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Old 17 May 2006, 13:57   #1
Country: Other
Town: Stanley, Falkland Is
Boat name: Seawolf
Make: Osprey Vipermax 5.8
Length: 5m +
Engine: Etec 150
Join Date: Mar 2006
Posts: 3,655
Theoretical question prop vs gear ratio

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.


BogMonster is offline   Reply With Quote
Old 17 May 2006, 15:28   #2
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,850
Originally Posted by BogMonster
As I say it is probably completely academic as I doubt you can even buy different gear ratios for engines, I just wondered.
No it's not and yes you can. Maybe not for outboards though.

I would like 4" less pitch.
jwalker is offline   Reply With Quote

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