New harmonic in driveline

[Skipper]

Have a new med/high pitch vibration in the driveline. More like a hum. Happens intermittently, but is steady and more volume, that I can feel in the seat, when I turn the wheel slightly to starboard. Goes away if I turn more.
Cutlass bearing, stuffing box, and coupler were replaced, and shaft trued to coupler, in July of '14. About 200 hours ago.
Has been smooth as a Benz since, until recently.
It's not something that anyone would likely notice, except me because it's a change in the boat's behavior.
Rudder has zero play in the wheel.
I thought it might be the shaft zinc but the issue didn't change after I replaced the zinc a week ago.
Bottom is clean.
Motor is tops. Universal M18 2 cyl.
Any ideas?

 

[jviss]

Deteriorating motor mounts? Have you re-checked the alignment?

 

[Bill Roosa]

big chunk of marine growth on the hull? These can cause all kinds of weird stuff

 

[Skipper]

Deteriorating motor mounts? Have you re-checked the alignment?

They look fine. Looks can be deceiving, but would this account for the hum when only slightly turning the rudder to stbd?

 

[Skipper]

big chunk of marine growth on the hull? These can cause all kinds of weird stuff

The bottom was just cleaned.

I'll add that I can see the rudder and prop from the dock finger. There's nothing odd or different there.

 

[jamhass]

Motor mounts. Don't just do a visual or push-on-it test, recheck the torque on the securing fasteners. Similarly, double check shaft coupler etc. Sometimes, we don't get things as secure as we think, and since you recently worked on these areas .....

 

[uncledom]

Is your shaft zinc wearing evenly all around?

 

[Skipper]

Motor mounts. Don't just do a visual or push-on-it test, recheck the torque on the securing fasteners. Similarly, double check shaft coupler etc. Sometimes, we don't get things as secure as we think, and since you recently worked on these areas .....

Thanks. I'll check the mounts again.
The shaft work was done by a mechanic in the yard in Annapolis. It's been fabulous for 3+ years. I inspected it all up close about 3 months ago.

So turning the rudder slightly to starboard while under way will set the driveline up in a hum if the motor mounts are loose or deteriorating?

 

[Skipper]

Is your shaft zinc wearing evenly all around?

As I said, the zinc is new last week.
I thought that might have been it but no change.

 

[jviss]

It's been fabulous for 3+ years.

Nothing lasts forever.

 

[Skipper]

Nothing lasts forever.

I was referring to the fact that the coupler, which another poster to whom I was addressing suggested might need inspection, was likely not the culprit; since it was installed new by a reputable professional, and not by me as suggested, and the work had exhibited no sign of faulty installation for a lengthy period. And especially since I had recently inspected it. All the bolts are tight and wired, no play, no sign of change.

I'm really looking for someone who can readily identify the symptoms.
The inducing of the hum with the turn of the rudder only to stbd, and only when turning slightly, is a mystery that nobody has been able to directly explain.

 

[jviss]

You are going to feel feedback from the force of the prop wash hitting the rudder, and it seems that at a particular angle, you excite that particular vibration mode, or frequency. I don't find it that mysterious.

Try holding the rudder at that angle and changing the engine speed, and seeing what happens.

It could be the rudder vibrating. Maybe the rudder bearing is worn.

 

[SoSound]

If a small change in steering makes a difference, the closest component (fulcrum) in the driveline would be the cutlass bearing. But don't get suckered into tunnel vision or obsess over just one component or system, I found a sound several years ago that I'd have sworn was within the hull to actually be a resonance in the running rigging that I quieted with an adjustment to the topping lift (fixed by tightening the mainsheet).

 

[Skipper]

You are going to feel feedback from the force of the prop wash hitting the rudder, and it seems that at a particular angle, you excite that particular vibration mode, or frequency. I don't find it that mysterious.

Try holding the rudder at that angle and changing the engine speed, and seeing what happens.

It could be the rudder vibrating. Maybe the rudder bearing is worn.

It's mysterious when it starts suddenly after owning the boat for 7 years and 300+ hours of motor time.

I too have wondering about the rudder. It's skeg-hung and is set on a big solid bronze post. I've looked at the stuffing box. No leaks, no wild corrosion. Zero play in the steering. It seems sound.

 

[Skipper]

If a small change in steering makes a difference, the closest component (fulcrum) in the driveline would be the cutlass bearing. But don't get suckered into tunnel vision or obsess over just one component or system, I found a sound several years ago that I'd have sworn was within the hull to actually be a resonance in the running rigging that I quieted with an adjustment to the topping lift (fixed by tightening the mainsheet).

Does seem logical that it might be the bearing.
Although it sure would be an early failure.

 

[Skipper]

This is her bottom with rudder. July of '14.

 

[Skipper]

Actually that's a terrible angle. But it's the only bottom pic I have on this phone.
Here's a sister ship. Same rudder and prop config.

 

[CarlN]

Also don’t obsess on the rudder angle. Looking for harmonics is like chasing deck leaks.

Growth on prop? Polish it.

Nick or bend in prop from minor collision. Might not have felt it.

Prop slipped a bit on shaft due to poorly fitted key

Grounding strap wiper on shaft

Cutlass bearing (as mentioned) - uneven wear could be at back or front of bearing. Of course, you can’t see the front

Engine mounts. Just the slightest sag from age could do it. Alignment could be OK at rest but go out under load. Try loosening the top nut on each mount one at a time and see if they each are loaded. Try raising each up 1/2 a turn. Nuts should be equally hard to turn.

 

[jviss]

ry raising each up 1/2 a turn.

I think that will require a re-alignment.

 

[NYSail]

Check your steeling cables..... and maybe rudder post. Best thing to do is as soon as you hear it, keep it going, let someone else steer and go looking around everywhere