Here is a prop calculator I have used
http://continuouswave.com/cgi-bin/propcalc.pl
With the 6100, 15 pitch, 2.4 gear ratio and zero slip the calculated speed is 36 mph. The zero slip is not possible as some slip is always there but it gives an indication of upper limit. Slip is variable and depends on the setup and local water conditions, etc. You can back calculate the slip if you measure your speed with a gps and then use the calculator to determine the slip knowing all the other data.
The gear ratio you note might be in error so research it. I do not know what it would be. If you can't find it you can count the rotation of the prop to one flywheel revolution. As an example if the flywheel turns once and the prop turns 1/2 a revolution the gear ratio is 2:1. You will have to estimate the amount of rotation of the prop in your case it will turn less than 1 rev for 1 flywheel rev. But this should give you an idea if the 2.4:1 is correct.
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