Chasing down a vibration

[Jkraft]

As the title states, I am going to jump into chasing down a vibration my boat has. For information purposes. It the Yanmar 6LP 315 Engine Box Model

When I bought it, it was there. The owner told me that it is due to hull design. I don't think that is true because they would all do it. The owner before him tried to do some things to fix it but its still there. Here is what it does:

It is smooth from idle to 2400 rpm. Once it starts coming up on plane I get a shake/ vibration up until around 2900-3000 rpm then it smooths back out nicely.

Here is what has been done:
Prop Tuned and Balanced
Cutlass baring and brass housing replaced
New Shaft

What I am going to check next:
Engine Alignment
Motor Mounts

Any other ideas on what to look for? Its odd that it is smooth unless in that RPM range of 2400-2900.

Thank you,

[Norseman]

Try to go straight to neutral from 3000 RPM, if the vibration goes away immediately it may be engine/drive train related, if you have a few seconds vibration while the boat decelerate, it could be hull-related.

[Ben423]

I'm pretty certain the vibration has nothing to do with hull design. If there was a design issue, it would have been thoroughly discussed on this forum. My guess is that the source of the vibration is related to the motor mounts or drive train. If the motor mounts are original, it would be prudent to replace them anyway. Having one fail when you're underway is not fun.

Like most things, engine mounts start to sag as they get older, which affects engine alignment. I would start there. Please keep us posted on what you find.

[Pitou]

Vibrations can come from various components.
Another place: A worn rudder bearing can cause vibration from prop wash. I had that issue and R&R'd on my 28.

[Jkraft]

Try to go straight to neutral from 3000 RPM, if the vibration goes away immediately it may be engine/drive train related, if you have a few seconds vibration while the boat decelerate, it could be hull-related.

Will do!

[Jkraft]

I'm pretty certain the vibration has nothing to do with hull design. If there was a design issue, it would have been thoroughly discussed on this forum. My guess is that the source of the vibration is related to the motor mounts or drive train. If the motor mounts are original, it would be prudent to replace them anyway. Having one fail when you're underway is not fun.

Like most things, engine mounts start to sag as they get older, which affects engine alignment. I would start there. Please keep us posted on what you find.

Thanks Ben will do. Checking engine alignment and motor mounts this week. I can tell you this has been going on for 6 plus years or at least for the last two owners

[Jkraft]

Vibrations can come from various components.
Another place: A worn rudder bearing can cause vibration from prop wash. I had that issue and R&R'd on my 28.

Wow! There is a place I never would have looked. What are the indications of a worn rudder bearing? It does not have any play, or it didn’t last week when the boar went in.

[Pitou]

Vibrations can come from various components.
Another place: A worn rudder bearing can cause vibration from prop wash. I had that issue and R&R'd on my 28.

Wow! There is a place I never would have looked. What are the indications of a worn rudder bearing? It does not have any play, or it didn’t last week when the boar went in.

Just like in the shaft bearing, if it's loose in it's housing the bearing needs replacing.

[Jkraft]

Welp, here is the verdict and the back story. Let me know what you guys think. The pro came out yesterday and came out for a ride. He said my problem is definitely in the drive train and it’s not the trans or the motor. So that’s good news.

He started with engine alignment. He disconnected the shaft at the coupling. The motor was pretty far off. Not sure how far off but he said the front of the motor was way too low. We got that corrected. He said the shaft was super easy to disconnect and push back which indicates that there is no binding. But we will need to pull the boat to see if the shaft is favoring one side of the cutlass over the other.

Once all back together he still wasn’t happy. He said it’s way better but it appears that the shaft and the coupling are not mated property. He said it’s as close as we can get it now. He recommends pulling the shaft and coupler this winter and sending them out to be machined and matched.

Here is the history on the boat (after lots of digging and contacting previous owners):

In 2009 the owner experienced vibration under way and stopped to check. Two of the bolts on the coupling had backed out and hit the transmission and sheered off. All were lose. He tightened the other two up and ran home that way and had it fixed. He claims the vibration was gone after that.

Boat then switches hands and there is a gap in history of a couple years. New owner buys it in 2014 and has the vibration. He tunes the peop and sends the shaft out to be checked at black dog. They said the shaft is perfect. He replaced the cutlass including the cutlass housing. Still has the vibration.

After motor alignment it’s still vibrating too.

Is it possible the shaft and coupling weren’t mated properly from the factory? There is a possibility that the shaft was replaced at some point but I doubt it.

Please note also that the shaft was checked, not replaced. I was originally told incorrectly

Sorry for the book I just wrote. What do you think?

[RobS]

What is "way better" - what were his final clearances? to how many thousandths did he get the couplers aligned?

When he slid the shaft away did he check the tranny flange for run-out?

Did he check the pre-load on the engine mounts?

Yes it is possible there was coupler to shaft runout from the factory if Albin did not have the coupler fit and faced to the shaft.

Also possible that in the other times the shaft was R&R'd that the coupler was not reinstalled true to the shaft or the coupler itself has runout.

I have seen perfectly tuned props get tweaked during reinstall because someone chocks the prop against the hull at the blade end rather than down at the root. This will twist a blade just enough for a vibration.

[Jkraft]

What is "way better" - what were his final clearances? to how many thousandths did he get the couplers aligned?

When he slid the shaft away did he check the tranny flange for run-out?

Did he check the pre-load on the engine mounts?

Yes it is possible there was coupler to shaft runout from the factory if Albin did not have the coupler fit and faced to the shaft.

Also possible that in the other times the shaft was R&R'd that the coupler was not reinstalled true to the shaft or the coupler itself has runout.

I have seen perfectly tuned props get tweaked during reinstall because someone chocks the prop against the hull at the blade end rather than down at the root. This will twist a blade just enough for a vibration.

Welp I basically can’t answer any of these questions. I do know he mentioned something about 4 thousands at the coupler.

Last guy had the prop redone twice and I did it once. Really feel like I’m shooting Into the air here. The guy working on it is the one that is supposed to be the best around here. Guess I’ll be calling him Monday

[Bob Noodat]

If your "expert" did not bring a dial gauge with him and take several measurements using it, I would question the utility of his efforts.

[Tree]

I had a vibration on FP2 for the first year that I couldnt track down until the engine was pulled for the infamous failed core plug that was dripping oil.

Turned out to be 2 sheared bolts on the crankshaft pulley - was causing it to run out and was only noticeable under load at certain rpms

[Jkraft]

If your "expert" did not bring a dial gauge with him and take several measurements using it, I would question the utility of his efforts.

He had a digital gauge with him and was there at least two hours measuring and adjusting. I just don’t know what he did..... I’m not the mechanic he is.

[Jkraft]

I had a vibration on FP2 for the first year that I couldnt track down until the engine was pulled for the infamous failed core plug that was dripping oil.

Turned out to be 2 sheared bolts on the crankshaft pulley - was causing it to run out and was only noticeable under load at certain rpms

Thanks Tree! Will check that too