Engine vibration at 1900 to 2400 RPM's. Replaced engine mounts, engine alignment is correct, 2 blade prop has been balanced, new cutless bearing and bronze prop shaft appears to be straight. Shaft vibration also occurs under sail at hull speed (approx. 7 knots) with engine off and in neutral. Thinking of replacing bronze shaft with stainless shaft to provide a more ridged material possibly reducing shaft flexing and vibration. Please post any thoughts on this.
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Shaft Replacement Item
SSteel is somewhere about 1/4 to 1/3 more stiff than bronze so there should be less vibration for the same amount of trueness (no shaft will be 100-percent true). If the prop hit a small piece of debris it's possible it bent the tapered end of the shaft slightly. Is the vibration something recent or has this been since you had the boat? If you replace the shaft then check the coupling for trueness too. If non of the above works, another thing to include is a flexible shaft coupling. I installed a R&D coupling from PYI (pyi.com) which reduced vibration significantly. Used up about 1.25 inches of space between the couplings. A test installation can be done with the boat in the water by just removing the zinc closest to the strut and sliding the shaft back the appropriate distance to install the coupling. If this works then the shaft can be shortened accordingly (requires removal of the rudder).
SSteel is somewhere about 1/4 to 1/3 more stiff than bronze so there should be less vibration for the same amount of trueness (no shaft will be 100-percent true). If the prop hit a small piece of debris it's possible it bent the tapered end of the shaft slightly. Is the vibration something recent or has this been since you had the boat? If you replace the shaft then check the coupling for trueness too. If non of the above works, another thing to include is a flexible shaft coupling. I installed a R&D coupling from PYI (pyi.com) which reduced vibration significantly. Used up about 1.25 inches of space between the couplings. A test installation can be done with the boat in the water by just removing the zinc closest to the strut and sliding the shaft back the appropriate distance to install the coupling. If this works then the shaft can be shortened accordingly (requires removal of the rudder).
Vibration
Hello Antony, This, as I’m sure you are finding, is a strange one. Its very seldom that a shaft will vibrate in both a drive operation and a static/free wheeling state. Your drive position is pushing the shaft forward causing the engine to fall all about thus creating an environment that could easily cause vibrations. Free wheeling though is just the opposite. The propeller is free wheeling because of the drag being produced by the pulling of the propeller through the water. This pulling is also trying to pull the engine out the back end of the boat, which takes away loads that would commonly create vibrations. Does the vessel experience vibrations when in reverse moving aft? I would almost think that I would check the coupling for center and your cutlass bearing for center. Something during the rotation of the gearing is making the noise, I can’t believe it has anything to do with the shaft or the propeller. Those two items may simply be the catalyst/transmitter for the noise. (in my opinion) Hope this helps Kevin
Hello Antony, This, as I’m sure you are finding, is a strange one. Its very seldom that a shaft will vibrate in both a drive operation and a static/free wheeling state. Your drive position is pushing the shaft forward causing the engine to fall all about thus creating an environment that could easily cause vibrations. Free wheeling though is just the opposite. The propeller is free wheeling because of the drag being produced by the pulling of the propeller through the water. This pulling is also trying to pull the engine out the back end of the boat, which takes away loads that would commonly create vibrations. Does the vessel experience vibrations when in reverse moving aft? I would almost think that I would check the coupling for center and your cutlass bearing for center. Something during the rotation of the gearing is making the noise, I can’t believe it has anything to do with the shaft or the propeller. Those two items may simply be the catalyst/transmitter for the noise. (in my opinion) Hope this helps Kevin
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