5.7 Merc Alpha One Gen one violently shakes entire boat?!

[celebrity88]

Sorry if this is so long, but in desperate need of help!! I have a 1988 Celebrity Crownline Cruiser 28 with twin Merc 350s and Alpha Ones. I bought the boat cheap in the fall of 23 because it had a bad engine (starboard). Have been working on it since. Mostly myself, but last year I gave up and took it to a marina shop to address/repair outdrive and shifting issues in both drives. They said all was good, but I could not take it out right away as I had a stalling issue on the old (port) engine. When I finally got it out this past fall, the new starboard engine - which idled fine - would bog and die under load. But I'm not worried about that right now. When I came back into the marina, the old port engine -which was going fine - I shifted into reverse to dock, and then back in to forward and it made an awful clunking sound and shook the entire boat. Reversing, it was fine. Back into forward, violent shaking and clunking. It sounded like the driveshaft broke loose and was crashing around. When I took the drive off, I find the driveshaft and u joints are fine. Turning the shaft in forward/neutral/reverse, all seems fine, prop moves as it should, no noises. Searching, I could not find any matches that would help me narrow this down. When Googling, its AI tells me what the problem could be, things I thought of and checked, but suggests that it could be a shift linkage problem (which the marina did work on), but I can't imagine that would shake the entire boat so bad. Interesting thing: both engines are standard rotation, starboard drive is standard, and the port drive is counter rotating - which mostly the internet/forums/etc seem to claim was not something that was done. Crossing my fingers that someone has a suggestion.

 

[alldodge]

Welcome

Assuming the drives where not pulled to adjust the shifting, correct?

If drives were not pulled it's most likely the U-joints

When was the last time the drives were pulled and serviced, and alignment checked?

 

[celebrity88]

Welcome

Assuming the drives where not pulled to adjust the shifting, correct?

If drives were not pulled it's most likely the U-joints

When was the last time the drives were pulled and serviced, and alignment checked?

Thanks so much!

Both drives had been off for servicing lower units by me. I installed new starboard engine and I had marina align it. Then when both didn't shift correctly, I returned to marina and they also had both drives off and reinstalled, which was in September.

The problem drive is off at the moment, and an inspection shows driveshaft and joints are good.

 

[alldodge]

When I took the drive off, I find the driveshaft and u joints are fine.

Yep, missed that, and reason I break up my post

the old port engine -which was going fine - I shifted into reverse to dock, and then back in to forward and it made an awful clunking sound and shook the entire boat. Reversing, it was fine. Back into forward, violent shaking and clunking. It sounded like the driveshaft broke loose and was crashing around.

Things that can shake the boat in gear are the coupler, drive or prop.

Drives were serviced so no sign of lube discoloration, which for me leaves inside the drive. Take the top cap off the drive and see what the bearings look like

 

[celebrity88]

Yep, missed that, and reason I break up my post


Things that can shake the boat in gear are the coupler, drive or prop.

Drives were serviced so no sign of lube discoloration, which for me leaves inside the drive. Take the top cap off the drive and see what the bearings look like

Thanks! That was my plan. When I but in a new impeller I noticed play in the prop shaft, so I took it to the marina and they replaced bearings in the lower unit. If they'd seen anything in there that didn't look good, they SURELY would have replaced/charged me!!

Props are brand new...

 

[alldodge]

noticed play in the prop shaft, so I took it to the marina and they replaced bearings in the lower unit. If they'd seen anything in there that didn't look good, they SURELY would have replaced/charged me!!

You may have a great Marina, but most Marina's don't employ certified Merc Tech's because the Tech's can make more money working for a dealer

Also if they didn't have the Merc special tools and knowing there are differences between the Counter rotation port lower unit

 

[celebrity88]

You may have a great Marina, but most Marina's don't employ certified Merc Tech's because the Tech's can make more money working for a dealer

Also if they didn't have the Merc special tools and knowing there are differences between the Counter rotation port lower unit

They are Merc Certified Techs, and I had informed them that it was counter-rotating.

But that drive was working fine for a time, I had planning on running up the river to test out but pretty quick the other engine was not working right so I turned around and came back in. While maneuvering into my slip is when it started thumping.

 

[rolmops]

Could it possibly be that you hit something under water with your prop while docking?

 

[celebrity88]

Could it possibly be that you hit something under water with your prop while docking?

Doubtful...not just a thud or random. I know I'm not explaining the shaking well, but it was maybe like driving a car with a square wheel.

 

[Bondo]

They are Merc Certified Techs, and I had informed them that it was counter-rotating.

But that drive was working fine for a time, I had planning on running up the river to test out but pretty quick the other engine was not working right so I turned around and came back in. While maneuvering into my slip is when it started thumping.

Ayuh,....... Welcome Aboard,........ There's no short cuts to diagnosing what's going on,......
Determine which side it's coming from, by running one side, then the other,.....
Then pull that drive, 'n start the motor, if no thumping, tear down the drive, 'n see what's wrong,....

 

[nola mike]

Interesting thing: both engines are standard rotation, starboard drive is standard, and the port drive is counter rotating - which mostly the internet/forums/etc seem to claim was not something that was done.

I think this is typical on a chevy I/O.
Is the problem only in gear? Does it happen with the drives off? Have you tried swapping the drives to see if the problem follows the drive? Does the steering position make a difference?

 

[cyclops222]

Shaking a boat at idle speed ? Takes a lot of weight FLOPPING around.

 

[celebrity88]

I think this is typical on a chevy I/O.
Is the problem only in gear? Does it happen with the drives off? Have you tried swapping the drives to see if the problem follows the drive? Does the steering position make a difference?

It only happens with the port drive. I don't know that I'd want to swap drives with them being counter-rotating (then forward is reverse and reverse is forward...)
I have not started it up without the drive off (just took it off last weekend)...I may do that today, but I imagine I would learn nothing as it was thumping in drive, but not neutral or reverse. I'll run it to make sure though.

Thanks for all the input everyone, I'll update!

 

[nola mike]

It only happens with the port drive. I don't know that I'd want to swap drives with them being counter-rotating (then forward is reverse and reverse is forward...)
I have not started it up without the drive off (just took it off last weekend)...I may do that today, but I imagine I would learn nothing as it was thumping in drive, but not neutral or reverse. I'll run it to make sure though.

Thanks for all the input everyone, I'll update!

Only reason I say swap drives would be for testing. I've seen reports of couplers suspected of taking a set and causing a wobble. Without the drive on would expect that to go away. If you swap drives and it persists you're looking at an engine/coupler/gimbel/alignment problem instead of a drive problem. If it goes away then your problem is with the drive.

 

[cyclops222]

None of my Mercruiser 305Ls have ever shook the boat.
Does the FWD thump continue as you increase boat speed. Is a random thump or like definite repeat pattern.

 

[celebrity88]

None of my Mercruiser 305Ls have ever shook the boat.
Does the FWD thump continue as you increase boat speed. Is a random thump or like definite repeat pattern.

Well, the thump/vibration was literally so violent that I would not have tried...plus I was THIS close to my slip.
But definitely not random.

I just got back from the club and ran that engine. No noises or vibration or anything abnormal at idle or rpms.

 

[Bondo]

Well, the thump/vibration was literally so violent that I would not have tried...plus I was THIS close to my slip.
But definitely not random.

I just got back from the club and ran that engine. No noises or vibration or anything abnormal at idle or rpms.

Ayuh,...... Did the pace, mirror motor rpms,..??

Did you shift gears, running today,..??

 

[celebrity88]

Ayuh,...... Did the pace, mirror motor rpms,..??

Did you shift gears, running today,..??

I did go with the RPMs.

But today I just ran the engine, the drive is currently off.

I was going to pop the drive cap, but I've not had that off before and I'm not sure I could accomplish anything! The drive shaft and U joints look good, and turning it by hand there are no weird noises/clunks/grinds/ to be heard. I'm thinking of putting the drive back on in a tub and seeing what happens.

 

[cyclops222]

Did you have something wrapped on the prop that banged only in FWD ? Now has fell off ?

 

[celebrity88]

Did you have something wrapped on the prop that banged only in FWD ? Now has fell off ?

I suppose anything is possible!