By Thomas P.

The propeller rotates at the same speed with the propeller shaft, while the propeller shaft usually rotates much slower than the engine crankshaft turns, at the same throttle setting. This is due to the reduction gear which exists on all the outboard engines among their crankshaft and propeller shaft. 

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Gear ratio (or reduction ratio) is expressed by the number of crankshaft revolutions for one revolution of the propeller.

In other words, if an engine has a 2:1 gear ratio means that for one revolution of the propeller the engine crankshaft turns twice.
We can easily find the propeller shaft rpm by dividing the engine rpm with the gear ratio.

For example, if our engine operates at 4000 rpm with a 2:1 gear ratio, its propshaft will turn at 2000 rpm
(4000 rpm x 1/ 2 = 2000 propeller shaft rpm).
If our engine operates at 4000 rpm with a 2.50:1 gear ratio, its propshaft will turn at 1600 rpm
(4000 rpm x 1/ 2.50 = 1600 propeller shaft rpm).

The higher (numerically) the gear ratio is the slower the speed of the propeller shaft gets and thus the less the number of propeller’s revolutions is.

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Anyone could wonder why the manufacturers of the outboard engines don’t install quite small reduction gears so as to get the maximum rpm in our propeller and therefore the maximum final speed.

The answer is that if the gear ratio is decreased (numerically) a lot then the huge propshaft speed (which means too low torque would be produced) will make us use very small diameter propellers, so as the desired thrust could not be produced.
And of course, without thrust no speed can be achieved.

The purpose of the gear ratio is to reduce the speed of the propeller shaft so as a proper diameter propeller can be used which will give the appropriate thrust that our boat needs to move.

...keep Ribbing!

 

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