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Old 15-03-2011, 15:35   #1
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Shaft Alignment Problem ?

Our boat is being delivered from La Paz, Mexico to Panama and is currently in Ixtapa, Mexico with transmission problems. The transmission is a Yanmar/Hurth KBW21.

The transmission was getting difficult to shift, and it was not a linkage issue, so the crew pulled into Ixtapa for repairs. We had a local mechanic remove the transmission and he found the bearings for the input and output shafts were greatly worn and needed to be replaced. We also replaced the damper plate while the transmission was off.

Our problem appears to be with the prop shaft. With the shaft disconnected, the engine purrs and the transmission shifts smoothly. However, when the prop shaft is connected to the engine, there is significant vibration and the transmission is again hard to shift.

Using a guage, the shaft appears to be .4mm out of round. This may be from a bent shaft, but we cannot tell with the shaft installed.

We are almost to the point of hauling the boat, pulling the shaft, and checking it for trueness. However, the tsunami really silted up the marina and we are not sure we can get there. We have haul the boat to pull the shaft because there are crocodiles in the marina (no joking) so you cannot get in the water.

I'm looking for suggestions. Is there anything we should look at before we haul the boat? Any thoughts on how much out of true the shaft can be before it becomes an issue? Thanks!

Steve
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Old 15-03-2011, 15:44   #2
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Re: shaft alignment problem?

Quote:
Using a guage, the shaft appears to be .4mm out of round.
Not good. A really nice alignmnet should be within .003 .inches of perfection. With a shaft at .4 mm this is bad. You would want the total alignment within .1 mm (converting from Inches to mm). Normally that would be measured after it was connected to the coupling for a net alignment. If you got .2 mm it would probably pass. It seems like just a tiny bit but a spinning shaft is going to wobble and make trouble. > .4 mm is way too high.
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Old 15-03-2011, 15:50   #3
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Re: shaft alignment problem?

O.4 doesn't seem that bad (not good though), but it depends on many factors. The shaft, if bent, will probably be bent at the outer end which may not show up so much inside the boat.
You might have a damaged/ missing a blade on your prop. You might have a worn shaft bearing.
Can you turn the shaft by hand, does it have a tight spot?

If you cant look in the water, then hauling is probably your only answer, to diagnose any of these problems properly.
Sorry
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Old 15-03-2011, 19:23   #4
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Re: Shaft Alignment Problem ?

is the shaft hard to turn when disconnected from the transmission?
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Old 15-03-2011, 20:00   #5
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Re: Shaft Alignment Problem ?

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Originally Posted by never monday View Post
is the shaft hard to turn when disconnected from the transmission?
Ditto!

And what has changed to cause this problem, assuming that it was running good in the past. Bent shaft or strut? Cutless bearing seizing? Zink against strut? Motor has moved?
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Old 15-03-2011, 20:58   #6
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Re: Shaft Alignment Problem ?

The engine was replaced in Mexico by the previous owner. It is my feeling that the mechanic who did the replacement did a poor job of aligning the shafts.

The prop turns frrely by hand, and when the shaft is disconnected from the engine, it shifts easily. When connected, the transmission binds.
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Old 15-03-2011, 21:02   #7
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Re: Shaft Alignment Problem ?

Sounds like alignment time!
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Old 15-03-2011, 21:07   #8
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Re: Shaft Alignment Problem ?

One question I am not sure of is the application of flexible couplers. The boat has a rigid clamshell-type coupler now. Would a flexible coupler help the situation?

I pulled out my caliper so I can lay eyes on how much 0.4 mm actually is, and 0.4 mm is not much misalignment. We are talking about 16 thousandths of an inch. Seems to me a coupler should be able to handle this considering the propshaft rotation is around 900 rpm at cruising speeds.

Any thoughts on these couplers?
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Old 15-03-2011, 21:27   #9
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Re: Shaft Alignment Problem ?

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Originally Posted by steve77 View Post
One question I am not sure of is the application of flexible couplers. The boat has a rigid clamshell-type coupler now. Would a flexible coupler help the situation?

I pulled out my caliper so I can lay eyes on how much 0.4 mm actually is, and 0.4 mm is not much misalignment. We are talking about 16 thousandths of an inch. Seems to me a coupler should be able to handle this considering the propshaft rotation is around 900 rpm at cruising speeds.

Any thoughts on these couplers?
Try bending your shaft that .016". That takes quite a bit of leverage. A soft coupler helps but doesn't resolve the problem. It would get you down the road but would eventually wear out the soft coupler, and you may still have some vibration and some stiffness in the trany.

You're lucky the shaft coupler didn't work loose or the shaft crack and break. Which you may want to check. If motors are installed on the hard they need to be realigned after being in the water for a while. The boat settles into it's natural shape in the water. And after running the motor for a while it may settle into the woods a bit too.
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Old 15-03-2011, 21:39   #10
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Re: Shaft Alignment Problem ?

I had major shaft and cutlass bearings caused by poor alignment. I actually ate through a shaft in 7 days due to poor alignment (after having a new shaft and bearing installed with an alignment). Anyway, if you pull the boat to replace anything, let it sit in the water for 24 hours before the final alignment is done. The second yard to do my work did that and it's been great ever since. Part of the issue on my boat is having a very short shaft. That makes it even more important to have everything true. My best advice is find someone with a reputation for doing precise alignments. My alignment sucked- then I had a new bearing and shaft installed but it came with a horrible alignment. Then 7 days later I had a newer bearing and shaft installed with an amazingly precise alignment and the difference is astounding. I'm pretty sure the last guy said he accepts nothing less than .003" but tries for better. When you watch the shaft in gear now it doesn't even look like it's moving. I think I'll have a Gin and Tonic in his honor right now.
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Old 15-03-2011, 21:45   #11
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Re: Shaft Alignment Problem ?

Del's right. Let the boat sit in the water for a couple of days before doing an alignment.
.001" of misalignment is about max you should have, and the shaft should turn freely when connected to the trans.

Unless you understand all the angles (pun intended) have a pro get the job done right. Being in Mexico, it wouldn't hurt to get some referrals while you're at it.
There are good mechanics, but being as macho as the country is, if a guy doesn't know what he's doing, he's not likely to tell you.
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Old 16-03-2011, 02:32   #12
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Re: Shaft Alignment Problem ?

Quote:
Originally Posted by steve77 View Post
The engine was replaced in Mexico by the previous owner. It is my feeling that the mechanic who did the replacement did a poor job of aligning the shafts.

The prop turns frrely by hand, and when the shaft is disconnected from the engine, it shifts easily. When connected, the transmission binds.
That sounds like it may be your problem, as said already final alignment is done in the water.

Quote:
Originally Posted by steve77 View Post
One question I am not sure of is the application of flexible couplers. The boat has a rigid clamshell-type coupler now. Would a flexible coupler help the situation?

I pulled out my caliper so I can lay eyes on how much 0.4 mm actually is, and 0.4 mm is not much misalignment. We are talking about 16 thousandths of an inch. Seems to me a coupler should be able to handle this considering the propshaft rotation is around 900 rpm at cruising speeds.

Any thoughts on these couplers?
Flexible couplers do not compensate for poor alignment, they are used to reduce vibration and noise transmission. They still require precise alignment.
Aquadrives can run with some misalignment, but it's still good practice to get it properly aligned.
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Old 16-03-2011, 05:13   #13
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Re: Shaft Alignment Problem ?

Quote:
We are talking about 16 thousandths of an inch.
A good alignment would be .003 a run of the mill one would be .007. .016 isn't in the ball park. When done well you will actually hear the difference. Poorly done and it rattles the stuffing in the box loose and will break the flexible coupling. Just not something you really want.
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Old 16-03-2011, 07:59   #14
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Re: Shaft Alignment Problem ?

See ABYC 6.5.4.2 , and the Tables therein.
Here ➥ http://www.abycinc.org/committees/P-06.pdf
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Old 17-03-2011, 07:41   #15
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Re: Shaft Alignment Problem ?

The crew is having the boat hauled this morning to inspect the shaft, cutlass bearing, prop, etc. I'll let you know what they find.
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