Bad knock

[Ilanortho]

Posting again;
Bad knock I thought was transmission on Hunter 34 Yanmar 3GM30F/ KM3. Engine realigned, new packing gland over the winter. Cutlass bearing OK. Only in fwd gear and low RPM.
A very bad knock that can be heard off the boat even when docked.

Boat was pulled and engine realigned again as shaft was hitting stern tube.
Back in the water ran beautifully for at least 45 minutes coninuous, but after a hard sail, coming back in knock recurred and was even worse. Best heard from rear access port and seems like transmission although again only in fwd at low RPM but with great frequency. Sounds like metal on metal knock. During winter refit was told that engine mounts looked OK, but I'm starting to suspect they are the culprit given how this comes about.
Help!
Thanks. Ilan

 

[DougM]

If the situation is indeed a result of shaft alignment, I doubt that a proper realignment can be accomplished with the boat out of the water. Especially if the boat is on stands or a cradle over the winter, the hull is supported far differently than afloat. I would suggest aligning in the water after floating for a significant length of time.

Depending on the age of the boat, it certainly would make sense to replace the engine mounts regardless of their outward appearance.

To rule out the transmission, if you can find a length of wood dowel, place one end on the transmission case and the other very close to your ear to more or less telegraph any sound emitting from within while the transmission is operating.

 

[Ilanortho]

Thanks Doug. Ilan

 

[lee oakley]

I had simaliar thing but on 42 houseboat after shafts were aliagned and went out hard knock took back in had redone took back out wasfine after 15 min on niagrariver it started hard knock again got mad and floored it bad idea drive shaft broke and shaft fell out! any waynot alianed right!

 

[Ilanortho]

Hunter 34 Yanmar w/ Kansaki transmission:
Follow-up answer to all of you sitting on pins and needles:
Bad (awe-full) knock at only low RPM and intermittent so that no one can figure it out, was indeed the transmission. It was pulled and the cone clutch replaced with resolution of problem.
Here's the pearl: If you look very carefully at the prop shaft while it is knocking, you will see that when the knock occurs the shaft will ever so slightly spin a bit irregularly and not the constant smooth spin you would expect to see. As you throttle up centrifugal force causes the clutch to stop slipping.
As a thanks for all your comments I am posting the findings so future searches may unearth this as a possible answer.

 

[SFS]

Really glad you got it figured out. Intermittent symptoms are often a PITA to diagnose and treat.

 

[Ilanortho]

Really glad you got it figured out. Intermittent symptoms are often a PITA to diagnose and treat.

Thanks! Ilan