On small private aircraft the engine is placed at the front of the fuselage, or

Question

On small private aircraft the engine is placed at the front of the fuselage, or on the wings E.g. Stationair, Otter

On combat aircraft the engine/s is placed at the end of the fuselage E.g. Raptor, Typhoon

On larger aircraft constructed to carry cargo/passengers the engines are placed on the wings E.g. DC3, A380

The positioning probably has to do with the amount of load to be lifted into the air, and manouverability desired... which seems sensible.

Yet most water-going craft (boats, power-boats, merchantmen, naval craft) have their propellers/push-pull placed at the back of the fuselage.

What is the advantage in placing the driving component for boats in this position in addition to protecting them against damage? Can the propellers on a boat, with adequate protection say- a tubular enclosure, placed alongside the fuselage not drive the watercraft with equal efficiency and power?

Explanation / Answer

I doubt there's any single overriding physical reason for putting the propellor at the back of a boat. It's a convenient place to put the propellor, and it reduces drag (i.e. having any external propellor module would increase drag).

The requirements of a boat engine are vastly different to an aeroplane engine. It operates at much lower speeds in a medium that is much denser and more viscous. That means you don't need multiple propellors to generate enough thrust: you just use a bigger single propellor. It would probably reduce drag to put a single big engine at the back of an Airbus A380, but even if you could make an engine that powerful, you'd have a single point of failure and if a fan blade does go pop you've probably destroyed the whole aeroplane. As Quantas recently found out, having four engines a long way from the fuselage makes life a lot less stressful when an engine goes bang.

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