Shaft alignment-loud banging sound

[Endurance2010]

Loud banging sound is likely the result of shaft mal-alignment.

I'm currently in my haul-out and it appears the loud banging sound coming from below is a result of the prop shaft banging against the sides of the sleave where it leaves the packing. Initially, I was under the impression it was the cutlass bearing, which does need to be replaced as well. However, there is very little play with the cutlass bearing and I can replicate the banging sound when I pull on the shaft hard. This is likely the result of engine vibration movement and lack of regular haul-out maintenance.

The yard says it's a 2-4 hour job to re-align the engine and shaft. Will update after I have a good cry when my total yard bill comes due.

--Dan

 

[rfrye1]

Does it sound like............

Clackity-clackity-clackity? I have the same issues with my H376 ever since I bought in 10 yrs ago. I have replaced cutlass 3x's, even replaced motor mounts, replaced prop shaft too. Sometimes the noise doesnt show up. at lower rpm's (below 2500) no problem. I'm thinking about changing to a 3 blade prop next to see if that helps. Whats interesting is that if I take my time and slowly increase RPM over 20-30 minutes while motoring, it seems to be less of a problem, go figure....

Good luck, keep us posted. Bob

 

[Steve Dion]

This is only a half boat unit, so you are getting off cheap if that is all they charge.

Be sure that you shaft is straight before they start the alignment process. You also want to have your motor mounts checked too.

It is not going to be a pretty sight if they "think" they have aligned the engine and you have a bent shaft (about $100/ft) or your motor mounts are shot (about $100/mount).

Hopefully they are not out to SCREW you.

 

[neild]

if they align it before they put it in the water.. it should be done again after it's in the water... the boat shape changes..

 

[walmsleyc]

All good points above. My 2 cents about my Legend 35:

- It was banging against the log.
- It did it often when taking a hard turn (motor mount getting soft, engine leans, and shaft gets closer to one side of the log and knocks)
- Did it sometime after first starting (engine running a little rough at startup).

In addition to aligning the engine to the shaft, I also ensured that the shaft was centered in the log (check this while in the water). This minimized the knocking, and I now have very little of it.

A contributing factor is certainly my motor mounts. They are at the very end of the usable specification (they have sagged over the years). I also ended up adding shims under my front motor mount to allow me to get the front of the motor up enough to effect the proper alignment.

I have a shaft seal, and thus it is hard to check the centering of the shaft in the log while in the water. It certainly doesn't have to be perfect, but I was able to test fairly easily by physically pushing the shaft up, down, left, right, and judging/measuring how far I was able to push it in all directions. The amount I was able to deflect it in all directions allowed me to get close to centered. I had done this on land initially, and found that once in the water, the shaft had moved 1/8" up or down (can't remember).

I expect that when I replace the motor mounts, most traces of this knocking will be gone (it is now vastly improved).


Chris

 

[capejt]

I'll be watching

I'll be watching this this post closely, as I am going through exactly the same thing with my '79 H33.
Keep us updated!!

 

[sail_4_me]

I also plan to change motor mounts on my 37 Cutter

This spring I'm also changing mounts. The knocking always occurs at lower RPMs 600 - < 800 when I'm leaving or coming into the dock. Its nice to see that my suspicions may actually be valid. Curiously enough I have a Yanmar 3QM30F engine, with 3GM motor mounts!

I just hope the stainless shaft is not bent.

 

[Jim Haynie]

The last poster - Sail 4 me -- likely is dealing with another issue entirely. All 3 cylinder engines make horrible banging sounds when run at very low RPM's, so his shaft-log-alignemnt all may be perfectly OK and is just hearingthe normal effects of hitting the harmonic RPM rate. I never let mine get below about 900 and if it starts banging a bit simply increase RPMs a tiny amount to stop instantly.

 

[Steve Dion]

Dan:

If you end up changing the motor mounts be careful not to get talked into aftermarket mounts. If you have a GM engine they have different mounts fore/aft. If you have a JH engine they use different mounts port/starboard.

One of our fellow sailors had his engine replaced and the people that did the engine swap put the forward mounts on the aft and vice versa. It banged so hard that it threw the shaft out of the coupling. They had to have someone else fix it.

I would guess that new motor mounts and installation would cost between $600-1000. The mounts are over $100 each.

 

[sail_4_me]

In any event I have to replace the mounts, as they are 28 years old and bottoming out. See attached pictures. we'll see after install/adjustment if the vibrations and banging are reduced.

I agree RPM is a factor but at a particular speed, the banging is so pronounced it feels like the engine is hitting something and I can feel it on the side of the boat. I havent found any evidence ( other than these pictures ) to indicate where its coming from. My guess is that the steel on the mounts is bottoming out due to aged rubber, but the shaft alignment is also pretty close to the edge. I'll feel better when they are changed.

BTW I can run the engine at 400-600 and its smooth and quiet

 

[Brad Hoff]

I am having the same issue. It happens when I motorsail with heel to port. I also started shortly after I replaced my stuffingbox with a shaft seal. I think in my case the extra support the stuffingbox provided allowed my old and most likely worn out motor mounts to shift enough when heeled. Had it looked at and was suggested to replace my brass shaft with stainless and replace motor mounts. Right now I just try not to motorsail on a port tack.

 

[capejt]

Funny, mine starting acting up after I replaced the stuffing box with a shaft seal too!

 

[Maine Sail]

Funny, mine starting acting up after I replaced the stuffing box with a shaft seal too!

A PSS will not "hold" the shaft and keep it centered with a misalignment issue. It allows the shaft to do what it wants. The loud banging is most often the shaft hitting the shaft log due to improper alignment but can sometimes be attributed to a broken mount..

The shaft should pass through the shaft log in the hull perfectly centered and the shaft bearing completely parallel. If the mounts are worn they can settle and allow the shaft to come in contact with the shaft log when under power and when under sail.

 

[Bill1565]

Based on the photo of the shaft, it is not centered in the shaft log.

I have just resolved a similar problem on my boat after a year of experimenting. When we bought the boat, we had loud banging at all engine rpm in gear and out.
Replaced a worn martec 2 blade with a Campbell sailer 3 blade and alligned the strut to the shaft log out of the water. Elimimated much of the banging except at idle and again at around 2200 rpm. Had a mechanic replace the rear motor mounts and re allign engine. Eliminated banging at idle. Another mechanic, helping with an electrical problem, pointed out that bolts were loose on new motor mount, and one nut on motor mount was only finger tight. Turns out first mechanic stripped the holes on one rear motor mount and just forgot to final tighten hold down nut on a front mount after re-aligning. put in longer bolts set in resin, secured hold down nut and presto, all rattling is gone. Spent way more than we negotiated in price reduction, took way longer than I liked, but we finally got fixed.

 

[capejt]

Thanks Maine Sail! You've confirmed what I was already thinking. Although I must admit, my symptoms aren't exactly like others here. In neutral, all is fine. In reverse, all is fine. But at any speed in forward... thump, thump, thump! I finally got a good visual inspection of the mounts done last week and they do look quite shot. I was originally planning on just doing a shaft re-alignment and see if that cured it, but after taking a good look, I think I'd be better off going for new mounts. Of course, after your "snowballing" post, I'm a little scared. That kind of luck has followed me a long time!!

 

[Endurance2010]

Update - yard realigned the shaft and engine. All is well now. No banging, except when I take a hard turn and crank the RPMs. Total charged was about $350. As an added bonus, the engine no longer emits white smoke at 2000 RPMs.

 

[rfrye1]

I also found my problem (see my post above). My engine mounts are bad, and the engine is misaligned. Had my diver check strut and cutlass, all is ok. New mounts are ordered, being installed next week.

 

[sail_4_me]

Thought I would also reply. I pulled the engine and had a lot of work done, one reason being an upgrade from 150lb to 200lb rated motor mounts. My Yanmar 3GM30F weighs about 400lbs, going to the heavier mounts seems prudent. The heavier mounts also have larger bolts. My rear-left motor mount bolt was sheared off and is likely the root-cause of the banging, made worse by 28-year old, too-small mounts.

Add in a new stuffing box, new cutlas bearing, new exhaust hoses, this thing better be purring like a kitten when I launch!