Howdy,
Welcome aboard!
First of all, it might be helpful for you to have a look at the following thread for future questions.....
https://forums.iboats.com/forum/eng...-repair-i-o-and-inboards/473314-faq-for-i-o-s
Also, take a look at:
https://forums.iboats.com/forum/forum-rules-and-guidelines/407734-attention-forum-rules-attention
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There are additional reasons why a higher ratio drive is not used on a small block (read lower torque engine... )
While you *could* probably get it to work by using a lower pitch prop, it would be less efficient.
For most all recreational planing type watercraft, manufacturers have recommended (in general) to always select a prop-pitch that will provide maximum speed (optimum, weight, CG, at sea-level, trim etc) at maximum rated engine RPM. (those same manufacturers already provided the "right" drive ratio)
Propeller manufacturers design props to operate with a rather specific RPM range. (racing props and surface drives, etc excepted)
I.E, there is an RPM upper-limit where any prop will become inefficient......such that above that upper limit, the hydrodynamic drag becomes so great that turning it any faster doesn't give you much more thrust. (this is true for any prop turning in either liquid or air)
Same thing applies with pitch. There's a maximum pitch that either results in an aerodynamic stall in air, or cavitation in water. In air, there's also max RPM that produces prop-tip velocities that can exceed the speed of sound.
In all cases thrust is lost.
The point of all this is that to keep the propeller RPM in the range that is most efficient for the pitch you have and use gears to operate the engine in it's most efficient range.
If you get out of either the optimum range for the engine or the prop, you will lose efficiency.
Get out of the range for both, you lose even more......
The engine, drive and prop designers already "did the math"
Cheers,
Rick
