The medium is different, but the physics are the same.
Changing the physical & Gravitational environment Certainly does.
A Bulldozer spends a large amount of power overcoming the problems inherent in moving heavy loads around in a Gravitational field.
In an often Muddy Field. It uses a low RPM, High Torque engine.
An old Engineering joke: With the right transmission, you can move a freight train with a sewing machine motor.
Which is true, BTW. Although you might have to wait a couple days to see how Far the train moved...
A Tugboat is using a very large screw to gain power in a Fluid environment. Most of the Useful work is achieved because of the Inertia possessed by the water. A Lot of power is wasted because fluids don't provide much in the way of "Friction". And that tugboat actually Loses power if the screw turns too Fast because it can create an "air pocket" around the blades.
The PT boat has it easier. Once it is Moving, the engine just needs to Keep it moving. So, it uses a much smaller screw with a totally different blade pitch. A Tugboat sized screw would actually slow it down. The PT boat's screw is designed to minimize the cavitation (the "Air Pocket" problem) effect at high RPMs.
Al these differences are because the Tugboat & PT Boat propulsion systems are solving Different sets of Physics problems.
Aside from the fact that they are both in Water, they are very different.
A Spacecraft is operating in a Microgravity environment where Inertia is the major controlling factor.
A Very different environment, Physics wise, than Water. Or Air.
Oh, and as far as maintenance of high performance engines, I've done a bit of it.
What you are doing is Replacing the parts that Wore Out in a Very short period of time because of the high Stress placed on them.
You should see what just a few minutes of a high speed run does to a Torpedo engine.
You replace a Lot of parts. Most of them without even bothering to examine them because you Know they are trashed.
And because the Maitenance manual counts it as a Major Safety violation to even Attempt to re-use most of them.
In some ways it would almost be cheaper to just plug in a new engine.
But, that doesn't look so good when you write Specifications for them, so the specs require "Frequent Maintenance" instead.
So, I'll stick to my remarks about desinging an engine to tear itself apart for a short term advantage.
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