Prop Shaft Knock

[Richard Deep]

OK, this is driving me nuts. My 85 H31 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 Hunter Owners can help me out. Cuz whatever it is, it can't be good!

Thanks,

- Richard
S/V Discovery
San Francisco

 

[Island Girl 34]

My bet would be still on the cutlass bearing. My H34 has the same knock and has been identified as the cutlass bearing needs replacing. That is the first thing on my list for my winter pull out. It does not take much slop in that bearing for it to vibrate like that... Let us know what you find out.

Ken

 

[Don12364]

Alignment?

If the prop shaft does not pass down the centre of the log tube then it can knock on the inside of the tube. This could result from sagging engine mounts or loose engine mountings - assuming it was correctly aligned to begin with - which I doubt. The short length of flex hose between log tube and stuffing box allows this misalignment.
I had just this when I changed from a normal stuffing box to a Pack-less Shaft Seal (PSS) which has a face type sealing gland with nothing to journal the shaft. Occasionally the engine would vibrate a little more or the shaft would whirl and contact was made.

And it is very loud and scares you witless the first time.
No immediate harm results but it is best not ignored.

 

[Steve Dion]

Sounds like a cutlass to me.

You are looking for a lift, a cutlass bearing and a splash. It is usually about 1-1.5 hrs labor. The cutlass is probably around $100. You may as well repack your stuffing box too.

 

[Bill Roosa]

that's the knocks

If the frequency is less than the prop shaft RPM then it is almost impossable for it to be in the engine. also since it only happens when the shaft gets accelerated and loaded suddenly and not just loaded it is very likly that there are some loose/worn bearings that work fine when loaded smoothly but let the prop shaft bang around randomly when sudden loads are applied.
the cuttlass bearing is the only thing down there that could be causing anything like this.
Is the knock random or does it just happen regularly at a frequency less than the prop shaft RPM?

 

[Manny]

Cutlass bearing is a good place to start but a broken or worn engine mount could do the same thing, especially with sudden torquing of the engine. Use some long pieces of wood (or a pry bar with wood pads) as a lever under the engine mounts and gently lift to see if there is a lot of play or a broken mount. I had a similar issue with mine and found that a mount had a broken stud - the cutlass bearing was fine.

Manny

 

[Ken13559]

you can dive under and push the prop up and down to check the cutlass. you probably have 2-3mm of play due to wore rubber in the cutlass.

 

[JustDucky]

I had the same problem but let it go for a season. The cutlass bearing was shot and there was a large groove worn in the shaft. Had a new shaft put on and cutlass bearing replace. It runs smoothly now, all knocks and vibrations were gone this season.

 

[Nelsonhow]

Check the engine mounts first. Especially the side of the engine where the fuel filter and pump are. Diesel dripping onto the mount will make the mount spongy on the inside even though it look OK on the outside. Replace mounts either 2 at a time (front or back) or all 4.