So why do most airplanes have the props on the front but all ships have the props at the rear. Some planes have pusher props, so it can be done. But I don’t know of any ships with the props at the front.
What is the difference between water and air that precludes front-mounted ship props?
I don’t know that a front mount is precluded. I’ve never driven a ship, but I’ve used several types of flat bottomed fishing boats. When fishing (freshwater) it is fairly common to mount a small auxillary “trolling motor” on the front on the boat. The guy fishing in the front seat can operate the motor from his seat, and guide the boat into position. Note this is used while actually fishing, the regular rear mounted motor is used when going from launch to fishing spot and vice versa. I see no reason why a front mount could not power a big ship…but it may be awkward to steer, or inefficient or something.
I would think that, for a large ship, having the propeller mounted fore would greatly increase the turbulence under the hull, and producing vibration and sway. Just a guess, though.
On water craft, rear props are much easier on the drive shaft, bearings, and seals. Since they spend most time literally pushing, the thrust bearings and shaft splines are almost always in a state of compression. Also, they are less susceptible to fouling from aquatic growth, or breaking due to running into floating or partially submerged objects.
On large ships, external maneuvering pods are located almost anywhere, as they are self contained, and not connected (mechanically) to the primary engine(s).
Trolling motors are essentially the same as maneuvering pods, very low thrust, and completely self contained drive systems (motor and prop mounted in the pod).
Water is much more dense than air. IANA fluid dynamics engineer, but I’ll toss out some guesses.
When a propeller is mounted forward on an aircraft, it is acting on undisturbed air. If it is mounted aft, then it is in the turbulent air caused by the airframe. Since weight and cost are important, it makes sense to mount the prop where it will create the most thrust for the least power – in undisturbed air. It seems to me that this also has the advantage of weight distribution. In a Robinson R22 helicopter (engine mounted aft of the cockpit), there is a minimum pilot weight. If the pilot does not meet the minimum weight, ballast must be carried. The heavier an aircraft is, the more it takes to keep it in the air; so ballast costs fuel but does not add to performance. It just keeps things safely balanced. By mounting the engine forward, the cockpit and passenger area are closer to the center of gravity; thus, there is a wider balance envelope.
So why not mount a screw on the front of a ship? Well, there are some ships that have power pods forward. But we’re talking about a traditional aft-mounted screw. Hydrodynamically, I’m going to guess that it’s better to have the bows cut through ‘clean’ water than through turbulent propwash. I think that the movement of the hull through smooth water might be more efficient. There’s also structure to consider. In an airplane the interface between the engine and the prop does not need to be airtight. (Actually, since most engines are air-cooled, they can’t be airtight.) A ship needs to keep the water out of the hull. Mounting the screw forward would result in ram pressure on the shaft seals. There’s also bending moment to consider. As I said, water is dense. The shaft would have to be long enough to allow clearance for the screw to turn. As the ship moves through the water, and moves vertically in it, the shaft is subject to bending moments of many tons. You’d have to redesign the bows to provide support, which would not be an efficient shape to move the ship through the water. By mounting the screws aft, underneath the ship, you have the hull to support them and their shafts.
As I said, these are just guesses. I stand by to be corrected.
I believe some Icebreakers have a prop at the bow.
What about the idea that a front-mounted ship’s propeller is more likely to bang into something?
This was the first thing that occurred to me. With a plane, you can see where the prop is and what you might be about to mince. With a boat, unexpected shallows could result in not only running aground but mangling the screw.
And if you hit something, damage to your prop may be the least of your problems.
Whereas a ship moves through a medium not nearly as clear as the air; allowing the relatively strong hull to push various flotsam & jetsam out of the way makes a lot of sense.
For smaller boats weight distribution of the engine may be better in the rear
Excellent points, both!
The Wright Flyer and other planes from the early years of aviation had Pusher-propeller configuration. An important reason they fell out of favour was pilot safety when crash-landing. The US military had so many pilots being crushed by the engine in otherwise survivable crashes, that they made tractor-propellers mandatory in new military single-engined planes. That was a couple of years before WWI IIRC.
Pusher-props kept getting built of course, like the B-36, which highlights another problem with this engine-configuration. The 6 enormous 28-cylinder Pratt-&-Whitney Wasp Majors was designed with the assumption, that they would be foreward-mounted, with the carburator downstream from the 4 rows of cylinders. This would keep the carburator nice and warm even at 36000 ft. In the B-36 however …
On a ship a smoth bow breaks the water. And the water traveles down the side of the ship. The forefoot on a ship is important. And now with the bulbous bow on modern ships a prop would destroy any advantage.
Also on the back side of the prop bubbles form causing cavitation. Look at the wake of a ship going at full power. the water looks like it is boiling up in the wake. that is not smooth water. ON a ship with a full astern bell the water traveling up the side of the ship is not smooth at all and more will pile up on one side than the other on a single screw ship.
There’s also the issue that some boats (including almost all pleasure craft) are designed to ‘plane off’ meaning that at a certain thrust level a large portion of the hull lifts up and out of the water and rides more on the surface of it rather than pushing it out of the way. You can then back off the throttle and travel at the same speed using significantly less fuel.
This design requires the prop be mounted at the back of the boat.
Turbulence along the bottom is probably the most important reason to put the propeller in the aft. For traditional configurations, this leads to pushing prop. Some Azimuth thrusters are pulling, but even they are normally located in the aft. The only cases I know with props in front are ice-breakers and dynamic positioning drill ships, but they have at least as much power in the aft.
Is there another reason related to the steering of ships? No experience of ships but on canal boats the effect of the rudder in the water flow from the propeller greatly helps with steering. Without the prop pushing water over the rudder you need a much bigger rudder.
Some commercial clam, crab and net boats have an outboard motor mounted in a “well” in the front of the boat. This design keeps the spinning prop far away from the work area, lines and nets. These are mostly slow, shallow water work boats.
In the photo above, you can just barely see the top of the Yamaha outboard up in the bow. At dock, the motor is tilted up out of the water.
The largest source of resistance on a vessel is wave resistance on the hull. This is obvious when you observe the wakes left by a vessel as all the energy they possess was given by the hull of the vessel. bulbous bows help correct wakes by creating destructive wave interference; therefore, smaller wakes
This picture illustrates it well.
A propeller also creates waves/ turbulence and if it were located in the front the turbulence would further increase the resistance of the vessel.
As for the ice breaking vessel with a prop on the front. It was a design that is/was not very good and the prop was used to break the ice. Spoon shaped bows are a much better design.
As for why airplanes don’t put them in back:
I believe that it is due to the viscosity of the fluid in which they travel. Viscosity is a measurement of internal friction between particles, since air is much less dense than salt water then the resistance cause by the prop onto the plane should be negligible.
Also, dynamic positioning vessels have props in front however they are azimuth thrusters that may turn 360 degrees. Where they lose hull efficiency they more than make up for it with maneuverability which is much more important to these vessels as they often don’t need to travel great distances but maneuverability is a huge concern.
ex. Drillships while drilling cannot stray outside a certain radius, however they don’t often take long voyages.
there’s also the docking problem. ships are meant to hit a soft structure to stop them completely. often it’s the bow or the sides.
Is it possible it has something to do with the shape of the craft? A ship is shaped much differently than an airplane. I would think a front prop on a submarine would be a closer comparison.