Do I have a lower unit problem on my outboard?

Question

I just installed a ss prop on my Merc 70 hp. Now when I shift into gear, either forward or reverse It makes a bad rumbling noise at idle speed around 700 rpm. When I speed up to around 1200 rpm it stops and everything is great. Shifts fine. New thrust washer installed.

Checked gear lube and no metal. Didn't do this with aluminum prop. I know the ss is much heavier. Could the extra weight be the problem?

Answer 1

I had an outboard years ago and the propeller shaft was slightly bent. You could not tell anything was wrong with a light aluminum prop. But, with a heavy stainless steel it vibrated real bad. I not sure this your problem, but, that what it was on mine. With the engine in neutral, not running you can spin the prop by hand and may see it wobble. If you have Earmuffs and a garden hose you can start it and put it in gear and see. Sometimes it may be easier to see spinning by hand.

Answer 2

It could be a balance problem or it could be bent slightly in shipping.  I'd reinstall the aluminum one again and see if it still does it.  Also look to see if the shaft is straight like the first answerer stated.

Answer 3

Be sure the rubber hub is installed properly and not compromised.

Answer 4

Check the hub.  Also make sure the prop nut is tight.

Answer 5

I've seen this issue come up many times on many boats we take care of. It turns out, that with the weight of the propeller that the motor has to spin, and with the tempermental idle of a two-stroke outboard, that the shaft that runs from the bottom of the motor itself down to the propshaft is actually being jerked back and forth against the splines on the bottom of the motor (coupler) and usually, if your propeller is installed correctly, that is the case. I personally have never seen a stainless propeller on any motor smaller than a four stroke 90 HP Mercury or Yamaha, and most of the time Yamaha's come with aluminum propellers. The factories for all marine engine manufacturers send out different props for each motor, and not most of the time, but ALL the time, they send the engine/drive packages with the prop that performs the best in all situations. My recommendation to you would be to put an aluminum prop back on this motor and call it good. If it was a 225 HP Mercury w/ that rumble I wouldn't worry about it because a stainless prop comes with that motor from the factory (and the 225's, before the OptiMax did rumble). Happy boating!