Most, but not all, propeller aircraft have the propeller in front. Then why do all ships have the propeller in the rear?
Aircraft manoeuver by using control surfaces to deflect the passing ambient air. Ships manoeuver (disregarding thrusters) by directing the discharge thrust from the propellor(s). The control surfaces (rudders) must be positioned in close proximity to the propellors themselves, in a manner that still allows the discharge thrust to pass without slowing the vessel.
You could do it and still maneuver using some sort of ducted prop, but I don’t think it’s be as efficient. Water is a different fluid environment than air, so the rules are a bit different. In fact, that was one of the major advances of the Wrights. Many of their competitors were using standard formulae and tables that had been developed for ship propellors. The Wrights did their own wind-tunnel testing and calculations and developed a much different propellor design.
There are some vessels that have proplusion units at both the bow and stern, but those are the exception to the rule.
There is also the efficiency of the bow and hullform to consider (above and beyond the prop efficiency). You wouldn’t want to be dragging chaotically disturbed fluid flows along the entire length of the vessel. Especially if the bow has to take a different shape to mount the prop.
I am specifically ignoring historical reasons in the development from steamship paddle wheels to propellors. Maritime engineers have long shown the willingness and ability to accept new and different ideas when they serve a particular purpose better than older ideas.
Here is a related thread about pusher and puller aircraft.
For boats its also a safety issue. Being that you usually can’t see below the surface its not a good idea to be ‘leading’ with your prop, i.e. if you so much as bump anything you’re going to bend or destroy your prop and be dead in the water.
It was also a design issue. Its much easier for the output shaft of a boat’s engine to be pushing against the bearings of the gearbox/engine rather than having to ‘hold’ it from pulling itself out.
Lets give the Wrights full credit here: first they invented the wind-tunnel, then they used it to test various propellor designs. Nobody before them had actually created a device where they could actually test their designs. Their actual wind-tunnel (or a later revision of it, not quite sure) is on display at the museum in Ohio.
This is one of the 7 critical inventions of the Wrights that made air flight possible. Just about exactly 100 years ago!
Or better yet, let’s give proper credit where it’s due:
(From http://www.hq.nasa.gov)
Thanks for the replies. I hadn’t thought about the rudder placement issue, and the safety issue. Makes sense.
Would the turbulence have a significant effect on hull drag? Do boat/ship hulls achieve laminar flow otherwise? (I’ve gathered that laminar flow is extremely difficult to achieve on an aircraft, but you have significant reduction in drag if you do.)
Turbulence can have a significant effect on drag, but it’s not necessarily a simple relationship. Viscous drag may be much higher in a turbulent boundary layer because the velocity gradient at the surface is much higher. However, a turbulent boundary layer resists separation much better than a laminar BL, which may narrow the wake and reduce form drag. The latter is a very important factor and many aircraft actually trip the boundary layer to turbulent and use vortex generators to energize the boundary layer to try to prevent separation and the potentially catastrophic loss of lift and control it causes.
In general, it’s safe to say that most real-world boats and aircraft will have turbulent flows. Some small low-speed craft may be able to maintain laminar boundary layers, but any ding, chip, or barnacle will trip the BL so most production craft are not designed to rely on laminar flow. There are some experimental designs intended to maintain laminar flow across the entire wing (there was a very strange looking F-16 at Edwards AFB with one laminar flow wing and one normal wing) but these are not typical in production craft.
Regardless of drag, viscosity may be one reason for the difference you see in typical aircraft and boat designs. One advantage of the front prop in aircraft is that the prop is biting undisturbed air. In push-prop designs, the prop is in flow disturbed by the fuselage so you may have considerable off-axis flow components to deal with. For a boat, boundary layers are typically much thinner so it’s much more practical to stick the prop a little down and out into relatively undisturbed flow.
Having the prop direct flow across the rudder obviously isn’t required for a ship to function (sailboats steer just fine) but it certainly helps, especially for low speed navigation. If you’re trying to drive a boat at very low speeds such as docking maneuvers, your helm may be fairly unresponsive if the rudder is moving at boat speed, but if the rudder is in the wash of the prop, the flow across the rudder is boat speed plus the velocity of the flow induced by the prop and the resulting force of the rudder is higher.
You know, I think this was my main problem - I kept thinking the props were behind the hull, when they are actually below the hull, as far as water flow is concerned. I guess that must make a big difference.