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Old 06-07-2019, 04:37   #16
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Boat: Fiberglas shattering 44' steel trawler
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Re: propeller shaft -stern tube impact noise ...advice please

If you've eliminated propeller inbalance, it sounds like you're back to the engine mounts. Its not happening on the hard makes sense since the prop has no resistance, ergo torque on the engine, when it's just moving air. I would want to be down there at the back end of the engine when this happens. If the engine jumps when the shaft starts banging, I can't think of a source other than engine mounts. The prop is balanced, the cutless bearing is intact, the shaft is straight (?), the engine is aligned, and we're back up at the engine mounts.

I know this is frustrating the hell out of you, but keep at it. There is a mechanical answer to this question, and it's not going to be complicated because there is a limited set of possibilities. Hang in there.
Never let anything mechanical know that you are in a hurry.
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Old 06-07-2019, 07:00   #17
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propeller shaft -stern tube impact noise ...advice please

I would guess the engine vibrates due to low RPM. It is normal that the RPM goes down when shifting to gear from neutral. When that happens the shaft come in contact with stern tube. When RPM increases the problem disappears due to higher RPM and less vibrations.

Check the aligning. Is the shaft in center of the stern tube ?

Check the RPM at idling. Perhaps it is to low.
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Old 09-07-2019, 12:50   #18
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Re: propeller shaft -stern tube impact noise ...advice please

thanks for the thoughts tkeithlu... I am kind of drifting back to the mounts. Observing the engine through this certainly shows a significant amount of movement. There is no powering through, and if pushed to when the banging starts it becomes somewhat scary. I can usually get it through the rough spot prior to this by back and forth throttle and at some point it just sneaks past, and accelerates nicely after which I can back down to whatever engine speed I want. It has however been getting tougher to get this to happen. The mounts are older, so replacement even if there is nothing obvious is not unreasonable simply from a maintenance perspective, but the mount expense certainly is daunting. As noted that there is never an issue until a few days after launch which gave me belief the mounts were fine, but after hopefully eliminating centering and alignment issues, propeller imbalance, as you say...back to the mounts.
As for the propeller, a replacement for the original fixed 3blade, and I followed Flex-o-fold's recommendations based on previous successful replacements on the same boat. Shaft dia is 1" and about 3' coupling to cutless bearing.
Oceansailor... not exactly... no evident problem out of neutral or in idle forward ... it is trying to get it throttled up from there that the issue happens.
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