I have no particular need to know this but just curious....
Which would be best - a high gear ratio driving a fine pitch propellor, or a low gear ratio driving a coarse pitch propellor? And why?
I was just looking at prop pitch vs gear ratios to work out what speed you might expect to get out of an auxiliary engine, the ones I was looking at seem to be 6" or 7" pitch, against the 19-21" generally found on bigger engines. Some of the "high thrust" low HP engines seem to have a different gear ratio to let them drive a bigger prop.
So I thought, if you could increase the gear ratio on a big engine (i.e. make the prop spin faster for a given engine speed) then would there be any benefit in doing this, say make the prop spin 3x the speed and use a 7" prop instead of a 21"? Or any drawback in doing this?
To my entirely inexperienced mind it seems that it might increase thrust at low speeds where a coarse prop will initially cavitate, but maybe there would be a big penalty at higher speeds with extra drag from the prop having to "chop up a lot more water" (no doubt there is a tech term for that!!)
As I say it is probably completely academic as I doubt you can even buy different gear ratios for engines, I just wondered.