Prop vibration that goes away on its own...

[scuttlebooty]

Recently, after the boat had been sitting for several weeks, but on two separate occasions, I experienced a strange vibration when shifting into gear.

The vibration is significant, and almost sounds like cavitation. It's present both in forward and reverse, and appears to be coming aft of the stuffing box and towards the propeller. However the vibration goes away on its own after a minute or so, and operates completely normally therin.

The prop was not significantly fouled, and I cleaned it anyway, but the vibration was still present. I jerked the shaft back and forth and there's minimal movement. Certainly less than before I replaced the cutlass bearing about a year ago.

The anode seems to be firmly attached.

Anyone have any ideas?

[a64pilot]

Check engine mounts, it sounds like something may be moving around and causing the vibration, but then shifts again and it goes away.
So I’m thinking drive shaft, which is of course indistinguishable from prop vibration wise.

However if it’s just present when shifting into and out of gear, if the transmission was slipping, would that cause a vibration? Maybe?

[scuttlebooty]

Originally Posted by a64pilot

Check engine mounts, it sounds like something may be moving around and causing the vibration, but then shifts again and it goes away.
So I’m thinking drive shaft, which is of course indistinguishable from prop vibration wise.

I was watching the gear box and shaft during the noise/vibration, and it didn't seem any different than when it went away. What should I be looking for? It also really sounded like it was coming aft of the stuffing box.

[Dauntless44]

Hi, i think i had the same issue at the end of last season. I pulled it and cleaned the prop ( which had a lot of growth) i was losing rpm snd power as well in spurts . I haven't seen it this year yet. Are you losing power?

[a64pilot]

In truth I’m guessing and suggesting to check something that is easy to check and requires no money.
When stumped I start by eliminating possibilities, I start with easy and cheap and progress to hard and expensive only after eliminating easy and cheap, even if expensive and hard is more likely.
To check mounts grab the engine when it’s off and forcibly try to move it, have someone watch the mounts looking for movement, except for the rubber deflecting there shouldn’t be any of course.

[model 10]

[QUOTE=a64pilot;
So I’m thinking drive shaft, which is of course indistinguishable from prop vibration wise.

Couldn't there be a second or third order vibration from the prop blades?

[bglad]

If it goes away as you gain speed then it could just be extra load on the propeller from a standing start. As hull speed catches up the propeller would be running more freely and likely more smoothly. Any little bit of scale or fouling might change the sound a bit.

[scuttlebooty]

It goes away on its own, but the fouling on the prop wasn't so bad. The prop has definitely been far more fouled with no vibration like this.

Additionally, the vibration is not present at all today, after trying again.

Even though it doesn't sound like it's coming from the gear box, could it be slipping gears? Or maybe it's corrosion on the shaft that is working itself smooth?

[a64pilot]

[QUOTE=Ecos;2934646][QUOTE=a64pilot;
So I’m thinking drive shaft, which is of course indistinguishable from prop vibration wise.

Couldn't there be a second or third order vibration from the prop blades?[/QUOTE]



It’s the going away on its own that has me a little stumped, usually something is the cause and until you remove that something the vibe continues.
Most of my vibration chasing comes from helicopters, and everything vibrates on helicopters and the vibe will often present in a different place than it originated, having travelled down the driveshaft, but where it presents has a harmonic at the frequency of the vibration. Same thing happens with cars, and of course boats, they all have driveshafts.
It’s either minor and normal, as these things do have vibratory “phases” meaning speed or RPM or perhaps a combination, only cure I know of is to a avoid one or the other.

Try to get to the bottom of what causes it to stop, is it as another poster said happen after the boat begins to move? There is defiantly logic to that question.

[captmikem]

I had this same problem and found the prop nut was not tight, prop appeared tight but was not. Tapered shaft with a key. Key wore a bit and the prop would vibrate, then as I throttled up, the prop would force itself fwd and tight on the shaft and vibration would go away. It about a quarter turn loose. Nut had been backed off slightly so the cotter pin would go thru the hole.
M

[scuttlebooty]

Well, the boat is tied to a slip right now, so these tests are stationary.

I just checked the prop again, and it seems firmly attached, and the cotter pin is in place, but one of the prongs is bent outwards.

Here's a possibly related event: The last time the engine was run prior to the vibrations was during a very bad storm at anchor that resulted in steep 5 foot waves and me at the helm battling against them to keep from dragging. In the chaos, a line was caught in the prop and cut, leaving a few wraps around the shaft and a dangling piece. However, I didn't notice this until weeks later when I went in to clean the hull. During the storm, and the subsequent re-anchoring, there were no such vibrations though.

[Painless]

We had a vibration in our prop only at a specific rpm. Close inspection out of the water showed a very almost un noticeable bend. We had Wilmington Propeller straighten and balance and it is gone. Maybe the line wrap bent the prop?

[thunderhoof]

Is your prop a folding type? Sometimes, while the prop is not driven during a shift, it will "un-unfold" one blade which will cause a severe vibration until it pops back into position.

[scuttlebooty]

It's not a folding type, I think it's a 12" fixed pitch 3 blade.

Well, if it were bent, how could I explain the fact that the problem goes away on its own, and even the next day is gone. But I bet if I leave the boat for bather 2 or 3 weeks and try again, it will be back..

[Secondshift]

Originally Posted by scuttlebooty

Well, the boat is tied to a slip right now, so these tests are stationary.

I just checked the prop again, and it seems firmly attached, and the cotter pin is in place, but one of the prongs is bent outwards.

Here's a possibly related event: The last time the engine was run prior to the vibrations was during a very bad storm at anchor that resulted in steep 5 foot waves and me at the helm battling against them to keep from dragging. In the chaos, a line was caught in the prop and cut, leaving a few wraps around the shaft and a dangling piece. However, I didn't notice this until weeks later when I went in to clean the hull. During the storm, and the subsequent re-anchoring, there were no such vibrations though.

Perhaps the wrapped line remnant acted as a dampener. Considering the line wrap I'd pull the prop and take it for a check up at a prop shop.

Regards