What are the advantages of a contra-propeller in marine use or aircraft.
In: Physics
Just so we’re clear we’re talking contra-rotating propellers where two propellers mounted *on the same shaft* spin in different directions and not *counter*-rotating propellers where two propellers mounted *on different shafts* spin in different directions
A contra-rotating propellers increase the efficiency of the propeller so you can turn more of the engine’s power into thrust. The when air(or water) is pushed by the first propeller it is pushed backwards but also outwards due to the spin, the second propeller facing other other direction counters that spin so there isn’t a net torque and more of the speed in the fluid is going backwards so there’s less energy lost just spinning stuff off to the side.
Contra-rotating propellers can also let you extract a lot more power out of a fixed propeller diameter which is useful for torpedoes that want to be narrow for speed, or planes that don’t have the ground clearance for a larger propeller on the front.
It does add extra weight and complexity ($$$) to the design so unless you’re trying to eek every last bit of performance out of what you have its often not worth it which is why they’re relatively uncommon
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