Strange rumbling noise - Maxum Boat Owners Club

rwc · 07-18-2018, 08:48 PM

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.

mmwjr · 07-18-2018, 11:12 PM

Gimbal bearing or u-joints. Does the pitch change when you turn the wheel?

rwc · 07-19-2018, 12:38 AM

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.

mmwjr · 07-19-2018, 03:20 AM

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.

Kristoff · 07-19-2018, 11:53 AM

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.

mmwjr · 07-19-2018, 12:29 PM

Quote:

Originally Posted by Kristoff

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.

Kristoff · 07-19-2018, 12:54 PM

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.

Kristoff · 07-19-2018, 12:57 PM

Also - forgot to ask, does it change with drive trim? If it does then U joints or gimbal bearing must be a strong suspect.

mmwjr · 07-19-2018, 06:00 PM

Quote:

Originally Posted by Kristoff

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.

Phillbo · 07-19-2018, 07:44 PM

When a coupler fails does it slip or just break?

mmwjr · 07-19-2018, 11:15 PM

Quote:

Originally Posted by Phillbo

When a coupler fails does it slip or just break?

There are two types of failure; rubber breaks or splines strip out.

Rubber failure can slip or be totally break.

Splines is a total failure and typically due to misalignment or not being lubed.

Phillbo · 07-20-2018, 01:53 AM

Is it possible the change in power could be from the coupler slipping?

Stick your head behind the engine and look for rubber marbles...

mmwjr · 07-20-2018, 02:21 AM

Quote:

Originally Posted by Phillbo

Is it possible the change in power could be from the coupler slipping?

Stick your head behind the engine and look for rubber marbles...

Never thought of that but if gimbal bearing or drive is locking up it can cause the coupler to fail.

rwc · 07-25-2018, 08:12 PM

The rumbling continues - usually starts after 30 to 45 minutes of operation and then recedes again once the boat cools down. It's not been as bad as the first two times, but I think that's partly because I'm hesitant to take the boat far or push it hard until I know what's going on.

I haven't experienced the power surge again. That seems to have been a one time event, so maybe that was a fuel starved carb due to the pitching and the rapid changes in rpm exacerbated this other problem? I'm not sure.

My current plan is to pull the boat and change the gimbal bearing (and inspect u-joints) and hope that solves it. I'm betting they are as old as the boat, so I don't feel bad about having to do it - I'm just annoyed that I didn't bite the bullet and have it done pre-emptively when the drive was pulled two months ago.

One other thought, I guess I should have mentioned - given that the drive was pulled two months ago and the bellows were changed, I'm assuming it's not something really obvious and something like rubber marbles from a failing coupler would have been noticed. Is it a possibility that the mechanic didn't align the drive correctly when it went back in and that's why this came up shortly after?

PS. Thanks for all your suggestions and thoughts. It has helped me decide what to do about it and given me some peace of mind. Much appreciated.

mmwjr · 07-25-2018, 11:00 PM

A misaligned drive will prematurely wear out the coupler and will put a side load on the gimbal bearing. IMO if misalignment is making the bearing noise the bearing was near end of life probably from a leaking bellows and age.

rwc · 07-25-2018, 11:17 PM

Thanks.

Sorry, one more question. Should I just go ahead and replace the u-joint at the same time or only if it shows signs of significant rust/wear?

I don't want to waste money/resources unnecessarily, but it costs $500 to lift/store the boat to make repairs, so if the u-joint is relatively cheaper, maybe it makes sense to do it rather than risk having to pull the boat again if it happens to be that or that wears out next.

mmwjr · 07-25-2018, 11:25 PM

If you believe they are original and the bellows was leaking before replacement I would have the u-joints changed.

rwc · 07-25-2018, 11:34 PM

Thanks. I'm hoping to find the time to get in there with the mechanic this time. Last time I was away when he did the work, so I didn't see what condition the bellows and surroundings were in when he changed them.

rwc · 07-30-2018, 11:39 PM

Looks like it was the u-joint. Apparently I suck at testing. Mechanic hopped onboard, cranked it hard over and revved it up to 1800 RPM in neutral. Immediately sounded like a dryer with steel-toed boots in it. He said "Your u-joint is failing. See you on Monday." and rode off into the sunset.

Replacing both gimbal and u-joint, just in case.

I guess I never had the wheel hard over with RPM over 1400 or so.

mmwjr · 07-31-2018, 01:18 AM

Well at least it’s the cheapest of what it could have been. Live and learn.