system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why on boats are the propellors at the back yet on planes they are on the front?
gpt
Some issues:

1. Turbulence.  When the medium through which your propeller moves is turbulent, you get wildly varying power levels.  So you want to avoid feeding very turbulent air/water into your propeller.  On a plane, the airframe itself generates a good deal of turbulence.  On a boat, the hull alone generally won't impart enough energy to the water to create significant turbulence because water is much more dense than air.

2. Steerage.  If you place the engine at the front, the resistance of the medium around your craft will tend to cause the craft to align along the direction of travel.  If you place the engine at the back, the resistance of the medium will tend to deflect the craft from the direction of travel. With a plane, you need some pretty significant control surfaces to overcome this effect because air doesn't pose much of an obstacle to yawing uncontrollably.  On a boat, it's nearly impossible to deflect its course in this manner because you'd need to displace enormous amounts of water to move sideways.

3. Boat propellers only work underwater.  Even with the denser medium, it still makes sense to put the propeller on the front of the boat - the water just makes it less impractical to put the propeller on the back, it doesn't make it *better*. What makes it better is that the front of your boat leaves the water in situations where you need powered propulsion (climbing up a wave) while the back of your boat leaves the water in situations where you don't (climbing back down the wave).  In the first case, gravity is opposing your direction of travel so a loss of propulsion means you fall back.  In the latter case, gravity is assisting your direction of travel so a loss of propulsion means you keep moving forward anyways.