Knocking propeller shaft

[davidivorwynphillips]

I would appreciate advice from anyone who has had this problem!
Boat is a Westerly discus + conventional stern tube, cutlass bearing, hurthe gearbox and Mercedes OM636 diesel.
Goes well until revs hit 1900rpm or more then I get what sounds like the shaft knocking the stern tube. I have had the propeller off (to reduce its pitch - a regular westerly problem), have taken the shaft out (checked for straightness), engine aligned and flexible coupling put in, engine feet replaced....all without effect. So I am at a loss but gather that this is sometimes a perennial difficulty.

 

[vyv_cox]

My shaft tends to do the same thing, particularly in a following sea. In my case it is due to the weight of the Bruntons prop in combination with excessive clearance between the prop and P-bracket. The clearance between shaft and stern tube on the Sadler is very small, so a small deflection causes them to touch. When your engine was aligned was the shaft placed centrally in the stern tube? It is best to do this first, holding it in place with wooden wedges before aligning the coupling flanges.

 

[davidivorwynphillips]

Thanks for replying - and great to hear of your experience! Yes I did centralize the shaft in the tube prior to alignment. There is no P-bracket, rather the conventional cutlass as per a Westerly. I'm wondering whether at higher revs the forwards pressure on the engine might be distorting the position of the shaft causing it to knock but am not sure how to test this?

 

[Eeyore]

I have exactly the same set up and exactly the same problem, except my knock appears at anything over 1500rpm! It only became apparent after the last haul out, when the engine and gearbox had to be removed for another job. I am now out of the water again and can see a bright spot on both the shaft and the fwd end of the stern tube, so when the stern tube goes back in, I am going to make absolutely sure it is in line (although I haven't figured out how yet). Checking the engine alignment only showed it to be perfect when trying to discover the source of the knock - it's actually the stern tube that is not in the boat straight.........at least that is my reasoning!

 

[Concerto]

Did you check the alignment in the water or out? Hulls do have different shapes depending upon its support.

Has the shaft had a replacement bearing. Wear could cause a wobble at higher revs.

You mention that the pitch of the propeller has been changed. Could the balance of the propeller be slightly out and at higher revs causing a wobble.

Or possibly a combination of shaft bearing wear and slight out of balance propeller having different points of wobble that resonate and get amplified above 1900 rpm.

 

[davidivorwynphillips]

Thanks for your suggestions.
Have checked the engine alignment in the water - obviously difficult to make sure the shaft is centralised as too much water would come in but given the heavy Westerly build I doubt that there is too much hull distortion going on between being in or out. I only have a cutlass bearing at the distal end and this seems to be tight. I think I might get the prop balance checked - there is a company offering a device to check this (rather as you do for car wheels). But I'd be disappointed with the lot who did th repitching of my prop as they said is was well balanced.

Apart from this I'm rather at a loss!

 

[Tranona]

Have you checked the engine mounts? The knocking is not unusual but as it is at specific revs suggests the engine is moving at that particular point.

 

[davidivorwynphillips]

I've had new mounts some years ago. I suppose one way would be to look at the engine for movement when the vibration starts, but not sure how it would be cured.

 

[NormanS]

I have exactly the same set up and exactly the same problem, except my knock appears at anything over 1500rpm! It only became apparent after the last haul out, when the engine and gearbox had to be removed for another job. I am now out of the water again and can see a bright spot on both the shaft and the fwd end of the stern tube, so when the stern tube goes back in, I am going to make absolutely sure it is in line (although I haven't figured out how yet). Checking the engine alignment only showed it to be perfect when trying to discover the source of the knock - it's actually the stern tube that is not in the boat straight.........at least that is my reasoning!

But it's the stern tube to which the rotating elements have to be aligned!

 

[davidivorwynphillips]

Yes but there is an arguement that although under static conditions the shaft and stern tube can be well aligned, movement caused by the ' centralising' tendency of engine rotation can cause a degree of malalignment especially when the tolerances in the system are tight. If you see what I mean!

 

[NormanS]

Unfortunately probably the majority of yacht engines are not installed to the precepts of good engineering practice. With a good installation, the propeller thrust, fore and aft, will be taken by a fixed thrust bearing. A short length of shaft, fitted with suitable flexible or universal couplings, between the thrust bearing and the engine gearbox, will then cope with any movement on the engine mounts, without having any effect on alignment.

 

[Neil_Y]

Tricky as we supply hard bearings, but as rubber bearings are by their nature able to distort under pressure, it is possible fitting a hard bearing will restrict the movement of the shaft when compared to rubber or elastomeric materials and this would restrict the shaft when it tries to flex under thrust loads. I would also look very closely at the amount of forward movement at the mounts or if any parts of the engine might be close to hull or the boats structure. Mark mounts with white tape to see how much they move, if they reach their limit I would expect a hard knocking sound.

 

[Eeyore]

But it's the stern tube to which the rotating elements have to be aligned!

Correct, but if you have a flexible traditional stuffing box and the inboard end of the stern tube flange is adjustable, and non of it is accessible with the engine and gearbox in place - it becomes a nightmare when fitting as there is nothing to 'line it up' with. Please don't say centralise the shaft and block it up with piece of wood while installing the engine as that is impossible also, that space is taken up by the exhaust muffler!