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Old 07-18-2018, 07:48 PM   #1
rwc
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Default Strange rumbling noise

Hi all,

A few weeks ago, I took the boat out and was going through some heavy chop under the bridge about 45 minutes from home. I was going maybe 8 knots at 2300/2400 RPM due to the chop. All of a sudden, the RPM dropped suddenly to about 1500, surged back up, then down, and back up again and the sound changed. It went up an octave and became loud and rumbly.

I dropped the power right down and made for a nearby marina that happened to be a hundred or so yards away. Even at low power, the noise was louder than it should be and rumbling. I checked the oil pressure and temp gauges and they were fine. I was concerned that I'd lost oil pressure in the drive, so limped along as slowly as conditions allowed (narrows under the bridge was NOT a good place to shut down or lose power).

I tied up and spent a half hour inspecting things, checking gauges and fluids, etc., but found nothing. I started up again and the engine/drive noise was back to normal. I made the trip back to my home marina, starting up slow, then applying a bit more power once I was close to home to see if it would happen again, but nothing.

Later, I made a couple of short trips out to see if it would happen again and it didn't until Sunday. After 45 minutes or so, I was on plane at about 3600 RPM and the engine/drive noise suddenly changed again - no power surges this time, but the same higher pitch and rumbling. I slowed right down and limped back to the marina.

Tested it for 20 minutes or so again yesterday, and it was back to normal with no change in sound.

Any thoughts on what it might be? My mechanic has asked me to open the cover and see if I can determine whether the noise is coming from the engine or the drive next time it happens, but he is stumped as to what might be causing it.

Thanks.
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Old 07-18-2018, 10:12 PM   #2
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Gimbal bearing or u-joints. Does the pitch change when you turn the wheel?
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Old 07-18-2018, 11:38 PM   #3
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No, it's pretty constant once it starts. No change with steering, relative decrease (in intensity, not pitch) with power.

Any thoughts on why it would suddenly appear after a certain time, then go away again? I figured it has to be something heat related.
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Old 07-19-2018, 02:20 AM   #4
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When parts are failing they can do strange this on and off then just totally let go.

Removing the outdrive and running the engine for a minute will isolate the issue between the engine and drive.
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Old 07-19-2018, 10:53 AM   #5
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What engine/drive combo do you have?

What you describe is unusual.

Rumbling is normally something to do with an outrdrive not an engine. As Once an engine starts to rumble, it does it all the time. Harmonic balancer or big end bearings etc... it doesn't come and go.

The change in power output is strange though, as that does sound like an engine issue.

I wonder if you have two separate issues that sometime occur at the same time?

If you have a carb'd mercruiser, then surging in choppy seas is usually a carb issue. Different things can cause it but the outcome is that the engine is starved of fuel and runs rough.

Shouldn't rumble though.

When it happens, what happens when you put the boat in neutral and give it some revs? That will take the drive out of the equation.
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Old 07-19-2018, 11:29 AM   #6
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Quote:
Originally Posted by Kristoff View Post
What engine/drive combo do you have?

What you describe is unusual.

Rumbling is normally something to do with an outrdrive not an engine. As Once an engine starts to rumble, it does it all the time. Harmonic balancer or big end bearings etc... it doesn't come and go.

The change in power output is strange though, as that does sound like an engine issue.

I wonder if you have two separate issues that sometime occur at the same time?

If you have a carb'd mercruiser, then surging in choppy seas is usually a carb issue. Different things can cause it but the outcome is that the engine is starved of fuel and runs rough.

Shouldn't rumble though.

When it happens, what happens when you put the boat in neutral and give it some revs? That will take the drive out of the equation.
"When it happens, what happens when you put the boat in neutral and give it some revs? That will take the drive out of the equation. "

Not true; the drive shaft is always turning therefore the gimbal bearing and u-joints are turning. If an Alpha the clutch is at the bottom of the drive so the only thing not turning in neutral is the prop shaft. If a Bravo the clutch is at the top but the forward and reverse gears are tuning it's just the vertical shaft that's not engaged.
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Old 07-19-2018, 11:54 AM   #7
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Yep fair point.

I was thinking more like there wont be any load on the gear box as the prop wont be turning, but yes you are of course correct the driveshaft through the gimbal bearing will still be turning regardless of which drive the boat has.

Sorry for the lack of clarity.
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Old 07-19-2018, 11:57 AM   #8
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Also - forgot to ask, does it change with drive trim? If it does then U joints or gimbal bearing must be a strong suspect.
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Old 07-19-2018, 05:00 PM   #9
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Quote:
Originally Posted by Kristoff View Post
Also - forgot to ask, does it change with drive trim? If it does then U joints or gimbal bearing must be a strong suspect.
I asked if it changed when steering which would be the same change and he said no.
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Old 07-19-2018, 06:44 PM   #10
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When a coupler fails does it slip or just break?
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