140 HP MC1 Cavitation, Vibration, Gear Noise

I have pulled out a 1974 OMC Stringer and replaced it with a 1971 MC1. Kept the same 140 HP GM 3.0 engine. Today was the first day on the lake. 3 things I noticed.

1. With the drive trimmed in any setting other than full down, at about 3000 RPM as the boat starts to plane the prop would cavitate. Did I get the drive to high ? To Low? With it full down she accelerates just fine.

2. There is a vibration like something out of balance? As I speed up it smooths out. Could the prop be out of balance? it is pretty well worn. Should be replaced anyway, or at least reworked

3. I serviced the drive prior to reinstalling. The oil that came out was nice and black. When the drive is running I notice a distinctive gear whine. Is this normal on an MC1 or should the gears run more quitely?

She seemed to shift fine. The control box lever is sticky, but the actual shifting of the drive seemed to be ok. I got a little engine burp sometimes when shifting to neutral, but not all times so I figure the shift cutout switch is working and doing its job. Any thoughts or comments are appreciated.

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

Bottom of the anti cavitation plate should be even with the bottom of the portion of the hull that is in front of it.

Black smelly oil usually means the bearings and/or gears could be pitted. Pull the top cap off and look at the gears and bearing. If they are pitted looking probably your problem. A drive with good gears and bearings should be fairly quiet.

The shake could be your prop. Even one that looks fine can do this if one of the blades is out of phase, this would be out of hydrodynamic balance.

Did you check the propshaft for runout? Is it bent?

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

reavesga said:

....
1. With the drive trimmed in any setting other than full down, at about 3000 RPM as the boat starts to plane the prop would cavitate. Did I get the drive to high ? To Low? With it full down she accelerates just fine.

....

Click to expand...

Did you locate the X dimension 13-9/16" up vertically from the line along the keel?
It sounds like you're too high.
See page 12
http://www.scribd.com/doc/36234870/MERCRUISER-3-0L-ManualAlphaOneMaintenance-86017211

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

Forgot to mention, at some point in this drive's history someone put a "whale tale" on the anti cavitation plate. Wondering if this could have a negative effect?

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

Is the whale tail still on there? or just the holes in the a/cav plate?
Seems like many people have tried those, in a vain effort to get up on plane faster. Unfortunately, it makes more drag.

One other thought, has the prop been repitched? I tried one a long time ago, was either a 19 or 23", twisted to give me 21". It would cavitate if you turned your head too quickly, or sneezed.

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

Whale tale is still installed. I am going to remove it. I do not know the history on the prop. Think I am going to replace it. At wot I was turning almost 5000rpm which is way to much for that setup. I think the book says 4500 max.

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

wire2 said:

Did you locate the X dimension 13-9/16" up vertically from the line along the keel?
It sounds like you're too high.
See page 12
http://www.scribd.com/doc/36234870/MERCRUISER-3-0L-ManualAlphaOneMaintenance-86017211

Click to expand...

x dim is correct. Engine could not go much lower in the boat as there is a beam in the way. I could drop it about an 1" maximum. Overall it is lower than the OMC I pulled out.

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

Ok...I got back from my trip and went out to the boat. I put a straight edge along the bottom of the boat and lowered the drive. With the drive all the way down the plate on the drive is 0.5" above the bottom of the boat. Is this going to cause me problems?

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

Could very well be your problem.

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

Could the prop be out of balance? it is pretty well worn. Should be replaced anyway,

Click to expand...

Ayuh,... I'd start There....

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

Arrgg.....

Prop is not in good shape so I am going to start with changing it out as this is the least expensive and easiest thing to do. Looking at a Solas or Comprop 4 blade. Currently there is a 15x17 3 Blade aluminum on there. All of the prop finders recommend at 14x19 or a 14x21. Any thoughts on this would be appreciated.

Weight = 2000 lbs with gas and crew
WOT = 5100 RPM (should be 4800 max)
Prop was a 15x17 3 blade aluminum
Cavitation plate 0.5" above bottom of boat.

Primary mission is good cruising (25 kts with reasonable fuel burn). Occasionally I pull a tube as well. Don't need great acceleration as it is always young kids on the tube right now.

I have also removed the whale tail. If the prop is cavitating now I sure don't want to raise the stern any higher. I have to admit when I was running, the deck attitude was very flat compared to how it used to run. Felt too flat to be honest. She used to really stand on her stern as she launched out of the hole. In testing last weekend she came up much more level.

Re: 140 HP MC1 Cavitation, Vibration, Gear Noise

Ayuh,.... Try a 19" prop...