Why do Aeroplanes have the propellers at the front and boats have them at the back?

Hi alanjbrown,

Over the years, a number of planes were developed with propellars that "pushed" from behind the wing or fuselage, but the general rule is that the propellar goes in front.  Efficiency dictates that the flow of air to the propellar encounter the least restriction.  

Boats propellars are another matter.  They operate at significantly lower RPMs and push against water, not air.  Water is much more dense than air so the action of a propellar in the water is much more efficient than a propellar through air.  Plus, the shape of the hull can actually "funnel" water to the propellar(s).  Another consideration is that a stern propellar is much less subject to accidental damage than a bow mounted one.



Kent

......they do?   That explains why my boat always goes backwards.

I have been flying this last week 4 times on McDonel Douglas (may be misspelled!) and they all had the propeller in the rear, and most of the mid range airplanes I saw had it on the rear...

Hi Sergio_Hdez,
Yeah, there were also some old water plane designs with double-engine gondolae. A classic one is the Do-X:
http://www.airliners.net/search/photo.search?aircraft_genericsearch=Dornier%20Do-X&distinct_entry=true


Cheers,

Stefan

i think because areoplanes are designed to be a much higher speed so they need the power at the fron but boats have them at the back because water is able to move them forward so the propeller at the back of the boat is an added bonus to help it move quicker

i did that with some RAF officers so should be right

I always though that this was a stability issue. If you 'pull' something through a medium, it should be more controllable than pushing it. With the propeller at the front, if you adjust the flaps you are only moving the point of stability, the plane will naturally find its own new stable angle of attack. With a propeller at the rear, flap adjustment would be much more complex.

Its a bit like the difference between driving an articulated lorry backwards as opposed to forwards.

Boats are actually the same because the equivalent of the 'flaps' is the rudder which is still behind the propeller.

Paul

On airplanes the propellers and engines are usually on the front, as that's the easiest way to get lots of cooling air to the engines.

A few planes, like the huge old B-36, had "pusher" props at the back of the wing, but these planes also had to have huge power-robbing fans inside the wing to push air thru the engine's cooling fins.  And the engine cowlings on the back of the wing cause lots of drag, as that's usually the thinnest part of the wing, which doesnt match up well to a large engine cowling, which is the "wrong way" for best streamlining.  Also the air coming off the wing will interfere with the propellers, giving the plane a very noisy "BLLLLAA" sound, which results in a lot of unnecessary vibration and wear.

On a boat it's best to have the propeller near the engine.  Boat engines are very heavy, and you don't want the weight in the front, as that keeps the boat from assuming the right stance in the water at high speed, plus it can result in the boat nosing down too much when it hits a big wave trough.

I’ll say a little bit more because I’m kinda into airplanes.

The air that a propeller pushes back is accelerated somewhat and because the prop is rotating, it imparts a slight rotation to the air. If you saw a diagram of the airflow off a prop, the arrows describing the flow would try to make it look accelerated and have a corkscrew shape. With the prop on the front, you have this fast corkscrewing air trying to flow over the fuselage increasing the drag of the fuselage.

With the pusher design, the fast corkscrewing air is just blowing back & not affecting the drag of the airframe.

There's a nice pair of pictures of the B36 Peacemaker bomber at http://www.stinsonflyer.com/consolac/b36-3.jpg and http://www.stinsonflyer.com/consolac/b36-4s.jpg

Any pictures of boats with the prop at the front?

A Jet engine is just a series of about 1,000 small propellers all working together.  Many of those sit right on the wings or inside the tail fusalodge.  

In short, they do have propellers in the backs of planes.

The turbine blades inside a jet engine do not propel the airplane - they merely act to compress and condition the incoming air and to provide ancillary power (e.g., electrical); the thrust is generated by the expansion of the burning gases. Therefore they should not be called propellers.

>The turbine blades inside a jet engine do not propel the airplane

That was true of older jet engines, but alost all nowadays are "high bypass" turbines, where the front compressor section has a larger outer section that just pulls air thru around the outsides of the engine.  

So we *are* partially back to propellers on the front.

Couldn't this be a weight distribution problem? One would guess it was easier to manage the heavy engine if it were situated in front, balanced by the weight of the fuselage and tailfin/stabilizer when the plane was on the ground and during take-off (critical moment), especially in early, rather lightweight constructions, where the engine must have been a big part of the overall mass of the plane. Where the engine sits, the propeller must be close, as long propellor shafts sounds like a bad idea inside a smallish craft. Then tradition prevailed...
/RID

The aircraft that have engines on the back have alot of the fuselage extending forward of the wing and aircraft that have engines on the front have alot of the fuselage extending aft of the wing. As long as the balance point of all the weight is near the center of lift of the wing ( actually slightly forward) you are OK.

So weight distribution is critical but you just design the dimensions for where you intend to place the engine.

It was done both ways early on too. The Wright flyer was a rear mounted engine and those early WW1 airplanes were front engines.

The WW2 P-39 had an engine behind the pilot and a drive shaft that went forward under his seat and between his legs to a propeller on the nose. This kept much of the aircraft's mass near the center so it could maneuver quicker.

Rear mounted propeller would be more efficient, but pieces of a large propeller would be left on the runway during take off rotation.
The center of gravity is not a big problem since the wing can be moved back. Low speed pitch of the longer nose can be controlled with canards. Look at the successful Rutan designs.

front-propelled boats would have to fight against the water flow caused by the props. THis is also a factor in plane design, but faster flow past the wings increases lift, so it's not all negative. Boats don't need to trade velocity for lift, as planes do (e.g. by taking off and landing into the wind.).

Also the lightness of air alows it do be pushed aside with less effort than water. An increase in front-on velocity results in a very small increase in lateral resistance. Wind resistance is proportional to velocity-squared, but water resistance is closer to velocity-cubed because of water's mass and its resistance to lateral translation -- to being "pushed aside".

The difference in resistance also means that plane propellors are designed to push air "outward-and-back" compared to water propellors, which push the water more straight backward. This further decreases the headwind effect of the plane propellor, compared to the "headwater" effect of the boat prop.

By the looks of all the posts it makes sense to have the propeller on the front of a plane because it lifts the nose without having to have extra aerofoils.

On a boat the reason for having the motor at the back is certinaly (in performance boats at any rate) that it lifts the nice of the boat allowing water to glide underneath the boat - like half an aerofoil :D


I do love the way computer orientated know everything about everything

whooee ....

super good responses to a very interesting question.

this is the comment  to which I could best relate :

Actually, the reason the prop is on the front is to be a fan to keep the pilot cool. You can tell this because he sweats so much when it quits  ;-)



leojl

Boat props have to be in the water to work, if you put the prop on the front it would bounce in and out of the water as you hit waves, reducing your power and the life of the your engine and propeller.

Actually, there are lots of boats with the props in the front, watch any fishing show.  They put their electrics in the front.  I suppose it's because it is much easier to manuver in close quarters with the moter pulling your front around than pushing from behind and there are no waves to bounce it out of the water.

Paddlewheel riverboats had their "propellers" on the sides.

Boat props in the stern makes sense because the prop should be close to the engine (who really likes a long drive shaft?), placing weight distribution overall towards the stern, not bow.  Imagine a bow heavy boat in rough seas.....

Aeroplanes generally have propellers in front so that the pilot can see it.  If the engine slows or stops, he can see it quickly.  The Cessna Skymaster had front and back props on the fuselage centerline.  If the rear engine quit on final approach, the pilot would not get a visual indication that he had only 1 engine.  Scary.

where did the questionaire go??

i think most ppl miss the one point of 'elasticity' - air is more 'elastic' than water - (that is it could be compressed or expanded more easily than the water) - as a result we can place the props for the plane infront / at the back of the plane .... while the props for the boat is actually placed below the boat (yes & at the back).... elasticity is the reason y we dont have to place the props of the plane above or below (i donno how this would fit!!) the plane - but it would work (a very little) better in clear air flow......

if the props are in front we have less expansion of air infront of the props and more compression at the back (which is where the restriction is)....
if the props are at the back - more expansion infront (this is where the restriction is) and less compression....

now work is done by the props against the air to impart motion / conteract drag.... if we have compression at the back of the props - no prob (relatively) - a little more work will have to be done by the props to work against the compression......
if we have expansion on the other hand.... we will have to do some extra work as before ..... but there will be less air to do work against in the first place !! .... the props will spin faster ..... but will do less work because there would be less air to do work against.....

with the boat,  however - the props are placed in clear water flow - below the boat, the main reason for placing the props at the back is for reasons mentioned before - if at the front, it will bounce out of the water; usually we want the front of the boat to lift out of the water and not dive into it !! - so we place the heavier engine at the back - and angle the props accordingly....

hope this questionaire comes back....
Nav.

The huge B-36 bomber comes to mind.  See an small diagram at:

http://www.cowtown.net/proweb/b36net/

"Aeroplanes generally have propellers in front so that the pilot can see it."

This is definetly not the reason they are in the front.  1. Insturmentation would tell a pilot if there was engine trouble.  2. Any halfway decent pilot would be able to tell there was an engine out by the way the plane was handling.

I think it probably has to do with turbulent or laminar flow of the air over the wings so the designer can get the drag and lift characteristics they want.  Try Googling "aerodynamic X flow" where X is equal to turbulent, laminar, and air" and see what you come up with.

Apologies for not having posted or awarded points earlier, but after the first fifty or so posts, a small problem has arisen. Just who here gets the points?

fixnix and m8rix have both provided me with links to pictures of vehicles going against this norm (with m8rix getting credit for the good explanation as well), but which of the many, good reasons given here should get the points for answering the first part of my question?

I'm going to increase the total points to 200, with 50 each going to m8rix and fixnix (unless someone can ceom up with better examples), and 50 each to the two best answers (or 100 to the obvious best).

Found this thread a bit late in the day. Very interesting as I worked as an aerodynamacist for a number of years and I don't have a solid answer.

A few observations:

Boats' propellers are normally just ahead of the rudder and therefore quick bursts of power provide a very good means of low speed steering.  Low speed steering on aircraft can be by steerable nosewheel or by differential braking.

I afraid gn0's point on the "elasticity" of air is not really valid. Surprisingly at subsonic speeds air acts as a virtually incompressible fluid. As the speed of sound is approached this ceases to be so and the changes in characteristics are often referred to as compressibility effects. Whilst supersonic aircraft propellers have been used normal aircraft propellers are designed to operate at tip speeds sufficiently below the speed of sound to avoid compressiblilty effects.

There have been quite successful designs of both puller and pusher props on aircraft and stability isn't really a problem for the reasons others have stated.

I was always amazed at the design of World War 1 fighter aircraft which had machine guns which were synchronised to fire through the prop without hitting the blades.