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). There is no flexible coupling nor room to fit one. 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 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 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 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 quiet 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.

 

[Daverw]

Sounds very much like misalignment, have the engine mounts gone soft? Or even one of them and it’s allowing the alignment to move the shaft center or bend the shaft and allowing it to hit the stern tube

 

[stromatolite]

Sounds very much like misalignment, have the engine mounts gone soft? Or even one of them and it’s allowing the alignment to move the shaft center or bend the shaft and allowing it to hit the stern tube

Thanks for the reply. It's possible the mounts may indeed be the issue, although inspection and poking with a screwdriver suggests all mounts are similarly firmly resilient and no evident fracture of the rubber to steel bonding. For what it is worth, this is a fresh water installation, albeit the mount age is theoretically on the high side at 10yrs plus. The coupling/shaft alignment was originally trued by a machine shop and engine/shaft coupling faces upon reassembly close to 1thou , although re-checking it good advice as the first few days of operation are fine. The impact issue comes shortly after leaving forward idle speed but if I can get it throttled up just right, disappears once up by a couple hundred rpm or so. One puzzle is there are no problems when reducing rpm back to idle speed. Wouldn't soft mounts always produce movement and contact throughout that rpm range?

 

[sarabande]

Perhaps the 'intermittency' of the problem is a clue ?

What clearance does the new propeller have by the deadwood ? Does the folding prop sometimes partially unfold when starting to go ahead. It does sound as if the prop is the cause if the engine mounts, seal and cutless are OK.

 

[Spyro]

I had the same problem when I first got my boat but as you suspect it was caused by the shaft not being central in the tube. the worst knock I used to get was on start up. As you say if you centred it when out of the water then the boat flexing may be the problem now. With the boat in the water how about undoing the coupling and moving the shaft up/down/left/right and checking how much it travels either direction before hitting the tube?

 

[scottie]

A

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). There is no flexible coupling nor room to fit one. 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 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 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 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 quiet 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.

Don’t worry about lack of flex coupling as with a flex gland and mounts it is not good practice to have one.
Checking the mounts is a sensible particularly if there is a fuel filter over one of them .
Yanmar mounts regularly feature here so are fallible.

 

[vyv_cox]

Even with an Aquadrive we encounter the same problem. 2000 rpm (engine) seems to be the 1st critical when knocking is at its worst. Mainly in following seas and making sharpish turns.
The PSS may be the problem here. It provides no radial support, unlike a packed gland. If the problem persists even with correct alignment you could consider a supplementary slipper bearing on the inboard end of the stern tube.

 

[scottie]

Even with an Aquadrive we encounter the same problem. 2000 rpm (engine) seems to be the 1st critical when knocking is at its worst. Mainly in following seas and making sharpish turns.
The PSS may be the problem here. It provides no radial support, unlike a packed gland. If the problem persists even with correct alignment you could consider a supplementary slipper bearing on the inboard end of the stern tube.

I have always thought the Albin seal with its seals and white metal bearing excellent if probably expensive setup

 

[stromatolite]

Thanks for the responses and ideas. To answer the questions:
Clearance to deadwood? not sure what deadwood is but the shaft is supported by a strut carrying the cutless bearing about 30" from stern tube exit. Gap prop to strut is 1 shaft diameter. 1inch
Prop unfolding? can't easily evaluate in water of course, but blades are geared and moving one moves them all. Sounds Tolkien-ish. Don't believe this is the problem as the impacts occur only when powering up to get the boat moving beyond idle speed (say 1knot) and presumably the blades are already open at this point. I can cruise around slowly with the engine just in forward with no issues... powering up starts the process.
Undoing the coupling to check clearances ... good idea, although practically hard to move the coupling back to disengage it once the PSS bellows is compressed. Given the as-found shaft out of centre measurements (originally suspected cause), the clearance now it is centred should be much better than before yet issue still occurring.
PSS vs packed gland lateral resistance? This I have wondered about, although there is not enough distance between stern tube and coupling to fit a traditional seal and maintain the packing. The PSS was chosen in part as a replacement for the original damaged lip seal arrangement because it had less length than many other options. Given the proximity of stern tube to engine, I'd wonder about transmitting some significant force and movement through the seal... whether packing or a lip seal like a Las-drop. I was thinking of this approach prior to finding what I thought was the smoking gun of out of centre shaft. I may have to re-examine this route if I am faced with having to pull the shaft in the fall.

And finally I do observe a fair bit of motor movement when the knocking occurs... so perhaps their is something wrong with a mount that only shows up when first asking torque out of the engine rather than something further down the drivetrain. Nothing obvious on inspection however and as this impact issue doesn't show up again when slowing to the same boat speed or any roughness at high power loads, or reverse suggested to me the mounts are okay.

 

[Heckler]

Thanks for the responses and ideas. To answer the questions:
Clearance to deadwood? not sure what deadwood is but the shaft is supported by a strut carrying the cutless bearing about 30" from stern tube exit. Gap prop to strut is 1 shaft diameter. 1inch
Prop unfolding? can't easily evaluate in water of course, but blades are geared and moving one moves them all. Sounds Tolkien-ish. Don't believe this is the problem as the impacts occur only when powering up to get the boat moving beyond idle speed (say 1knot) and presumably the blades are already open at this point. I can cruise around slowly with the engine just in forward with no issues... powering up starts the process.
Undoing the coupling to check clearances ... good idea, although practically hard to move the coupling back to disengage it once the PSS bellows is compressed. Given the as-found shaft out of centre measurements (originally suspected cause), the clearance now it is centred should be much better than before yet issue still occurring.
PSS vs packed gland lateral resistance? This I have wondered about, although there is not enough distance between stern tube and coupling to fit a traditional seal and maintain the packing. The PSS was chosen in part as a replacement for the original damaged lip seal arrangement because it had less length than many other options. Given the proximity of stern tube to engine, I'd wonder about transmitting some significant force and movement through the seal... whether packing or a lip seal like a Las-drop. I was thinking of this approach prior to finding what I thought was the smoking gun of out of centre shaft. I may have to re-examine this route if I am faced with having to pull the shaft in the fall.

And finally I do observe a fair bit of motor movement when the knocking occurs... so perhaps their is something wrong with a mount that only shows up when first asking torque out of the engine rather than something further down the drivetrain. Nothing obvious on inspection however and as this impact issue doesn't show up again when slowing to the same boat speed or any roughness at high power loads, or reverse suggested to me the mounts are okay.

“Powering up” could be the key! The prop pushes the shaft, which pushes the gearbox and engine which transfers the push to the boat through the mounts! So the maximum push is on the power up to get the boat moving. This could be bending the shaft or pushing the engine upwards due to soft mounts.

 

[stromatolite]

“Powering up” could be the key! The prop pushes the shaft, which pushes the gearbox and engine which transfers the push to the boat through the mounts! So the maximum push is on the power up to get the boat moving. This could be bending the shaft or pushing the engine upwards due to soft mounts.

True enough Heckler, thanks. Although even trying to very gently accelerate causes the problem at that initial stage... I suspect I can put much larger forces on the drive train after the point when the knock has been slipped past by a very fast throttle up ... and no issues. I've tried going through fast, slow, out of reverse, and little change. Given the sensitivity to this low initial speed range there may be some combination of whip, misalignment, mount softness that causes the problem. Alas for every theory there seems an operational aspect that doesn't seem to add up. Most puzzling is why centering a shaft that was 2mm high and thus increasing the clearance not clear or even reduce the problem? If nothing else, examining these various ideas is helping to clarify my thinking. I might try to rotate the coupling 180degrees and see if that helps. Then lower the motor another 1mm. After that taking up Golf, as I will be more than fed up with cramped location alignments.