Propulsion problem

[rwalden]

We have a 1993 SCR 2700 that was just purchased this year (first time boaters), only 653 hours on the 7.4L mercruiser. While cruising on Lake Michigan with 1ft waves I saw three 2-3ft waves coming from the wake of another boat. We were cruising at 20 – 22mph with 3500rpm on the tachometer when hitting the waves at a 45 degree angle. A second or two later I looked at the tachometer and it was reading 4800rpm and the boat started slowing down. I turned around to look at the aft of the boat and there was a fountain of water coming up from the prop, it also sounded as the prop was sucking in air and cavitating. I immediately throttled back and took a look at the back to see if I noticed anything and all looked ok. When I tried to get back on plane the fastest I could go was 9-10mph at 3800rpm. If I increased the throttle I would not go any faster and the fountain would reappear. The engine ran very smooth, no vibrations and no grinding sound. I still had forward and reverse at idle speed so I was able to get it back into my slip at the marina. Looking at the transom with the engine off I adjusted the trim and trailer several times to make sure it was working and in the down position. I looked at the prop and noticed a slight ding on the end of one of the blades, very minor. I took the boat out again to see if I still had the problem, sure enough the same thing. Any ideas on what could be the problem?

[shrew]

Where to begin???? The other boats wake or the size of the waves wouldn't have caused this issue, despite the sequence of events. This seems more like a coincidence than anything else. What you're describing sounds a lot like a spun hub on the prop. The rubber hub acts similar to a shearpin. If you hit something with the prop while it's spinning, you'll spin hte hub rather than cause significnat damage to the drive. This would require replacing the hub, which isn't that big of a deal. However, you also mention that the prop seems like cavitating as well. The description of the 'rooster tail' would seem like the drive is raising up while running. This too would cause a similar cavitation issue. When engaged in forward the thrust from the prop should drop the drive all the way down. This year I watched a buddy with an air bubble in his drive hydraulics. when he put the boat into reverse, there was no pressure in the line and the reverse thrust raised the drive all the way up. he bled his drive trim and everything was fine. In your case, even if you lost hydraulic pressure, moving forward wouldn't normally cause the drive to raise.

When this occurs were you able to put the boat in neutral and verify the position of the drive?

[rwalden]

I did try checking the drive in neutral however it was hard to tell the position due to the exhaust in the water. It seemed ok; I didn't notice anything like the prop any closer to the surface, it seemed normal from the four times I've taken her out. I'm so new to the boating experience that I'm really not sure what is normal. When I asked a few boaters at the marina the first diagnosis was a spun prop as well, the only thing that was confusing is the rooster tail. I'm concerned about taking it out if it is a spun prop. I'm afraid of wearing out the rubber and the prop falling off leaving me stranded. The boating season is just about done for Lake Michigan so I'm pulling it out of the water to look at it and wait until spring. I'll post what I find out.