vibration

[Dave]

Hi Kevin, and all:I have a 1996 C30 MkIII with the Universal 25XP and a 3 bladed bronze prop. When I aquired the boat last year, it had a huge vibration at low RPM. Upon checking the shaft alignment, I found it way out of tolerance. The manual calls for no more than .003 of an inch difference between the shaft and the transmission coupling. I had to move the enging a few degrees to the right and make various adjustments to the motor mounts, but it now matches up within tolerance, all the way around the coupling. I have a nice smooth running engine all the way from idle up to 2500RPM. from 2600 to 3000RPM, I have a "rythmic" knocking sound that will go away if I lift up on the stationary shaft log,(Just aft of the stuffing box.) I believe the shaft is touching the top of the shaft log at high rpm and lifting up on the shaft, gives it the small increase in clearance it needs. I plan on having the prop cleaned and balanced this winter, and then test running it in the spring. If the problem still exists, should I lower the aft mounts a turn, even though they match up perfectly now? I hate to pull the shaft, if that doesn't work, but that is all I can think of that is left. Any ideas?Thanks in advance, Dave

 

[Kevin L. Woody]

A whole lot of banging going on

Hello Dave,It sounds like you are experiencing the same ordeal I had on my own boat Coyote in that the shaft may not be centered in your stern tube/shaft log. Once you place the engine into gear and you begin to motor away, the engine begins to lower or compress the forward mounts and raise the aft mounts. The higher the Rpm’s, the greater the torque, the greater this movement or diving takes place. The danger of whacking the tube with the shaft is that it could potentially crack the fiberglass tube and we now have water entering the vessel. What I experienced was that the shaft was actually rubbing on the tube prior to hearing it whacking the tube. In reality I was using the stern tube like a cutlass bearing and supporting the shaft. This rubbing deteriorated the stern tube in that one area to the point it was apx. half the thickness of the rest of the tube. You too may have the same scenario taking place. If you drop the aft mounts a turn it will place the coupling out of alignment again causing the running gear vibration to return in some form. (Possibly not enough to cause much noise) I would remove the stuffing box by loosing it, slide it forward on the shaft, and try to see if in fact the shaft is not running true or centered. If the shaft is not running true just lowering the mounts will not take care of the problem because we will then have a strut out of alignment. With the strut out of alignment, you may then experience accelerated cutlass wear. Another thing to check is the engine mounts. If they have gotten soft the engine movement can increase allowing the engine to move a greater amount. A stiffer mount may reduce the running gear movement helping to keep the shaft in column. In Coyotes case I had to drop the engine on new stiffer mounts to center the shaft to the stern tube. We then loosened the strut and adjusted it to an aligned shaft, coupling and transmission. This fixed my shaft banging problem. I hope this helps. Kevin

 

[Dave]

Thanks, Kevin

Hi Kevin:Thanks for the fast reply. I think you have nailed the problem. We pull the boat next week, so i can remove the stuffing box and check the position of the shaft.Fair WindsDave S/V Westwind