Logo

Mercruiser Stern Drive noise

Fish Prospector

New member
I recently replaced my Mercrusier Alpha1 Stern Drive with a genuine new Mercruiser unit.
It seems to run fine but there is a kind of harmonic humming noise at cruise speed, 3000-3500rpm.
I broke the unit in as per mechanics guidelines and have replaced the break-in oil. I have put about 40 hours on it since replacement. Gimble Bearing was checked and is in good shape. I am using a Merc aluminum 16x16 prop which has some cavitation burn marks on it but is otherwise in good shape. I did not have this noise with my original leg. Any ideas what may be causing this and if I should be concerned?
 
I would sure check the drive oil level, as well as check it for water and metal shavings ASAP.
It also sounds like it could be gimbal bearing or U joints as well.
Have you greased the gimbal bearing lately?
Was the alignment checked?

Rod
 
Yes the drive oil level is checked regularly. I find that it needs a few ml's of oil to top up every 5 to 10 hrs.
There was no evidence of water in oil when it was changed at 20 hrs and only minor shavings as might be expected from break-in (OK'd by boat mechanic) but I will check again.
The gimble brg was checked, including alignment, and was determined to be OK when the unit was installed and was greased when the oil was changed.
 
Some props seem to make a characteristic purring sound, but if you have not changed props, that can't be it.
I don't have any other ideas.
Any chance it's coming from some other area, like the front of the engine?

Rod
 
I am not discounting your concerns regarding the humming you hear.

But i never not heard a humming from a alpha drive.

The upper bearing under the top cap hums. The gears turning hums also.

Maybe you just didn't pay attention before.
 
Back
Top