Progress Update - Finding the Cause
It has been quite a while since I got all the advices (with gratitude) from the forum (last thread being #123 on 27 June 2018), and as a gesture of appreciation, I will keep all updated on the current status.
This is the first thread, to report on the progress/status of finding the cause of the vibration. A second thread will follow to report on the progress/status of shaft replacement, so all forum members can take a breather in between.
To summarize all previous threads (up to #123), which started off with my initial concern of the shaft quality causing the vibration on my DD542 (while DD462s with almost identical design has none):
1. Whip Bearing equally dividing the shaft can cause harmonics.
2. Shaft may sag between Whip Bearing and Thrust Bearing during alignment, resulting in wrong Thrust Bearing location.
Also mine (and all DD462s) is really a Flange Bearing with no rubber padding. The mounting can amplify the noize, and the bearing cannot be adjusted if incorrectly aligned (therefore Aquadrive suggested).
3. Stern Tube may have a sag during alignment (possibly causing Whip Bearing No. 2 to be stuck).
4. The shaft is supported at three points: Cutless Bearing, Whip Bearing, Thrust Bearing (Whip Bearing No. 2 will be removed). Aligning at three points is tricky.
5. Prop blades exert differential pressure (and imbalance) as each passes under the hull.
6. Carden Shaft may have play.
7. If it is the shaft that vibrates, it can be observed if pronounced, with laser, scoring, strobe light, dial gauge, “wiggling”, etc. A lot more specifics can be confirmed, before going for an expensive trial and error boat lift and shaft change.
8. Hull “panting” is unlikely. Rudder can also cause vibration.
9. Vibration measurement and data analysis if done professionally, can locate the source of vibration.
10. I would do a sea trial and attempt to cover all advices given to me.
11. Bill Kimley at Seahorse asked me to find an vibration analyst, and said he would replace the shaft with a name brand in Hong Kong.
Here is the summary of progress and status on finding the cause of vibration:
12. Shaft sag during factory alignment is unlikely. Seahorse puts a temporary Whip Bearing at the end of the Stern Tube to support shaft weight. The bolting location of the Thrust Bearing is then decided. On the other hand, Stern Tube sag remains at least a theoretical concern for my longer DD542 (no problem for the shorter DD462).
13. Seahorse seems to be experienced with 3 point alignment (Cutless Bearing, Whip Bearing, Thrust Bearing, and the temporary Whip Bearing) for DD462s, and DD542 is done the same way.
Strictly speaking Seahorse aligns Cutless Bearing and Whip Bearing. The location of Thrust Bearing is then decided according to the line defined by Cutless Bearing and Whip Bearing, and the weight of the shaft is supported by the temporary Whip Bearing to preclude sagging.
14. Carden Shaft removed and re-checked for phasing and play. All checked out.
15. My 6 cylinder engine, 3:1 Gear Box, and 3 blade propeller theoretically re-inforce each other at multiples of 3 (pointed out by a professional vibration consultant). However with the Carden Shaft disconnected, no vibration was felt with the engine running in gear (therefore Gear Box resonance is unlikely). Also as already noted, changing the prop to 4 blade did not eliminate vibration.
16. Here is the big one: The shaft equally divided by the Whip Bearing would create a frequency pair, the frequencies are nearly equal yet different. This would result in the acoustic phenomenon “Beat”, which accounts for the cycling of vibration intensity felt at high loading (see Thread #1).
In other words, unless the frequency pair comes from another source, we could say with high confidence, the shaft is vibrating. However the cause and the most effective remedy of the shaft vibrating remain to be decided. It may well be the shaft material too weak, or it may be excessive load, or it may be alignment, etc..
17. Jeff Fritges (a professional vibration analyst recommended by JustBob at MV Mahalo, Thank you !) recognized the “Beat” phenomenon above. To determine the exact component/cause, he suggested to do measurement and data analysis. Bill Kimley at Seahorse decided no, because he will replace the shaft and re-align the entire drivetrain anyway. Bill also wants a thicker shaft. The next thread will report on the progress and status of shaft replacement.
18. Bill Kimely took the advice of a fellow DD owner, to take a ride on my boat (about an hour), along with a friend who is a professional mechanic on ocean liners. We did that on 23 July.
Again with the smaller fixed 4 blade prop yielding 50% power, we all observed the vibration is not bad at low power, but very bad at medium power (50%), with no “beat” phenomenon (either because power is not high enough, or because Whip Bearing No. 2 now broke up the original frequency pair created by Whip Bearing No. 1 at the middle of the shaft, or both). Also the mechanic friend noted the vibration to be concentrated at the aft of the Aft State Room (Master State Room). I also took the opportunity to check the salon and Forward State Room. There was none .
19. I took the boat out immediately after Bill Kimley left, for a one week cruise, hoping to do a sea trial in an empty typhoon shelter, using suggestions given in the forum.
The one hour ride with Bill, and this cruise is the very first time the boat is taken out after the last yard visit in March 2017, where Whip Bearing No. 2 was inserted and the prop changed to a smaller one.
For the first couple of days we were anchoring for some fun, but we lost the Gear Box control, then the Engine Display. The causes are now determined and problems fixed, but at the time I was spooked enough that I returned to port without going to the empty typhoon shelter.
20. After the cruise with the engine running for no more than 24 hours, I dived to clean some hull side barnacles I missed before. By habit I turned the prop by hand. Each turn is something like 90 degrees before the blade is rotated out of its lever.
To my surprise at the END of EACH turn, I felt the shaft or something in there vibrating. I pushed the prop up-down and sideways, but there was no play. I got back on deck, started the engine, and the whole boat shook violently when I put it in gear at lowest rpm. I immediately put it back in neutral, then in gear again, and the vibration was gone. Next day I dived and hand rotated the prop again, there was no vibration. Another day passed and I started the engine, put in gear, and the boat shook violently. Then as I did before, putting gear in neutral then back in gear, the vibration is again gone. I did not do a follow up dive.
In fact this violent shaking happened occasionally before and would disappear by itself once geared out then in. I was told at the time a garbage bag caught on the prop can do this. This is the first time I confirmed nothing was caught on the prop, it shakes, then shakes itself off.
However something must have gotten loose during the <24 hr cruise, that this shaking happened three times over 4 occasions of engine start within 2 weeks. My gut feel is it takes some time with the drivetrain inactive, for something to “settle”, in order to build up the mechanism of shaking. I wonder if it is caused by Whip Bearing No. 2 stuck axially, yet not fastened in place radially. Someone suggested it could be the Gear Box rubbing plates. I will experiment with engine start at varying frequency (say once a day vs once a week), to see how long it takes for the build up.
21. Anyway I hope Bill Kimley’s promise to replace the shaft with a thicker one, and re-align the entire drivetrain (in Hong Kong) would fix all. That’s another update in the next thread to come in a couple of days, some of it more frustrating than others. Take a breather for now and thank you all for reading.
