Right Prop?
 

I can't turn but 5200 rpms with a 15 pitch ss prop. The motor is a 2 stroke 150 Yammy on a 20' MA. That motor should turn that prop in the correct range 5500-6000 don't you think?

Are you trimming the motor up to maximize the RPM's at full boogie? My merc 150 has the correct W.O.T. rpm on the sticker,where are you getting 6k as the max?I'm not saying it isn't,just need more info to catch up to where your at.

You might want to check your owners manual before you try getting 6000 rpms. I have a four stroke now, but my 2 stroke yam I think, wot was 4500-5500 and they recommended the top half of that range. (5000-5500)

actually should swing a 17 but agree that it ain't 6000 - trying checking the engine height

Not enough rpm's
 

Yeah, thats trimmed out with a half tank of fuel and 2 people. The engine is as high as I can get it without remounting, not quite an inch above the keel. How can I check my tach?

well it should be fine as long as it reads fine a idle and planes at about 3000 it should be correct