I'm post-survey, but still pre-purchase of a 2000 C36MKII with a Max-Prop 3-blade feathering prop. I've read enough of the forum posts to get an idea what rpm and hull speed I should be getting but it's really weird, and I can't tell what might be causing the issue other than prop pitch.

In the sea trial, I was only getting about 4.5kts under full power in flat lake conditions. Free rpm was around 2850 and loaded rpm was about 2800 (50rpm droop -- not enough).

We asked seller to repitch to confirm, and he's been having a diver do it with no success. Here are the things they've tried...

15x18 4.5kts 2800rpm
15x20 6.7kts 1800rpm
15x18 6.8kts 1900rpm
15x16 7.0kts 16-1700rpm (yikes!)

It's weird! I can't imagine an engine related issue, or it wouldn't get to 2800rpm, I wouldn't think. Compression problems when loaded?

Any ideas?