Yanmar 1GM10 Knocking in Gear

[KCee1]

I have scoured the Marine Diesel Engines book by Nigel Calder, gone on countless sites but still cannot come up with a solution to the knocking and vibration of my Yanmar.
In neutral the engine purrs like a kitten, even at the highest revs ... at the beginning of last summer in gear the knocking and vibration started at 4 knots, (everyone told me it was normal knocking) but slowly it has been becoming worse at lower and lower speeds, until at the end of the summer I had to tow a stalled large sailboat to the marina and now the knocking is horrendous at only 1 knot. Club people have listened to the engine with instruments, surveyed the shaft, etc etc.. and proclaimed nothing seems apparently wrong. Admittedly there might be some motor mount deterioration but to look at them they seem fine.
Can anyone give me some advice?

[jimbunyard]

Likely the the transmission drive/flex plate has gone bad.

[Wotname]

Does it also knock as badly when going astern?

[KCee1]

I haven't noticed the sound changing in one direction or the other... no

[jimbunyard]

You may be able to test the drive plate, depending on how the gearbox engages, by, with the engine off, putting the transmission in either forward or reverse, and trying to turn the engine via the propshaft and some appropriate apparatus (pipe wrench, big screwdriver, big channellocks).

The engine compression should provide enough resistance to reveal a worn or broken driveplate through either clunking/rattling/excessive play at the transmission/flywheel endplate junction...

[Briney Bug]

Might also want to think about the prop- out of true, or something caught on a blade; shaft might also be out of align.

[alaskaflyfish]

Check your cutlass bearing. I have a 1gm10 and will agree that they knock and shake more than other engines. A flex coupling on the prop shaft/transmission flange also helps.

[KCee1]

Thank you everyone for your replies - I will print these suggestions and make a list... I'll look up how to get to the cutless bearing... prop, shaft, etc, all external pieces seem to be in fine form, says all my Club co-sailors...

[boeing1]

I would first check condition and security of you engine mounts and your prop alignment.

[hamburking]

cutlass bearing: With the engine off (key out, and in stowed in the vee cabin), grab the prop shaft near the strut and try to move it side to side. There should be no play, or maybe like 1mm.

prop shaft: with the engine off (key out, etc), and transmission in neutral, rotate the prop by hand. It should turn easily with no resistance at any point of rotation.

engine mounts: watch the engine while starting and at low rpm...if the engine "dances", you may have some bad mounts.

It could also be something weird, like too much resistance...if you have a ton of growth on your hull, but I would think you would also not be able to acheive higher rpm, and might have black smoke too. It does kinda fit with your towing story though.

If its the transmission, you may have a big job ahead. I had a bad (slipping) clutch plate on my 2gmf. That combined with a crooked mechanic cost me way too much to fix.

And finally, and this is most unlikely, it could be something called "piston slap". I only know about this because its common on honda fit engines, of which I have one. It only occurs at low rpm though. Very annoying knocking sound.