I have a 1994 Mercury 150 hp 6 cyl Black Max OB. This is the first time for this problem. Under 3000 RPM, no problem. However just after the boat planes out initially the RPMs increase as normal, but then RPMs tend to surge up about 800 RPMs, then drop back a few times, with each time it goes a little higher, and then it would redline at about 5200 - 5500 RPM however the boat itself slows down significantly as if the prop isn't turning at all. It seems like the motor is running / revving fine but no power to the prop at this point. Through this process I did not change the throttle position at all. We were going over some smaller waves 1 - 3 ft. The lower unit alwasy stayed in the water. My initial thought is that the prop bushing is slipping but didn't know if it could be something else? Any suggestions?

most probably prop bushing slip, very easy to check, just open up the prop and look at the bushing.

Kuran, Thank you for the feedback. I will certainly check it out as soon as I can. I may buy a new prop as well as get a new bushing put in if that is the issue. Can you advise the correct prop (Dia & Pitch) for this outboard or maybe a range of props that are recommended? The application is a 20' Trophy / Bayliner about 4000 lbs when loaded. Currently I have a 13.5" x 17" I believe.

if your previous prop performed 5600 to 5800rpm at WOT (wide open throttle), u should keep that size. any additional pitch would decrease about 50-150 rpm (refer to prop box), and so otherwise.