Vibration while reversing - Volvo saidrive 120 - engine penta 55

[elias]

Hi again !
Does anybody ever noticed when reversing under load ( example checking the mooring ) a vibration that might come from the gearbox ? It stops when the boat starts gaining speed while reversing .

[Symphony]

You have in a way answered your own question. Doubt it is anything wring with the gearbox. The engine is being overloaded when you are trying to move the boat with the prop when the boat can't move. As soon as there is waterflow when the boat starts moving the vibration will disappear - just as you have observed.

[elias]

Hmmm perhaps you are right , guess I m still sensitive after a failure last summer of an adapter shaft/cylinder inside the gearbox and let me with no propulsion . Anyway thanks for the advice !

[JEN-et-ROSS]

Hi Elias.
   Symphony is probably correct, with the boat stationary the prop. blade angle of attack will be far too high causing the water flow over the blades to stall and cause significant turbulence that you feel as a vibration. As you gain speed the AoA decreases and the blades unstall, so no turbulence, therefore no vibration...Bill

[elias]

I m thinking that I feel it more intense cause the propeller is clean( the boat just fall back in the water ) an pushes far more water than before where it was dirty

[IslandAlchemy]

Probably cavitation.  Maybe soft engine mounts.