I think the first thing you should do is find out what Yamaha says is the proper RPM for that engine @WOT. Assuming that the engine itself is running properly, once you have that info then you can start dialing in the prop to meet it by switching props. Until then it's really impossible to guess at what your WOT should be. 
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
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