The torque that is produced by the engine is converted to thrust by the combined effect of the gear ratio with our propeller. Physics teaches us that the Torque is proportional to the horsepower (Hp) and inversely proportional to the rotation speed (Rpm). Thus, the higher the horsepower or the lower the rpm of the propshaft is, the greater the torque that is produced gets.
As the torque at the propeller shaft increases, so a larger diameter propeller can rotate.
It is obvious that as the gear ratio of an outboard engine gets higher (numerically) so much slower the propeller shaft rotates, so much greater the torque is applied to it, and the more large-diameter propeller can be rotated.
On the other hand, we know that the larger the diameter of the propeller is the more water is pushed back, the greater the thrust is generated and hence the greater the boat propulsion is.
Combining all the above parameters (gear ratio-torque-thrust) we conclude that as much as the lower unit gear ratio increases numerically (at the same horsepower) the greater the torque at the propeller shaft gets, the larger diameter propeller the engine can turn and therefore the greater thrust will be produced.
So we can accept that the gear ratio of an outboard engine acts as a regulator of torque and thrust.
An obvious question could be why then in all the outboard engines are not installed quite high (numerically) gear ratios so as very large diameter propellers can be used and thereby achieve a tremendous amount of thrust.
The answer in this case is:
- the use of long pitch propellers would not be possible because then the applied load for the engine would be too much. So we would be forced to use very short pitch propellers and then the final achieved speeds would be disappointing.
- the reduction gear would be huge, and as a result the size of lower unit should be too big to accommodate.
- very large diameter propellers and too big lower units would create a tremendous drag which is inversely proportional to the speed.
Therefore, because the diameter and pitch of a propeller work together, manufacturers install proportionate gear ratios which enable the most efficient combination of diameter – pitch, so as to get the best combination of thrust - speed at specific horsepower, specific hulls and uses.