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Old 29-01-2015, 05:39   #1
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Shaft Misalignment Causing Knock?

My Yanmar 3 banger knocks when I significantly increase or decrease the throttle. The manual for the engine shows a number of possible causes for this problem, including misalignment of the prop shaft to the engine.


How do I tell if this, and not, for instance, a mal-adjusted fuel injector is the cause?

All help greatly appreciated.

Thanks,

G2L
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Old 29-01-2015, 05:42   #2
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Re: Shaft Misallignment Causing Knock?

Check the alignment. Also take a good looks at the mounts and see if they're sagging or otherwise worn out.

Does the knock disappear once the new throttle setting is reached?


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Old 29-01-2015, 06:07   #3
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Re: Shaft Misalignment Causing Knock?

Quote:
Originally Posted by Gone2long View Post
My Yanmar 3 banger knocks when I significantly increase or decrease the throttle. The manual for the engine shows a number of possible causes for this problem, including misalignment of the prop shaft to the engine.

How do I tell if this, and not, for instance, a mal-adjusted fuel injector is the cause?
As SM points out... Check the alignment... Pull apart your flange and see what she looks like... Do some searches regarding shaft/engine alignment....

Run her up in gear disconnected and see if you get a significant sound change... The engine of course won't be loaded up, but you may be able to tell something...
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Old 31-01-2015, 21:24   #4
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Re: Shaft Misalignment Causing Knock?

You set your engine alignment at the flange behind the transmission. Boats with a long shaft run may have several flanged sections, sometimes with pillow block bearing supporting the shaft. If you have multi shaft sections, start at the stern.
I set clearances at .002" or under. When I commercial fished, I found a shaft .004 or more out of alignment would wear the rear transmission bearing, made a knock that effected fishing and could make it difficult to get a proper seal on the packing gland. Sometimes poor alignment will cause wear in the cutlass bearing or even wear the shaft. I have seen transmission tail shafts ruined because of a bind in the shaft. I found most boat yards aren't very good at alignment. They like to throw in some mickey mouse fiber, plastic spacer between the flanges and call it good. When I anchor or tie up in a current of a few knots or more, I have to leave the transmissions in gear, otherwise the shafts slowly continuously spin. Having the shaft in line also saves fuel. When I bought this boat it used about 14.5 gallons an hour @ 10 knots. The realignment saved about a gallon an hour and with other changes I'm down to 10 gallons @ 10 knots.
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Old 31-01-2015, 22:39   #5
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Re: Shaft Misalignment Causing Knock?

When delmarry replaced his engine with a Yanmar 3 cyl. he found he had to replace his shaft and bore out the shaft log to accommodate it. As above, check your alignment and check the shaft log to see if there is evidence of the shaft hitting it. Also check your engine mounts. You may be able to get by with firmer engine mounts.
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Old 31-01-2015, 22:48   #6
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Re: Shaft Misalignment Causing Knock?

I vote for bad engine mounts.

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Old 03-02-2015, 02:10   #7
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Re: Shaft Misallignment Causing Knock?

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Originally Posted by Sailmonkey View Post
Check the alignment. Also take a good looks at the mounts and see if they're sagging or otherwise worn out.

Does the knock disappear once the new throttle setting is reached?


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As noted above, "yes" the knock usually goes away once the new throttle setting is reached. Will need to check the manual regarding how to check the alignment. Any quick suggestions?

Thanks for your help,

G2L
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Old 03-02-2015, 02:11   #8
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Re: Shaft Misalignment Causing Knock?

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Originally Posted by HappyMdRSailor View Post
As SM points out... Check the alignment... Pull apart your flange and see what she looks like... Do some searches regarding shaft/engine alignment....

Run her up in gear disconnected and see if you get a significant sound change... The engine of course won't be loaded up, but you may be able to tell something...
Got it.

Thanks
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Old 03-02-2015, 02:17   #9
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Re: Shaft Misalignment Causing Knock?

Quote:
Originally Posted by Lepke View Post
You set your engine alignment at the flange behind the transmission. Boats with a long shaft run may have several flanged sections, sometimes with pillow block bearing supporting the shaft. If you have multi shaft sections, start at the stern.
I set clearances at .002" or under. When I commercial fished, I found a shaft .004 or more out of alignment would wear the rear transmission bearing, made a knock that effected fishing and could make it difficult to get a proper seal on the packing gland. Sometimes poor alignment will cause wear in the cutlass bearing or even wear the shaft. I have seen transmission tail shafts ruined because of a bind in the shaft. I found most boat yards aren't very good at alignment. They like to throw in some mickey mouse fiber, plastic spacer between the flanges and call it good. When I anchor or tie up in a current of a few knots or more, I have to leave the transmissions in gear, otherwise the shafts slowly continuously spin. Having the shaft in line also saves fuel. When I bought this boat it used about 14.5 gallons an hour @ 10 knots. The realignment saved about a gallon an hour and with other changes I'm down to 10 gallons @ 10 knots.
Interesting. Now you've got me worried. The packing leaks a bit, but nothing too significant. Seems within specs at 1 drop every 8 secs or so. Time to get greasy.

G2L
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Old 03-02-2015, 02:19   #10
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Align the shaft by lifting the engine?

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Originally Posted by DeepFrz View Post
When delmarry replaced his engine with a Yanmar 3 cyl. he found he had to replace his shaft and bore out the shaft log to accommodate it. As above, check your alignment and check the shaft log to see if there is evidence of the shaft hitting it. Also check your engine mounts. You may be able to get by with firmer engine mounts.
Or a couple of extra washers to lift the engine a bit???

That has been suggested by a friend, but not sure he is an expert at such stuff.

Thanks for the input,

G2L
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Old 03-02-2015, 02:22   #11
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Mounts look Primitive

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I vote for bad engine mounts.

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"Bad" may not be the right word. "Primitive" is more like it. Just looks like a couple of brackets, with a bolt and washers. Will check more closely, however.

G2L
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Old 06-02-2015, 02:01   #12
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Re: Shaft Misalignment Causing Knock?

Shaft alignment is nothing to take guesses at. Just putting in new mounts or a washer or two will only make problems worse unless you accurately check the alignment.
You slack the bolts holding the flanges together, and insert a feeler gauge top, bottom and the two sides. If the engine is in alignment, the feeler gauge will be within .002" in all 4 places. If you have too big of a gap at the bottom, the front of the engine needs to go down or the transmission up, but still keeping the bolt holes and flanges in line. If the gap is on the st'bd side, move the engine front to st'bd, etc. It can be a time consuming task, making little moves until you reach a reading within .002". You can get shim stock (brass sheeting of various thickness) at better auto parts stores. Probably Ebay, too.
Misalignment will at least cause bearing wear. If the packing seal is over tightened to stop drips, the packing can wear a groove in the shaft. As I said before, I have seen the transmission tail shaft ruined and the transmission full of metal shavings. If you turn the shaft by hand and feel even a slight a bind, you have a problem.
This isn't an impossible job to learn. You just need to be accurate and careful. Also, an alignment done out of the water on blocks often will change when you float your boat.
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Old 06-02-2015, 23:00   #13
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What about revving the engine in neutral?

Quote:
Originally Posted by Lepke View Post
Shaft alignment is nothing to take guesses at. Just putting in new mounts or a washer or two will only make problems worse unless you accurately check the alignment.
You slack the bolts holding the flanges together, and insert a feeler gauge top, bottom and the two sides. If the engine is in alignment, the feeler gauge will be within .002" in all 4 places. If you have too big of a gap at the bottom, the front of the engine needs to go down or the transmission up, but still keeping the bolt holes and flanges in line. If the gap is on the st'bd side, move the engine front to st'bd, etc. It can be a time consuming task, making little moves until you reach a reading within .002". You can get shim stock (brass sheeting of various thickness) at better auto parts stores. Probably Ebay, too.
Misalignment will at least cause bearing wear. If the packing seal is over tightened to stop drips, the packing can wear a groove in the shaft. As I said before, I have seen the transmission tail shaft ruined and the transmission full of metal shavings. If you turn the shaft by hand and feel even a slight a bind, you have a problem.
This isn't an impossible job to learn. You just need to be accurate and careful. Also, an alignment done out of the water on blocks often will change when you float your boat.
After reading the above, I have a much better idea of how the job should actually be done.

Also, there is a related video on Utube, which shows an owner actually making the adjustments. Between the two, I think I now understand how its done.

On another note, I talked to the broker who sold me the boat. He used to sail it himself, w. the previous owner. He thinks the problem is injectors, and as someone mentioned up the thread, they seemed to think that the key would be increasing and decreasing the engine speed while in neutral.
What do you think of this test as an indicator?

Thanks again,

G2L
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Old 07-02-2015, 00:04   #14
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Re: Shaft Misalignment Causing Knock?

If it is the injectors, it could be the "crack" or "pop" pressure is a bit low on one or more of the nozzles. I found that this problem manifested itself most obviously on our engine when increasing the throttle. It was not a problem when decreasing throttle though.

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Old 21-02-2015, 04:11   #15
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Update: Problem Seems to Be Injectors

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If it is the injectors, it could be the "crack" or "pop" pressure is a bit low on one or more of the nozzles. I found that this problem manifested itself most obviously on our engine when increasing the throttle. It was not a problem when decreasing throttle though.

Matt
I tried two of the suggested tests above, and they seem to indicate that the problem is the injectors.

Firstly, I turned the shaft, and it seems very, very smooth. Also, I started the engine and revved it in neutral. The knock occurred when I increased power. Decreasing power did not produce the knock.

Lastly, I also talked to the broker who sold me the boat. He was a good friend of the owner and had raced the boat on weekends for a number of years, right up to recently. He firmly suggested that there was no alignment problem, and though this is no actual proof of the situation, it does back up what the tests seem to indicate.

I will update this thread after I get a good diesel mechanic to take a look.

Thanks to all for your advice.
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