R6 Vibration at lower RPM and lack of speed

[Markus_de]

Hello to the cobalt forum. I own a Cobalt R6 model year 2021. I have had the boat since it was new. For this reason I know it very well. The boat has recently had vibrations in the drive at 2500 to around 3000 rpm. I had no contact with the ground and the propellers don't look bent at first glance. The engine actually runs as usual. When I accelerate with the clutch disengaged (although this is apparently limited to 2500rpm) I can't find any problems with the engine. From the feel of it, the boat also needs more speed to get into planing motion and the maximum speed is also around 10-12 knots lower. the boat has the 350 HP 6.2 MPI mercruiser with bravo 3 - stock props. 150 hours, always regularly serviced. what could be the problem?

[jhnmdahl]

Something with the engine not running right? Outdrive misaligned? Old/bad gas? Could be lots of different things - may be time to have it checked out if troubleshooting isn't your thing.

[Pilotmike]

First thought is gimble bearing or yoke bushings, but 150 hours is very early for this to be a problem unless the alignment has been out since installation. Another feeler test would be At low speed and turning at tightest radius, do you feel and vibration in the boat or floor? That could also help iso the issue to the drive.

[Markus_de]

OK, I think it was bad fuel. Since months I fill up at the same gas station at the lake. Yesterday I used another one for the first time. And could not believe that all vibration is gone....

Could not believe that bad fuel causes that much problems!

[bruceb58]

Are you removing the drive annually or semi annually and checking alignment and u-joints?

[Markus_de]

No, I haven't done that yet. Is that required by Mercury's service plan? Boat has about 150 hours and all the usual maintenance is done regularly...

[billk2632]

Not sure if/how it is spelled out in the current maintenance schedules - but with an outdrive engine (Merc, Volvo anyone) it is a good practice to pull the drive at least every other year for an inspection of the U-Joints, Gimbal bearing, bellows etc, check the alignment and grease things before re-reinstalling the drive. If the boat is in the water and not dry stored every year is best. It's a preventative inspection that will save you big time by catching a small problem before it becomes a failure. An out of alignment or a small leak in the bellows can silently do a lot of damage that you won't realize until the big bang comes.

[bruceb58]

The guy I bought my boat from never did it either and my drive alignment was WAY out. Alignment will change the most in the first year or so as motor mounts settle in.

[BobRae99]

I'd have a careful look at the props. I had a tiny dent (about the size of a baby finger nail) in one of the blades that produced vibration in the same rpm range you mentioned. Once that was corrected the vibration went away.