Variable pitch is commonly used so the engine keeps at its best rate - 102 in this case. There can be highly damaging resonances at other speeds of the propshaft, engine itself or parts of the hull. Snag is I don't think variable pitch can go round as far as reverse. Given a very simple propeller it may be possible, but since the prop is key of MPT (miles per ton) it's design is sort of 'one way'.
I think the big gear is used for driving auxilliary machinery once the big beast is running - compressors, generators.
These things often run on cheap heavy oil and need centrifugal pumps to seperate the oil from crud and sand. The fuel also needs heating so it can flow through pipes.
Martin
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"need centrifugal pumps to separate the oil from crud and sand"
Blimey - we petrol consumers clearly don't know how lucky we are!
Interesting about the variable pitch constraints. I suppose a big epicyclic reversing gearbox a la Model T is out of the question?
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Ships are huge polluters for exactly this reason and they are getting worse because sulphur is now removed from lighter fuels and is therefore left behind in the heavy oils these things use. Air pollution from shipping is so bad it is causing crop failures around the Mediterranean. But try forcing them to clean up with legislation and suddenly Bolivia has a huge merchant navy. Oh, wait a minute...
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"Sounds like a variable pitch propellor might be an idea..."
Or a decent gearbox with reverse. One out of a Hillman Imp should be able to handle the power.
V
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