be fixed or reduced in magnitude?
If the vibration begins at relatively high speed and increases in noise level as you go faster, most likely the problem is due to propeller cavitation. Usually it will sound like gravel passing through the propeller and if you hold your hand or bare foot to the inside skin of the hull over the propeller, you will feel the vibration. With heavy vibration, touching of the inside skin of the hull over the propellers, may even give a "burning sensation". The greatest vibration amplitude will be peak at a distance about 20% of the propeller diameter forward of the propeller for mild cavitation. The peak will move aft of the propeller if there is heavy cavitation.
This type of vibration problem is best mitigated by improving flow (reducing turbulent flow) into the propeller and changing to different propeller dimensions, style and/or number of blades along the possibility of changing the reduction gear ratio. Generally it is best to seek technical advice from a professional knowledgeable in the design of marine propellers.
