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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?

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There have been quite popular airliners with body mounted engines: en.wikipedia.org/wiki/Boeing_727 –  mmc Jul 21 '12 at 19:59
    
@mmc: Neglected to think of those ... albeit I would argue fuselage mounted engines on liners may have been a minority. I could be wrong though. –  Everyone Jul 22 '12 at 14:01
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4 Answers

You are looking at and comparing two different mediums: air and water. Air is all around the aircraft. If water were all around the boat then you could put propellers on top of it if you wanted to. I doubt you would have many passengers. The deeper in the water the props are the more "bite" they can get without creating a vortex and becoming "air-locked" for lack of a better word. To answer your question, yes, you could put them there but about all you would do is churn up a bunch of water without really going anywhere.

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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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The single point-of-failure argument carries weight. It also made me think of the power-train involved; moving the propellers to the side would require a connecting device - potential for loss of power there. I would debate the drag argument on the grounds of a streamlined cowl/enclosure. –  Everyone Jul 22 '12 at 8:25
    
as for big aircrafts putting the engines on the wings is more convenient, because one can make modifications like changing fuselage length later more easily. –  Yrogirg Jul 22 '12 at 8:35
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There are boats with the propulsion on the side, such as a side-wheel steamer.

enter image description here

A boat could have the propulsion on the front, but then it might have to back up to dock. But then again, who knows?

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It could have propellers on the side, but that might be a problem in tight spaces.

If the propeller was on top, it would be an air-boat.

enter image description here

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The main reason is mechanical: the size of the the engine, shaft and propeller require them to be aligned. However many modern ships do have auxiliary engines located in 360° oriantable pod and located at different places under the boat hull. They are used to rotate the boat.

https://www.google.com/search?num=10&hl=en&site=imghp&tbm=isch&source=hp&biw=1366&bih=643&q=pod+propeller&oq=pod+propeller&gs_l=img.3..0i24j0i5i24.1161.6141.0.6328.17.15.2.0.0.0.182.1472.6j9.15.0...0.0...1ac.DntMyneXvoU

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Many new ships have only azimuth thrusters, typically 2 at the front and 2 at the back - and no single main rear prop –  Martin Beckett Jul 22 '12 at 19:47
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