Prop Shaft Knock

[Richard Deep]

OK, this is driving me nuts. My boat has developed a strange knocking (thumping?, slapping?) sound that I think is related to the prop shaft.

It doesn't happen all the time, and seems to be related to how heavy a load is on the engine (2GM20F). It happens in forward and reverse, usually when I rev up quickly (like when pulling into the slip and using reverse to stop, or after pulling out of the slip and switching to forward motion). It also happens when I'm motoring if I increase RPMs too quickly. I can get it to stop by backing down and then gradually increasing RPMs (I usually motor around 2600-2800 RPM).

I've looked at the shaft going into the stuffing box, and can't detect anything out of line, even when the shaft is turning. I've had my diver check the cutlass bearing, and that seems OK.

The frequency is the knocking is less than the shaft RPM. For example, if the shaft were turning at 60 RPM, the knocking might be 10 times a minute.

I'm completely baffled and I'm hoping the collective wisdom of the sailing community can help me out. Cuz whatever it is, it can't be good!

Thanks,

- Richard
S/V Discovery
San Francisco

 

[RAD]

I have the exact same symptoms with my ODay and I'm having the cutlass bearing replaced this winter and next spring I'm going to do a shaft alignment and hope this solves the problem.
When I bought the boat I had a constant knock that at first I thought was a tranny problem it wasn't then checked the prop and shaft until I aligned the shaft with the engine and all was good for two seasons and I've also learned from this site that the rubber engine mounts can get soft and need replacing so I replaced two out of four (the easy ones)

 

[Dave from Mi.]

Knocking

Hi Richard;

That happened exactly as you described it on my Catalina30 with the Universal M25XP with a Hurth trans. The cause was the shaft coupling was not in alignment with the coupling on the transmission, causing the shaft to vibrate and occasionally hit the sides of the shaft log. It is also caused by the rubber in the motor mounts becoming older and less firm. As you rapidly accelerate the RPM's the torque causes the motor to move on the mounts.

The cure for me was to realign the shaft to recommended tolerances +/- .004" for my rig, and I added a shaft coupler "donut" by PYI. Made everything nice a smooth at all RPM settings

Good luck
Dave

 

[Dave from Mi.]

Twin postings

Hi Rad;

We both posted same thing at same time, Your a little faster than me at typing;-))

Fair winds
Dave

 

[Tim R.]

I have had similar problems before that turned out to be the zinc coming loose. Your diver should have seen this though.

 

[Rich (P303)]

When you hear the knocking... if you decrease throttle... put into neutral... put back in gear... and apply throttle needed... does it go away?

If so... Could be worn cones in your transmission.

I have a 2GMF with a problem too. I think mine needs a rebuild.

Did you put your hand on the packing gland? If it is very hot... could be the coupler and/or set screws loose.

 

[J Page]

Mine knocked (ysm12) until it broke. No forward, no reverse, nada. Was the shaft coupler. Cause was broken motor mount, scoreing the coupler and eventual failure. Replaced shaft, motor mount, and coupler all at the same time. Required dropping the strut. Expensive, but quality work and now not ever on my mind.
Lugeman

 

[walmsleyc]

There are a number of posts and the archive (and responses to this exact question). As someone has already noted, it could be knocking against the log. The reason for knocking could be many (bent strut, engine shifted, hull changed shape, worn cutless, etc. etc.)

Mine was knocking against the log. With the engine off, if you have easy access to the shaft and log from the inside, grab the shaft with your hand, near the log, and carefull apply pressure in 4 different directions. If you find that in one of the directions, it does not take much pressure to cause the shaft to touch the log, then this "could" be your problem. Because of most stuffing boxes, you wont be able to visually check that your shaft is centered in the log, but that's what you really want to check.

If that is your problem, you may need to adjust you engine supports to achieve proper centering, AND ensure that everything else is in good shape and aligned.

Cheers.

Chris

 

[Mike B]

Any chance you have a folding or feathering prop? I had a 3 blade Autoprop on my last boat and if one of the blades hung up due to barnacles or the need to grease the blade bearing, I would get the same symptoms. The cure was to clean and service the prop.

 

[Deadline]

So one easy fix is to realign the engine.
The yard can do this fairly inexpensively.
Or you can do it yourself, if you consult the Archive.

 

[RAD]

realign the engine

You can do this yourself but you have to have a lot of patience and be very tenacious and allow a lot of time to do the job right.
Oh by the way did I mention patience

 

[Nereussailor]

Sounds like your motor mounts are worn out and need replacing. It will throw everything out of alignment and cause plenty of damage. It happened to me on my H30. by the time I figured it out it was too late. ( I was in the middle of Green Bay when it went) I ended up replacing the cutlass bearing, shaft log tube, motor mounts, and transmission. While I was in there and everything was apart, I put in a PYI dripless seal too. Don't wait, or you'll end up spending more than you want to. It might not be too late to save your tranny.

 

[higgs]

Check the prop strut, too.