‘Puller’ props on airplanes provide high speed air over the wings and control surfaces during take off and landing. This shortens the runway distance and helps maintain control at low air speeds.
Depending on the rudder configuration, it may be difficult to manuever a boat with a ‘puller’ prop. Small planes can rotate around their landing gear through the rudder with careful braking. In a boat anchors are the only brakes.
Only with a poor deckie at the con.:smack:
well, you can always have that. 
With wave action the bow often can be completely out of the water. Obviously you are going to have problems if your propeller is constantly in an out of the water.
A technique used when when docking to push your boat sideways is a short burst in reverse, The discharge from the props essentially pushes the hull. This is called propwalk When the boat is in reverse, you may assume that the physics are similar to a bow mounted screw going forward.
So this is essentially the problem with the bow mounted screw. The discharge acting on the hull. Not exactly the sideways push, although that can’t be ignored,but that you are setting up an adverse current as a result of the prop discharge for the hull to overcome.
Air planes with the props up front use the air discharge for lift , and given the much lower density of air (1 to 800 if I remember correctly), not exactly a significant detriment.
FWIW, I’ve seen a plane with a propeller behind the cockpit. It was at a fly-in of private pilots at SPI. It was a teeny-tiny experimental, probably home-built, craft of some sort. About the size of a Mazda Miata.
This post has me wondering about all the differences between boat and airplane propellers. When fluids are different, the relative scaling between different geometries and speeds and sizes is typically treated via dimensionless numbers, especially the Reynolds number - and the Reynolds number is based entirely on size, speed, and kinematic viscosity. The thing is, IIRC, air and water have pretty similar kinematic viscosities (because the ratio between their densities is about the same as the ratio between their dynamic viscosities). So, at first blush, I’d expect boat and airplane propellers, and also blowers and centrifugal pumps, to be pretty similar. Obviously, though, they aren’t - I wonder what’s wrong with this way of looking at the problem?
Also, there’s been ongoing research for passenger/cargo jets with moving away from turbofans to unducted fans or “prop-fans,” which in some configurations are “pushers.”
Air is a compressible fluid while water isn’t.
Yes, but even little rubber-band balsa aircraft have propellers that look very different from boat propellers. Surely the pressure developed behind that plastic propeller is a tiny fraction of an atmosphere, and the air is behaving practically incompressibly. There must be some other motivation to shaping them differently.
airplane propeller blades are airfoils, and create "lift’ just like the craft’s wings. Boat propellers do neither. their roles are functionally and fundamentally different.
OK, we’re on to something here. How and why are they different? It’s pretty apparent it would be wrong to use an airplane propeller on a boat or vice versa, but why?
The design of an airplane propellor and a boat propeller are so fundamentally different (and complicated) that I would not even attempt to explain it here. If you’re really really really interested, round up the book “Propeller Handbook, The Complete Reference for Choosing, Installing, and Understanding Boat Propellers”, by Dave Gerr. This is a 143 page book containing all sorts of formulas, graphs, discussions, explanations, etc. about boat propeller design, copyright 1989 by International Marine Press.
And this is just about boat propellers. I’ve probably got one about designing airplane props here somewhere, but I don’t feel like going thru all my bookshelves just now.
It is interesting to note that one of the things that enabled the Wright Brothers to sucessfully design and fly their airplanes was the wind tunnel research that they did to allow them to design the first propeller. Modern testing has determined that it had an efficiency within just a few percent of the most modern prop designs. Those guys were smart!
Sort of a hijack, but why don’t ships bypass propellors? Instead of heating water to make steam, and driving a turbine (which turns the prop)-why not shoot the steam out the back? You would be more efficient (no mechanical losses in the turbine and driveshaft).
Has this ever been done?
The water necessary to make all that steam would have to be fresh water, and carried on board. Tons and tons. And to boil all that water would take a LOT of fuel.
I rather expect that the main result would be a lot of steamy bubbles at the stern, and just a bit of forward motion.
There are “jet” boats, usually designed for shallow river use, that use a high speed water jet instead of a prop. These work, but IIRC their efficiency is quite a bit lower than a well designed propeller, at least at the speeds that normal boats travel at. A jet is actually better, but only at very high speeds of around 70 or 80 knots. And even there a super-cavitating prop (think racing hydroplanes) is considerably better.
Also as the steam was jetted out of the nozzle into water it would begin to condense decreasing its volume. A lot of energy would go into heating the water cooled nozzle rather than pushing the ship. In a turbine most of the energy goes into turning the blades. That is less slip, friction, and the heat of the exhaust steam.
Looks like (if you can trust Wikipedia), the dynamic viscosities for water and air are about a factor of 13 different. Also, the velocities are going to be very very different for boats and airplanes, two orders of magnitude or more. Finally, even if the Reynolds numbers for a boat and airplane are similar, that just means the nature of the flow (turbulent versus laminar) is roughly similar. But because air is so much less dense, the plane will have to move much much more air to get the same thrust, compared to the boat, so the propellers will have different objectives. Not surprisingly to me, the one that needs to move more fluid is larger.
Which also leads to jets versus propellors:
In the big picture physics point of view, jets and propellers do the same thing: push air/water backwards, which pushes the craft forwards. Simple conservation of momentum says to get the same thrust you can either push a lot of fluid backwards slowly (like a paddlewheel or large slow propeller) or push a little bit of fluid backwards at high speed (a jet). But simple conservation of energy (mass x velocity squared) says that the high speed fluid takes a lot more energy for the same thrust. Therefore jets are going to be much less energy efficient than propellers. The only complication is that the craft can’t move forward any faster than the air/water is being pushed backwards.
So to get high speeds, you need a jet, but at low speeds a propeller is way more efficient. Most boats don’t go fast enough to need a jet, so they stick with propellers. Airplanes do want to go faster, so they use jets. But passenger/cargo planes, which care about efficiency, almost all now use turbofans rather than pure jets, because they’re much more fuel-efficient.
So why are plane propellors wings but ship propellors not? I can see how an airplane propellor would get additional thrust as the airfoils generated “lift” in the forward direction. So why not have the ship propeller do the same?
Also, there’s the different shape of the two types. Airplane props are much skinnier than ship props. I’ve read that ship props have that long gentle curve on the front edge to cut more gently through the want and prevent cavitation. Otherwise the prop would just thrash the water like a blender with no propulsion. So why aren’t airplane props like this? Is there less cavitation with a compressible fluid? Does the airfoil shape address the issue?
I notice that most cooling fans seem to use a ships propellor shape even though they push air. Why is that?
“cavitation” doesn’t happen in a gas. You can have flow separation which will hurt the efficiency of the propeller. Cavitation in water is to be avoided because it can actually destroy the propeller.
One advantage for forward-mounted airplane props is that the landing gear doesn’t kick up debris into them. A secondary concern, certainly, but not unimportant.