Losing speed around 3k RPM

[robinsbd]

I took the boat out for a long run up the river. I was able to get on plane several times and usually cruise around 3k RPM. At the end of the day, I headed back down the river and when I raised the RPM to about 2800 RPM, the RPM shot up a bit and my speed dropped off. If I slowed it down to 2000 - 2500 RPM, I could get moving again. After that, I never could really get above 17MPH without having it slip on me. The engine is running fine. My first thought is that I spun the prop hub. It's possible I trimmed up too much as well. My other worry is that the coupler could be going out.

How should I proceed with troubleshooting?

Thanks!

 

[Bondo]

Ayuh,..... Try another prop to eliminate that,.... trim down to eliminate that,......

If it still slips, it's probably the coupler,.....