Acceptable prop shaft/engine vibration, or not?

[stevebythesea]

Hi, The attached video shows vibration at the engine/coupling on our yacht (A Catalina 28MKII). At tick over, there is no engine vibration. In gear (forward or astern), vibration as shown. Increase revs by roughly 500rpm, no vibration.
I am wondering whether this is relatively normal. If not, is it serious or can it wait until she comes out of the water in October (when I will be replacing the coupling and stuffing box.
Thanks in advance.
Please copy the link below and paste into your web browser.

https://www.dropbox.com/sh/fg548tu6fix5z2s/AAAcdg48co36Pe7sHjuYjm6ra?dl=0

 

[RichardS]

The link does not work for me, I'm afraid.

Richard

 

[davidej]

Nor me.

 

[colind3782]

Nor me but very little vibration is "acceptable" it will soon wreck the cutless bearing etc.

Check your engine mounts and alignment?

 

[stevebythesea]

Thanks Richard,
I have edited the link - hope it works now.

 

[stevebythesea]

Thanks, It works now.

 

[stevebythesea]

Thanks it works now

 

[stevebythesea]

Sorry about the link, it works now.

 

[RichardS]

Thanks Richard,
I have edited the link - hope it works now.

I can't see or hear anything unusual. Most small diesel engines vibrate like that at certain revs.

Richard

 

[<152587>]

The "cathedral" engines as fitted to ships suffer from this, warnings are in place not to operate the engine in the critical rev range. The engines will develop harmful harmonics in the critical range. However, are you sure that you have got complete combustion? A slight mis-fire will lead to your symptoms. A power check by slackening back the injector feed pipe on each cylinder in turn, the revolution drop noted should be fairly equal on all cylinders.

 

[jiris]

Hi, The attached video shows vibration at the engine/coupling on our yacht (A Catalina 28MKII). At tick over, there is no engine vibration. In gear (forward or astern), vibration as shown. Increase revs by roughly 500rpm, no vibration.
I am wondering whether this is relatively normal. If not, is it serious or can it wait until she comes out of the water in October (when I will be replacing the coupling and stuffing box.
Thanks in advance.
Please copy the link below and paste into your web browser.

https://www.dropbox.com/sh/fg548tu6fix5z2s/AAAcdg48co36Pe7sHjuYjm6ra?dl=0

Hard to tell from the vid if the vibrations are caused by uneven running of the engine or misalignment between the gearbox and prop shaft. The symptoms you describe could apply to either. I would start checking the engine (smoke, difference between temperature of individual exhaust ports etc.) and progress over engine mounts to the coupling. If all seems to be O.K., check if you can turn the shaft (in neutral, obviously ) by hand very smoothly. If you feel different resistance in certain parts of the turn, it may be a sign of misalignment and more accurate idea about its extend will be gained after uncoupling the shaft.
At this point you can probably get away with running the system carefully, avoiding prolonged use in the revs range where the vibrations occur, but it will almost certainly be getting worse and cutless bearing, packing box, coupling and gearbox will suffer. There are lots of different opinions about how accurately the coupling has to aligned. The fact is, the more accurate the better, but some people are entirely unrealistic with their thousands of a millimetre figures - they simply forget to take in account that engine mounts are flexible and the drive train moves very significantly depending on load. So, some common sense is needed here. By all means, the vibrations I saw on the video seem to be over the limits I would call "acceptable" - I can be wrong, but I'd suggest you would find some marine engineer to inspect it.

 

[zoidberg]

One thing further to consider, after checking all the above.

One or other of the engine mounts themselves may be slightly mispositioned and/or too high or too low on its post. A careful inspection may also reveal that the rubber(s) is degraded, warranting replacement.

There are some further tips by Beta Marine in the text above.

 

[Plum]

Hi, The attached video shows vibration at the engine/coupling on our yacht (A Catalina 28MKII). At tick over, there is no engine vibration. In gear (forward or astern), vibration as shown. Increase revs by roughly 500rpm, no vibration.
I am wondering whether this is relatively normal. If not, is it serious or can it wait until she comes out of the water in October (when I will be replacing the coupling and stuffing box.
Thanks in advance.
Please copy the link below and paste into your web browser.

https://www.dropbox.com/sh/fg548tu6fix5z2s/AAAcdg48co36Pe7sHjuYjm6ra?dl=0

Very difficult to compare from a video but as far as I can see your engine vibration looks just like my 3 cylinder diesel when in gear at idle. Suggest you check your idle speed is within spec. If it is, I would not worry.

Www.solocoastalsailing.co.uk

 

[JimC]

The vibration in the video didn't look so bad to me.

 

[coopec]

It works now. (thought you'd like to know)

Very interested to view OP views as lining up the prop shaft/engine as it is one of my next tasks.

David Pascoe is a yacht surveyor and has written a book on various aspects of boating. Here is a short list of problems which he says may cause vibration.

Here's a short list of the problems that can be caused by engine/shaft alignment faults:
Rapid cutlass bearing wear.
Misaligned strut galls shaft, requiring shaft replacement.
Causes stuffing boxes to wear out and leak, not infrequently sinking the boat.
Bent or broken shafts
Drive system vibration that can damage transmissions, engine mounts and the boat hull itself.
Vibration causes damage to other systems.
Transmission failure caused by increased stress on the rear output shaft bearings and gears.
Loosening of struts, causing leaking and possible sinking.
Oscillating propeller shaft causing stuffing box clamps to loosen and work free, flooding or sinking the boat.
Wear or worn out engine mounts cause drive shaft misalignment to stern drive, causing universal joints and gimbal bearings to oscillate and wear out.

http://www.yachtsurvey.com/Alignment2.htm

 

[Neil_Y]

Doesn't look bad to me, an engine on rubber mounts will always wobble around more at some rpm than at others, the stern gland is where you will see this movement as one part is fixed and the shaft/engine are moving around. If you imagine motoring in rough sea the inertia of the engine mass will cause it to move around a lot more than this and the shaft flex will allow this to happen.

The shaft has enough flex to cope with this movement. When you engage forward the thrust on the shaft will try and get it to bend, like pushing the boat along with a pole, it will want to bend. This is where the bearing supports resist this tendency to flex but soft bearings or the normal running clearances will allow some bend and the engine being on rubber will also allow it to move and twist as the shaft bends.