propeller shaft -stern tube impact noise ...advice please

[stromatolite]

I have a Albin Nova 32' sailboat - powered by a Yanmar 2GM20 with a PSS shaft seal and 3blade flex-o-fold prop (a replacement for the original fixed 3blade). When increasing speed from idle in forward gear a banging noise occurs from the propeller shaft impacting on the stern tube. It is not every time (sometimes it runs through perfect), but usually I have to work the throttle back and forth and eventually it will slip past this point and run smoothly throughout the entire power range. Decreasing speed to idle does not re-exhibit the impact issue, nor does reverse, and otherwise the motor runs with minimal vibration even at high speed or in neutral. Consequently although a broken motor mount could be a possible cause, they look in great condition and no evidence of a problem (not to mention expensive for casual experiment). Prior to launch this year I pulled back the PSS seal and checked the clearance between shaft and stern tube and found the shaft 2mm high - and so thought I had found the problem. Alas, despite lowering the engine to centre the shaft and realigning, the issue continues. The problem only occurs a few days after launch which made me think boat flex was the final straw to remove clearance when the shaft was originally high (and so very puzzled why it is still happening now with presumably far more clearance with the shaft centred and re-aligned in water). Shaft is straight, propeller blades are geared together and opening correctly, 1 shaft dia gap to the cutless (no wear or movement noted here), and the zinc in good shape and relatively close to the strut support. Curiously when I changed the original lip-seal a few years ago to the PSS I found the shaft to be quite low. All advice was it should be centred. Perhaps I need to lower the shaft some more, but wondered if the collective expertise here might have other suggestions as to issue and solution? Much appreciated.

[GordMay]

Greetings and welcome aboard the CF, stromatolite.

[senormechanico]

Are you SURE the motor mounts are OK? I went several years after buying a boat with a Yanmar 3hm35 with no problems at all except one THUNK when the engine was shut down.

Almost by accident, I found that one motor mount bolt was broken and the engine was just being held in place by 3 mounts and gravity on the 4th one.
After replacing the mount, the THUNK was gone.
Oh, and the shaft ought to be centered in the shaft seal.

[wsmurdoch]

During the pre-purchase survey the surveyor said he thought he could hear a broken engine mount under my Yanmar 3HM35F, but he could not locate it. Six months later a mechanic put a 2x4 under the engine and using it as a lever lifted the engine a little. It was then easy to see that the bolt on the forward port mount was broken. He was also disturbed that the gap between the base of each of the other three mounts and the upper metal bit were too small indicating that the rubber had lost its bounce or had torn. The manual sayed it should be greater than 3/16" at the center. We replaced all four and realigned the engine.

[stromatolite]

Thanks for the welcome and comments.
I cannot be totally sure the mounts are ok, but they look fine, no oil or diesel, have similar firm resistance to screwdriver poking, and resist lifting up. Fresh water service too. But... there is no sign of vibration after getting through this low speed spot, it doesn't reoccur when descending through it, and most curiously it always takes a couple days after launching to have the problem show up. One might anticipate a broken mount would be more consistent. But I will try lifting them with a crowbar and see what happens. I'll try realigning it again too. This has me so puzzled as virtually every theory seems to have an operational aspect which doesn't make sense. I am wondering if the change from old lip seal to PSS took away a bit of critical sideways shaft movement due to resistance of the rubber hose, but ... given very short distance between stern tube and coupling its not as if that effect would seem significant.

[tkeithlu]

There is no justification for the propeller shaft hitting the shaft log. At at least the bottom end (I have cutless bearings at both ends) the shaft is centered by the cutless bearing, and at engine end, the shaft is centered by its attachment to the engine, which sits very nearly in one place, given your engine mounts. So, can you replicate the problem while you are at the shaft? Is it hitting at the top of the shaft log (your engine had to jump several inches, it's out and out unmounted) or at the bottom (your cutless bearing is REALLY shot). I think you've got several people looking and this, incredulous that your engine mounts are OK.

[stromatolite]

thanks tkeithlu. Totally agree. Fortunately it is only just out of forward idle speed... once past all is smooth as silk, and sometimes it doesn't happen at all. The cutless bearing (a single, strut mounted one) is in good shape and has no sideways movement. Clearance between shaft and stern tube was about 5mm all around once centred, so it implies the motor is moving this much for sure. It is a Yanmar however. I'm starting to get more suspicious of the mounts, but I thought that cause would be more consistent and show up at other speeds or when returning back through this slow range... but perhaps I gave them more of a pass than I should have. After renewed thinking on mounts I am also wondering what happens if the motor is not evenly loading all four, ie, primarily supported across a diagonal between a front and back corner. Sort of how a 4 legged chair with a short leg is at times supported only by two. I would think this would encourage some vibration particularly at lower speeds. No idea how one checks this.

As for replicating the problem on the hard... no I can't move either end of the shaft anywhere near enough to touch...

[senormechanico]

Originally Posted by wsmurdoch

During the pre-purchase survey the surveyor said he thought he could hear a broken engine mount under my Yanmar 3HM35F, but he could not locate it. Six months later a mechanic put a 2x4 under the engine and using it as a lever lifted the engine a little. It was then easy to see that the bolt on the forward port mount was broken. He was also disturbed that the gap between the base of each of the other three mounts and the upper metal bit were too small indicating that the rubber had lost its bounce or had torn. The manual sayed it should be greater than 3/16" at the center. We replaced all four and realigned the engine.

Interesting. That was the SAME mount on my previous boat's 3HM35F that I described.

[skipperpete]

Hi stromatolite
Nicely spotted with the unloaded/lazy engine mount, it might well be the problem. A few years ago I needed to establish the cause of a driveline vibration on a large engine so I bought a pair of load cells to go under the mounts and a readout panel. I wanted to know how much weight the rear mounts were carrying as opposed to the front so we could fit heavier mounts at the rear. What became immediately obvious was that the load was very unevenly distributed like your short legged table and this was not obvious despite several independent alignments ....and it was surprisingly difficult to adjust out that imbalance.

[tkeithlu]

You may be hitting a sympathetic vibration - a speed near idle (in this case) where the natural frequency of the shaft (it's got one) matches the vibration of the engine, is driven to bigger and bigger amplitudes, and is not any longer damped out by the rubber engine mount. Sympathetic vibration took down the Tacoma Narrows bridge and Comet jet airliners.

It sounds as though you've narrowed this down to the engine mounts well enough to justify replacing them. If that is your conclusion, do replace them all - the three that may not have failed still were subjected to abnormal loads and travel making up for the fourth, and you don't want to have to repeat this repair. In the grand scheme of things a set of mounts is not more than a ripple in the sea you pour money into.

Best of luck with it. Tell us what you find.

[transmitterdan]

Is it possible that one blade of your prop is “sticking” and not unfolding the same as all the others? If a folding prop has 2 blades unfolded but not the third you could get some violent whipping of the shaft. Once it unfolds things will smooth out.

You should be able to watch the motor while someone else throttles up. If there is a broken mount causing the problem it should be visually obvious. Look at the shaft where it attaches to the transmission. It should not move to the naked eye. You can also fasten a flat metal strip so it is close to but not touching the coupling. At all speeds there should be little or no movement of the coupling.

[tkeithlu]

Very good thinking, Dan. That fits the evidence very well.

[JimsCAL]

I would also recommend checking the prop. I have a Martec 2-blade folder and sometimes doesn't open completely and I have similar banging to what you describe. A shift into reverse and then forward fixes the problem.

[stromatolite]

At risk (hopefully not) of annoying those trying to help, the propeller is believed not likely to be the problem. I am of course not sure of anything regarding this vibration problem, but the prop is geared and moving one blade moves them all. On the hard it works smoothly and no hint of any issue. It also is a bit puzzling why it always works perfectly for a couple days after launch. I've tried almost every combination of forward, reverse, slow, fast etc and nothing seems to work to reliably get things past this rough spot. Once through, it all runs well. This year, after re-centering the shaft in the stern tube, all indications and logic were I had solved things. After a couple days with still no problem, went out the shipping channel under power with no issues ...yay!... hoisted the main and noted a batten slide was not right and so returned out of the channel to fix it. On re-acceleration it started banging around again. For what it is worth, I put a crowbar on the engine mounts and they all seemed firm and attached. Alignment checked and within 0.002". The engine does bounce visibly around at this low speed so the resonance theory has some merit (or a mount problem I can't find). Alternatively perhaps the batten slide is the cause.

[transmitterdan]

What is the shaft diameter and material. What is distance from strut to coupling and strut to propeller? This might be an unfortunate combination of shaft diameter and unsupported length.

Is this the original prop? If not what was the original prop? The reason for asking is that maybe this prop is requiring more torque at low speed and results in “whipping” of the shaft. Sometimes owners try to overprop to improve fuel economy when motor sailing. But they don’t increase shaft size to be compatible with larger deeper pitch props.