system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why do submarines and boats have propellers in the back but airplanes have propellers in the front?
gpt
Some airplanes have the propellers in the back.  Some airplanes have them in the front *and* the back.  Some have them on the sides (when they're mounted on the wings).

It boils down to the fact that air inside an airplane is okay, but water inside the boat is a bad thing.  Propellers on an airplane are (usually) directly attached to the engine, and air in the engine is good.  You need it for fuel mixture as well as cooling.  On a boat, however, you don't want water getting into your engine or the hull at all, if you can help it, so the props are at the rear so the force of the prop doesn't push water into the seals around the shaft.

There's more to it than that, but that's the eli5 version.