Eli5: why are propellers on the front of planes but the rear of boats?

Question

The thought just came across my mind and I simply cannot think of a good answer, especially not knowing much about the mechanics of it all.

In: Engineering

Answer 1

Airplanes normally operate in one state of matter – gas (air), though they use wheels to taxi around at airports between flights.

Boats operate in two states of matter – liquid (water) and gas (air) almost 100% of the time (outside of drydock).

While behavior of moving water (hydrodynamics) and moving air air (aerodynamics) share many characteristics, they do have some important differences. Water, for example, is essentially incompressible compared to air and is far more dense than air is -> this is one reason why airplane propellers and boat propellers are very different. Hydrodynamic behavior is also a reason why blunt shapes like modern submarines and bulbous bows on ships are common (much greater underwater efficiency) than the relatively sharper design elements on high speed airplanes.

Anyway, putting the main propellers on a boat up forward (along with maybe the rudders) also presents several challenges.

* Practicality. Propellers mounted forward on a ship would require the ship to have huge rudders back aft to provide enough force to change course (like, orders of magnitude bigger than they are now)-> huge rudders require massive machinery back aft to turn them -> massive machinery aft not only then requires compensation forward to maintain stability but it also limits the available weight and volume that can be used for whatever job the ship is being designed to perform.

* Safety. On the water and especially coming into port, is no such thing as the near-immediate stop on the water like you would taxiing an airplane or driving a car. Ships coming into port (unassisted) will typically make a slow approach and come close to the pier bow-first to put over lines and use their rudders and engines to bring the stern in. If those propellers and rudders were forward and there was a collision or harder than expected contact, then there’s a huge chance the gear would be damaged and suddenly a “hard bump” becomes a “boat drifting uncontrollably around other boats”. This issue also applies to hitting something uncharted (only 20% of the ocean is mapped) or just stuff floating around (like a log or a half-sunken container) as visiblity underwater is very limited.