Mark - The Prop calculator should give you a pretty good idea.
http://www.rbbi.com/folders/prop/propcalc.htm
Yamaha 150 gear ratio = 2.0
Prop Pitch = 19
With a prop slip of = .149
3000 = 23.0 mph
4000 = 30.6 mph (prop slip 14.9%)
4000 = 32.4 mph (prop slip 10%)
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