Post
by DickTuschick » Fri Jun 20, 2014 9:06 am
Hello to all,
I thought it might be time to provide a "recap" on this subject.
Shaft vibration has been an issue on some PDQ 34's and in almost every case it has been traced back to engine mis-alignment. In a few instances it actually originated at the factory where they did not do a good job when commissioning the boats but it is an issue that needs to be checked on every boat periodically in any case.
Engine out of alignment will cause vibration but this is often hidden by the flexible shaft couplings that are installed on all PDQ power cats. The flexible coupling can be both a good thing and a bad thing. On the good side, it does make the boat run smoother and quieter, but on the bad side "hiding" the vibration prevents the operator from knowing that he/she has a problem. If there is vibration, it can lead to premature failure of the dripless shaft seal and the cutlass bearing.
In every case where we saw a leaking shaft seal it was traced to engine out of alignment.
In extreme cases, an out of alignment condition can lead to a "wobble" in the shaft so this also must be checked if a problem has been detected. This "wobble" condition is facilitated by the fact that the shaft length between shaft coupling and aft cutlass bearing is at the maximum recommended distance. This is normally not an issue, but if the engine has been out of alignment for a long period, a "wobble" can easily be created.
What have we learned from all this ? Well, certainly that engine alignment should be checked periodically by a qualified mechanic, one that is skilled in the "art" of alignment. We have also learned that if your shaft seal is leaking or your cutlass bearing is worn, it most likely is caused by an out of alignment condition……but…if the cutlass bearing is really worn, this too by itself, can cause the vibration that then causes the shaft seal to leak.
As noted in an earlier posting there is an upgrade to the boat that has been designed by the boatyard in Stuart that does most of the work on the PDQ's, Stuart Yacht Builders. They have designed a Mid shaft bearing that is installed in the fiberglass shaft tube just aft of where the dripless shaft seal attaches to the tube. This additional bearing allows the shaft to be held straight between the two cutlass bearings and thus a better engine alignment can be achieved. Further, it does eliminate the possibility of a shaft "wobble" entering into the equation. The installation process includes checking the shaft straightness, checking the aft cutlass bearing, installing a new dripless shaft seal, and doing a very exacting engine alignment. In total this is a rather expensive upgrade and one that is definitely not required to have a well running boat. At present there are a few boats that have had this upgrade installed and all of satisfied with the outcome, myself included. I must add that in the case of our boat HERON, we did not have a problem of any kind as I was very consciences about engine alignment, but rather just wanted to "test" this upgrade so I could comment on it with some experience.
Hope this helps clarify this subject.
