With the specs you gave, 1st post I find potential speed to be 70.79mph. Im not trying to cast doubt tho just to say there are variables on the boat beyond arithmetic while #s are cut & dry. The 1.85 :1 ratio with the 12 x 23 prop @ 6500Rs could potentially go 76.4. I dont know much about the powerhead except to say it will need to produce power at much more rpm. for that 83mph+ mark. At 2:1, 23P youll need a minimum of 7700 Rs to reach 83.8 & thats before subtracting slip/resistance that can vari from about 90% [very efficient] or less.
What I do to get these #s is Pitch times RPM divided by 12, times 60 divided by 5280, times Ratio. [.5 in this case]= Potential Speed. To get ratio I divide the low # of gear teeth by the high #. for instance Yamato 14/15s = .933:1 using .933 to multiply. Best of Luck with that run for the record.
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