Often this is a practical approach. However, a larger prop turning more slowly can be more efficient. Particularly if it prevents the prop from cavitating significantly. I would not want to try pushing my boat with a 8" prop turning at 3200RPM, for instance. I would much rather be pushing the boat with say a 12" prop turning at half that speed while keeping the motor speed where it should be. YMMV of course. More than one way to skin a cat.
Some motors in some applications will work just fine without reduction gear. But remember, if there is no thrust bearing in the motor directly driving the prop, you will still need an external thrust bearing. Thoosa is the only one that I am aware of that has an integrated thrust bearing that can accept reasonable axial loading. Motenergy, Golden, etc will need a thrust bearing, and any gearbox suitable for marine propulsion use will be able to handle the thrust. So there is another reason for a gearbox.
However, a gearbox is not the only way to get reduction. Smaller boats in particular often use a belt and pulley system with a thrust bearing on the prop shaft. Motorcycle chains and sprockets work too, but they can sling oil. The one advantage of a chain is the chain can be broken to change a sprocket or whatever, or an ACTUALLY broke chain can be repaired or replaced, without needing access to the end of the shaft. For instance, it is a simple matter to mount a sprocket on the output of a transmission on a diesel, mount the motor up above that sprocket, and run a chain around the motor sprocket and the prop shaft sprocket. If you try the same trick with pulleys and belts, you have to decouple the shaft somewhere to slip new belts on should the old ones break.
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