Why are boat and plane propellers in different places?

Thanks to Andy Wheeler for the answer!

Will - As it turns out, there are a fair few answers to this question in which physics and practicality go hand in hand. Some aircraft have their propellers closer to the front because goods are often loaded on and off at the back. And putting humans near spinning blades isn't always a sensible idea. It's also common for boats to have propellers at the back to avoid them bonking into things. Halc on the forum also points out: ‘A small boat tends to lift its bow out to the water at high speeds. Not a good thing then if the propulsion is there’, which is a very fair point. But there are also good physics reasons for these respective propeller placements as well. And here to explain is the University of Cambridge's, Andy Wheeler.

Andy - The big difference between water and air is that water is very much more dense than air. So if you take a metre cubed of air, it weighs about a kilogram and a metre cubed of water weighs about a ton, so a thousand times more. So that means that propellers for aircraft need to be much bigger. And that's because the thrust you get from a propeller is related to the amount of the mass flow that passes through the propeller and the speed with which the propeller can push that backwards. And so that can affect practically where you could put propellers on an aircraft. You couldn't have a propeller that was too close to the ground that might then hit the ground or towards the back of an aircraft that might affect how the aircraft could land. Because the propeller is also attached to an engine and the engine tends to be heavy. And the distribution of weight on an aircraft is very important. So if you have too much weight towards the back of an aircraft, that will affect how the aircraft can be controlled in flight.

Will - And is there a case of there needing to be the right sort of air flowing through an engine?

Andy - That's right. So quite important to the performance of a propeller is how the flow is entering the propeller if you were to put a propeller behind something. And so the flow coming into the propeller is very distorted. That would affect the prop performance of the propeller. The propeller on the boat, because the water is a thousand times more dense than air, it's much smaller and you tend to mount that at the back of a boat. And I think that's more to do with where you'd want to mount the engine. So for instance, on a small boat, you might want that boat to pitch up a little bit so the weight of the engine can help with that so that the boat can then move more smoothly through the waves. So the viscosity isn't really the main thing that affects these big decisions on where you put the engine and the propeller and how you distribute the weight, for instance, in each case.

Will - I did anticipate this being far more a physics question, but it does seem to be more one of a practical applicational question.

Andy - One thing I'd like to say is that actually there isn't really any physical principle that tells us we can't put an aircraft propeller at the back. I mean, you'll see some aircraft do have propellers at the back or have jet engines at the back of the aeroplane. And as we move towards more futuristic proportion systems, we might start to see more electrically-driven propellers and those can be made much smaller and they can be distributed over the aeroplane. And that gives us a much bigger degree of flexibility in the design of the aeroplane. So we could have small propellers mounted on the wings, for instance, and that can help to blow air over the wing to improve the performance of the wing.