Why differences between propeller of boats and airplane?I got this "A propeller works by converting the rotation of the engine into horizontal thrust similar to a fan. The blades push the air backwards creating acceleration, and create a difference in pressure between the forward and rear surface adding to the movement. 

The above is, of course true for a boat propeller. For an airplane propeller Bernoulli's pincipal is involved. Bernoulli's principle states that the greater the speed of a fluid, the less the lateral pressure. So the propeller uses the same deal as the wing. " 

But my question why don't they design airplane propellers using the same way they design boats propellers? Why do they need take avdvantage of Bernoulli's pincipal in airplane propellers but no in boat propellers?

Best Answer:  Boat propellers work by transfer of momentum. Accelerating a large mass of liquid (water) backwards, 

Air is less dense. So moving air backwards contributes, but creating a low pressure on an airfoil gives more force to move the aircraft forward 

Plus, with water there is the problem of Cavatation 

http://en.wikipedia.org/wiki/Cavitation

Could some design of a propeller be used in both air and water?Propellers in water are smaller in diameter. They also move more slowly. On the other hand, aircraft propellers are larger in diameter, have narrower blades and operate at very high speeds. An aircraft propeller would break apart in water, while a water propeller would produce little to no thrust in air.

Rotational speed can be easily adjusted while moving between environments. However, is there such a thing as a propeller shape that is halfway between the two designs? Or would it simply be extremely inefficient in both environments?

Interesting, it implies water to air and air to water "craft" which is a target to engieers in the present time.

There's two important differences between air and water: Air is compressible, and the densities are about a factor of ~1000 apart - 1 kg/m³ vs 1 t/m³!

For most concerns where you use propellers, compression plays no role because the pressure diferences are very low. The densities, however play a large role.

The thrust can be described as F=m˙∗Δv, with m˙ beeing the mass flow - kg&/s or such - and Δvthe difference in velocity a volumeelement of fluid is accelerated.So, to achieve a similiar thrust, the same propeller would have to move 1000 times more air than water by volume. Hence the often larger and faster spinning propellers for planes.

On the other hand, in a heavier medium each wing of the propeller is subject to stronger torque (all else beeing equal):

Q=ρVα2D3fq(ND/Vα)

(Source) ρ is density, Va rate of advance (how much the propeller moves forward per revolution), D is diameter and N number of revolutions.Without going into the math it can probably be shown that in a heavier medium the propeller will experience somewhat more torque for the same thrust - I'm to lazy to try now. The propeller will be built with more robust (and possibly heavier) material than would be the case id it's an only air propeller.

That said, I believe that a propeller for both media is entirely possible, though challenging.

However, a propeller for a both media will need a drivetrain that can accomodate speed roughly a factor 1000 apart (that's not trivial).

One other reason why don't see a propeller for both media is that there is no vehicle that could make use of one.

Who said that the propellers in water are small? If you have a look at a nuclear submarine, you're likely to see that the height of the single propeller is almost the height of the submarine. The size of propellers doesn't matter at all. It's completely based on how denser the fluid is (enough to break the propeller)..!Water propellers have a lot of challenges to face. A ship is a lot of mass (to be thrusted) compared to aircrafts. Moreover, the blades also experience cavitation (a bubble-formation

phenomena) that causes wear of the material. So, the propellers should be strong and also be constructed in such a way that the equilibrium of the ship is maintained.On the other hand, aircraft propellers have no (or less) problems to face when thrusting through air compared to ships. The pressure exerted by the blades is far enough to lift the plane. The issue is different is water because "a whole mass" of water has to be pushed back.You're right about them when the function of both are replaced. The aircraft can't even lift a submarine's propeller. So, there can be no such thing as "inter-propellers" which can function depending on fluids (like the one in X-men). But, there are a lot of similar models like some sea planes which can move at a moderate speed in both air and water since it slides easily. But, no hetero-propellers...

Let's take a table fan with four blades. If its moving in the anti-clockwise direction, then the side along the direction of its motion is curved inwards so that they can push the fluid out. Let's assume this as thrust. Now, imagine how the fan swirls in both fluids. In air, it requires less mass of the fan itself so that it can thrust itself forward. In water, it requires more mass to push the denser fluid back and so thrust front. Hence, the mass & inertia matters here...

Could some design of a propeller be used in both air and water?Propellers in water are smaller in diameter. They also move more slowly. On the other hand, aircraft

propellers are larger in diameter, have narrower blades and operate at very high speeds. An aircraft

propeller would break apart in water, while a water propeller would produce little to no thrust in air.

Rotational speed can be easily adjusted while moving between environments. However, is there such a

thing as a propeller shape that is halfway between the two designs? Or would it simply be extremely

inefficient in both environments?

Interesting, it implies water to air and air to water "craft" which is a target to engieers in the present time.. – Force Apr 14 '13 at 20:32

There's two important differences between air and water: Air is compressible, and the densities are about a

factor of ~1000 apart - 1 kg/m³ vs 1 t/m³!

For most concerns where you use propellers, compression plays no role because the pressure diferences are

very low. The densities, however play a large role.

The thrust can be described as F=m˙∗Δv, with m˙ beeing the mass flow - kg&/s or such - and Δvthe

difference in velocity a volumeelement of fluid is accelerated.

So, to achieve a similiar thrust, the same propeller would have to move 1000 times more air than water by

volume. Hence the often larger and faster spinning propellers for planes.

On the other hand, in a heavier medium each wing of the propeller is subject to stronger torque (all else

beeing equal):

Q=ρV α2D3 f q( NDV α )

(Source) ρ is density, Va rate of advance (how much the propeller moves forward per revolution), D is

diameter and N number of revolutions.

Without going into the math it can probably be shown that in a heavier medium the propeller will

experience somewhat more torque for the same thrust - I'm to lazy to try now. The propeller will be built

with more robust (and possibly heavier) material than would be the case id it's an only air propeller.

That said, I believe that a propeller for both media is entirely possible, though challenging.

However, a propeller for a both media will need a drivetrain that can accomodate speed roughly a factor

1000 apart (that's not trivial).

One other reason why don't see a propeller for both media is that there is no vehicle that could make use of

one.

Who said that the propellers in water are small? If you have a look at a nuclear submarine, you're likely to

see that the height of the single propeller is almost the height of the submarine. The size of propellers

doesn't matter at all. It's completely based on how denser the fluid is (enough to break the propeller)..!

Water propellers have a lot of challenges to face. A ship is a lot of mass (to be thrusted) compared to

aircrafts. Moreover, the blades also experience cavitation (a bubble-formation phenomena) that causes

wear of the material. So, the propellers should be strong and also be constructed in such a way that the

equilibrium of the ship is maintained.

On the other hand, aircraft propellers have no (or less) problems to face when thrusting through air

compared to ships. The pressure exerted by the blades is far enough to lift the plane. The issue is different

is water because "a whole mass" of water has to be pushed back.

You're right about them when the function of both are replaced. The aircraft can't even lift a submarine's

propeller. So, there can be no such thing as "inter-propellers" which can function depending on fluids (like

the one in X-men). But, there are a lot of similar models like some sea planes which can move at a

moderate speed in both air and water since it slides easily. But, no hetero-propellers...

Let's take a table fan with four blades. If its moving in the anti-clockwise direction, then the side along the

direction of its motion is curved inwards so that they can push the fluid out. Let's assume this as thrust.

Now, imagine how the fan swirls in both fluids. In air, it requires less mass of the fan itself so that it can

thrust itself forward. In water, it requires more mass to push the denser fluid back and so thrust front.

Hence, the mass & inertia matters here...

Differences between Propeller and Jet AircraftDave Jackson   Other

Consider a jet and a propeller driven airplane side by side on the tarmac. The engines will of course

look very different. The jets wings will be swept back much farther and it will look sleeker and faster.

But most key differences will not be apparent.

Airplanes driven by propellers are ideal for applications where efficiency and cost outweigh speed

requirements. Small planes, regional aircraft and cargo planes are good examples. Don’t be mistaken

into believing propeller aircraft are slow and aerodynamically crude machines. On the contrary, some

are very high tech, pressurized and fast.

Propellers provide an efficient means to pull an airplane through the air. Unlike a screw type ship

propeller, these are basically spinning airfoils. They are fuel efficient and inexpensive. Propeller

equipped aircraft can generally fly slower and can use shorter runways. Gravel runways pose little

danger to the engines. Noise can be a disadvantage, however, and supersonic flight is quite out of the

question. Altitude is limited by the propellers need for thicker atmospheric air to “bite into”. Piston

engines will never be as safe and reliable as jet engines. Turboprop’s do away with the piston engine

by replacing it with a gas turbine. Although expensive, these are far safer and more powerful.

Some aircraft require much more power or speed than a propeller could ever provide. Jet engines

power large aircraft, fighters and anything else that needs to go fast or high at the expense of cost.

The largest GE jet engine produces 128,000 pounds of thrust. Unfortunately, fuel consumption is

sometimes measured in gallons/second – a truly horrific expense. Jets, being faster, require the

aerodynamics of a swept wing. The wing, in turn, requires a faster landing speed than propeller

aircraft. Jet engines are also incredibly expensive. The same Roll Royce engine costs $21 million.

Still, if you need to power a 747 or an F-22, there’s really no other way.

Jet engines provide enormous “kick in the pants” thrust by combusting jet fuel under very high

pressure and temperature. The turbine visible on the front of a jet engine is the first stage of the

compression process. Vast amounts of air (and anything else in the way) are ingested into the engine

and hurled back into the next compressor section. The constantly combusting, super heated gas

exiting the combustion chamber is forced through the rear turbofans at high speed. This movement

turns the rear sets of turbines to perpetuate the process. While much of the engines thrust is achieved

by the exiting gas, even more is derived from the spinning turbo fans. The thrust of exiting gas is

greatly expanded when afterburner sections are used on military aircraft.

As complex and precise as they are, jet engines have few moving parts. A jet engine can run for

thousands of trouble free hours, making them much safer than piston driven propeller aircraft.

Both types are evolving into what we need them for. Smaller, lighter, cheaper jet engines continue to

be developed. Propellers and piston engines are getting quieter and even more efficient. Regardless,

some facts remain written in stone. We will never see a propeller driven aircraft fly as fast or fly as

high as a jet. Dirt strip bush planes will never see a jet engine. But until a better invention comes

along, we will always need both jet and propeller driven aircraft

A B O U T P R O P E L L E R S …

James Watt of Scotland is generally credited with applying the first screw propeller to an engine, an early steam engine, beginning the use of a hydrodynamic screw for propulsion.

Mechanical ship propulsion began with the steam ship. The first successful ship of this type is a matter of debate; candidate inventors of the 18th century include William Symington, the Marquis de Jouffroy, John Fitch and Robert Fulton, however William Symington's ship the Charlotte Dundas is regarded as the world's "first practical steamboat". Paddlewheels as the main motive source became standard on these early vessels. Robert Fulton had tested, and rejected, the screw propeller.

Sketch of hand-cranked vertical and horizontal screws used in Bushnell's Turtle, 1775.

The screw (as opposed to paddlewheels) was introduced in the latter half of the 18th century. David Bushnell's invention of the submarine (Turtle) in 1775 used hand-powered screws for vertical and horizontal propulsion. Josef Ressel designed and patented a screw propeller in 1827. Francis Pettit Smith tested a similar one in 1836. In 1839, John Ericsson introduced the screw propeller design onto a ship which then sailed over the Atlantic Ocean in 40 days. Mixed paddle and propeller designs were still being used at this time (vide the 1858 SS Great Eastern).

In 1848 the British Admiralty held a tug of war contest between a propeller driven ship, Rattler, and a paddle wheel ship, Alecto. Rattler won, towing Alecto astern at 2.8 knots (5 km/h), but it was not until the early 20th century paddle propelled vessels were entirely superseded. The screw propeller replaced the paddles owing to its greater efficiency, compactness, less complex power transmission system, and reduced susceptibility to damage (especially in battle)

V O I T H - S C H N E I D E R P R O P E L L E R

Initial designs owed much to the ordinary screw from which their name derived - early propellers consisted of only two blades and matched in profile the length of a single screw rotation. This design was common, but inventors endlessly experimented with different profiles and greater numbers of blades. The propeller screw design stabilized by the 1880s.

In the early days of steam power for ships, when both paddle wheels and screws were in use, ships were often characterized by their type of propellers, leading to terms like screw steamer or screw sloop.

Propellers are referred to as "lift" devices, while paddles are "drag" devices.

S K E W B A C K P R O P E L L E R

An advanced type of propeller used on German Type 212 submarines is called a skewback propeller. As in the scimitar blades used on some aircraft, the blade tips of a skewback propeller are swept back against the direction of rotation. In addition, the blades are tilted rearward along the longitudinal axis, giving the propeller an overall cup-shaped appearance. This design preserves thrust efficiency while reducing cavitation, and thus makes for a quiet, stealthy design.

Propeller, mechanical device that produces a force, or thrust, along the axis of rotation when rotated in a fluid (gas or liquid). Propellers may operate in either air or water, although a propeller designed for efficient operation in one of these media would

be extremely inefficient in the other. Virtually all ships are equipped with propellers, and until the development of jet propulsion, virtually all aircraft, except gliders, were also propelled in the same way.

The blades of a propeller act as rotating wings (the blades of a propeller are in fact wings or airfoils), and produce force through application of both Bernoulli's principle and Newton's third law, generating a difference in pressure between the forward and rear surfaces of the airfoil-shaped blades.

Propellers of all types are referred to as screws, though those on aircraft are usually referred to as airscrews or the abbreviation "prop" or "props"(plural).

The distance that a propeller or propeller blade will move forwards when the propeller shaft is given one complete rotation, if there is no slippage, is called the geometric pitch; this corresponds to the pitch, or the distance between adjacent threads, of a simple screw. The distance that the propeller actually moves through the air or water in one rotation is called the effective pitch, and the difference between effective and geometric pitch is called slip. In general, an efficient propeller slips little, and the effective pitch, when operating under design conditions, is almost equal to the geometric pitch; the criterion of propeller efficiency is not slip, however, but the ratio of propulsive energy produced to energy consumed in rotating the propeller shaft. Aircraft propellers are often operated at efficiencies approaching 90 per cent, but marine propellers operate at lower efficiencies.

 A propeller's efficiency is determined by:

 

A well-designed propeller typically has an efficiency of around 80% when operating in the best regime. Changes to a propeller's efficiency are produced by a number of factors, notably adjustments to the helix angle (θ), the angle between the resultant relative velocity and the blade rotation direction, and to blade pitch (where θ = Φ + α) . Very small pitch and helix angles give a good performance against resistance but provide little thrust, while larger angles have the opposite effect. The best helix angle is when the blade is acting as wing producing much more lift than drag.

A ship propeller operates in much the same way as the aeroplane propeller. In the ship propeller, however, each blade is very broad (from leading to trailing edge) and very thin. The blades are usually built of copper alloys to resist corrosion. The speed of sound in water is much higher than the speed in air, and because of the high frictional resistance of water, the top speed never approaches the speed of sound. Although efficiencies as high as 77 per cent have been achieved with experimental propellers, most ship propellers operate at efficiencies of about 56 per cent. Clearance is also less of a problem on ship propellers, although the diameter and position of the propeller are limited by the loss in efficiency if the propeller blades come anywhere near the surface of the water. The principal problem of ship-propeller design and operation is cavitation, the formation of a vacuum along parts of the propeller blade, which leads to excessive slip, loss of efficiency, and pitting of the blades. It also causes excessive underwater noise, a serious disadvantage on submarines.

Cavitation can occur if an attempt is made to transmit too much power through the screw. At high rotating speeds or under heavy load (high blade lift coefficient), the pressure on the inlet side of the blade can drop below the vapour pressure of the water, resulting in the formation of a pocket of vapour, which can no longer effectively transfer force to the water (stretching the analogy to a screw, you might say the water thread 'strips'). This effect wastes energy, makes the propeller "noisy" as the vapour bubbles collapse, and most seriously, erodes the screw's surface due to localized shock waves against the blade surface. Cavitation can, however, be used as an advantage in

design of very high performance propellers, in form of the supercavitating propeller.  A similar, but quite separate issue is ventilation, which occurs when a propeller operating near the surface draws air into the blades, causing a similar loss of power and shaft vibration, but without the related potential blade surface damage caused by cavitation. Both effects can be mitigated by increasing the submerged depth of the propeller: cavitation is reduced because the hydrostatic pressure increases the margin to the vapor pressure, and ventilation because it is further from surface waves and other air pockets that might be drawn into the slipstream.

Cavitation damage evident on the propeller of a personal watercraft.

 

K O R T K N O Z Z L E S

The Kort nozzle is a shrouded, ducted propeller assembly for marine propulsion. The hydrodynamic design of the shroud, which is shaped like a foil, offers advantages for certain conditions over bare propellers.

Kort nozzles or ducted propellers can be significantly more efficient than unducted propellers at low speeds,producing greater thrust in a smaller package. For the Bollard Pull it may produce as much as 50% greater thrust per unit power than a propeller without a duct.

Tugboats are the most common application for Kort nozzles as highly loaded propellers on slow moving vessels benefit the most.

The additional shrouding adds drag, however, and Kort nozzles lose their advantage over propellers at about ten knots (18,52 km/h).

Kort nozzles may be fixed, with directional control coming from a rudder set in the water flow, or pivoting, where their flow controls the vessel's steering.

Luisa Stipa and later Ludwig Kort (1934) demonstrated that an increase in propulsive efficiency could be achieved by surrounding the propeller with a foil shaped shroud in the case of heavily loaded propellers. A "Kort Nozzle" is referred to as an accelerating nozzle.

T Y P E M

A separate shaft carries the oil distribution box, and additional intermediate shafts can be arranged between the propeller shaft and the OD box shaft and is optimised for all types of installation, from no ice to highest ice class, throughout the speed range. Underwater replacement of blades as well as feathering design are optional features. The propeller is delivered with a low noise hydraulic power pack and remote control system.

 

Propellersby Chris Woodford. Last updated: August 14, 2015.

If you want to move forward, you need to push backward; that fundamental law of physics

was first described in the 18th century by Sir Isaac Newton and still holds true today.

Newton's third law of motion (sometimes called "action and reaction") is not always obvious,

but it's the essence of anything that moves us through the world. When you're walking down

the street, your feet push back against the sidewalk to move you forward. In a car, it's

the wheels that do something similar as their tires kick back against the road. But what

about shipsand planes powered by propellers? They too use Newton's third law, because a

propeller pulls or pushes you forward by hurling a mass of air or water behind you. How

exactly does it work? Why is it such a funny shape? Let's take a closer look!

Photo: Most propellers have two, three, or four blades; this one on a US Navy E-2C Hawkeye has eight.

They're made of tough compositematerials mounted on a single-piece steel hub. Photo by Daniel J. McLain

courtesy of US Navy.

How does a propeller work?

Propellers, often shortened to "props," are sometimes called screws—and it's easy to see

why. To push a screw into the wall, you apply a clockwise turning force to the head with your

screwdriver. The spiral groove (sometimes called a helical thread) on the screw's surface

converts the turning force into a pushing force that drives the screw into the wall and holds it

there. But suppose, for a moment, that you wanted to keep on going...

If you were a beetle and you wanted to move through an infinitely long wooden wall, you

could use a screw thread on the outside of your body to do it. You wouldn't need a screw

running along the whole length of your body: you could manage with just a little thread on

your head—a kind of screw cap—to bite into the wood in front of you. Now suppose you

were a fly, not a beetle, and you wanted to go through air rather than wood. There's no

reason why you couldn't use a screw thread in exactly the same way to pull you through the

sky. In effect, you'd be a fly with a propeller—and that's pretty much what the

first airplaneswere. Planes took to the sky when the Wright brothers figured out how to

combine engine-powered propellers and wings so they could go forward and upward at the

same time.

A propeller is a machine that moves you forward through a fluid (a liquid or gas) when you

turn it. Though it works the same way as a screw, it looks a bit different: usually it has two,

three, or four twisted blades (sometimes more) poking out at angles from a central hub spun

around by an engine or motor. The twists and the angles are really important.

Photo: A propeller is like a cut-off screw and works much the same way.

Why a propeller has angled and twisted blades

Propeller blades are fixed to their hub at an angle, just as the thread on a screw makes an

angle to the shaft. This angle is called the pitch of a propeller and it determines how quickly

it moves you forward when you turn it. A propeller with a steep pitch moves you further

forward with each turn than one with a shallow pitch, just as screws with steep pitches bite

into wood faster than ones with shallow pitches. Like gears, screws are examples

of machines—devices that multiply and transform forces. A propeller (or screw) with a steep

pitch is like a gear on a bicycle that helps you go faster: one turn of the screw moves you

forward more than a propeller with a shallow pitch would do, much like one turn of the pedals

does when your bicycle is in high gear and you want to go fast.

However, it's slightly more complex than that because propeller blades are twisted as well as

angled: in other words, the pitch of a propeller blade changes along its length. It's steepest

at the hub (in the center) and shallowest at the tip (outer edge). Here's why. Look closely at

an airplane propeller and you'll see it resembles an airfoil (aerofoil), a wing that has a

curved top and flat bottom. An airfoil wing produces lift mainly by accelerating air downward

and it works most efficiently when it's tilted slightly backward to make what's called an angle

of attack with the horizontal. (Read more about this in our main article on airplanes.) Now

suppose you take two airfoil wings, mount them either side of a wheel and spin it around.

Turn fast enough, with the wings at just the right angle, and instead of generating lift you'll

produce a screwing effect and a backward force that pushes you forward. This is effectively

how a propeller works. To make it really efficient, the angle of attack needs to be different at

different points along the blade—greater near the hub and shallower toward the edges—and

that's why propeller blades are twisted.

Photo: The blades of a propeller are shaped like airfoil wings, make an angle to the hub, and are twisted so

they work with maximum efficiency right along their length. Photo by Eduardo Zaragoza courtesy of US

Navy.

Variable pitch

And there's a further complication! Simple propellers on small aircraft have their blades fixed

at a certain angle to the hub, which usually never changes (it can be altered by tinkering with

the plane when it's on the ground, though not during flight). But the optimum pitch of a

propeller varies according to how fast the plane is going, so fixed-pitch propellers are really

only effective when a plane flies at the same speed all the time.

Bigger and more sophisticated planes have variable-pitch propellers(ones whose pitch can

be altered by the pilot). Some propellers have automatic mechanisms so they adjust their

own pitch to match the plane's flying speed. Constant-speed propellers are a variation on

this idea. They're designed so they change pitch automatically, allowing the engine always

to turn over at the same (constant) speed. Planes with variable-pitch propellers (including

World-War fighter planes) have another useful feature: the ability to feather the propellers if

an engine fails. Feathering means turning the propeller blades so they're edge on, making a

very shallow angle to the oncoming air, minimizing air resistance and allowing the plane

either to keep on flying on its remaining engines or glide to a crash landing. On some planes,

the pitch of the blades can be reversed so a propeller makes a forward draft of air instead of

one moving backward—handy for extra braking (especially if the main brakes on the wheels

suddenly fail).

Photo: Bigger planes can change the pitch of their propeller blades during flight using gear mechanisms

like this. This is one of the four propeller hubs from a large C-130H Hercules plane undergoing

maintenance on the ground. Photo by Robert Barney courtesy of US Air Force.

Why airplane and ship propellers work differently

Airplane propellers (sometimes referred to as "airscrews," especially historically and in

Britain) have thick and narrow blades that turn at high speed, whereas ship propellers have

thinner, broader blades that spin more slowly. Although the basic theory is the same, plane

and ship propellers are optimized for very different speeds in very different fluids—faster in

air, slower in water—and a propeller that works well in one isn't necessarily going to work as

well (or at all) in the other.

Chart: You might think ship propellers are always bigger than plane propellers, but that's not really true, as

this chart shows. I've picked five examples of marine propellers (dark blue) and five aircraft propellers (light

blue) for comparison. The smallest real propellers you're likely to find are the ones on outboard motors; the

biggest are the rotors on large aircraft like the Bell Boeing Osprey. Perhaps surprisingly, even giant ships

don't have propellers quite as big as the ones on the Osprey. As a general rule, however, the bigger the

ship or plane, the bigger the propeller (or propellers) it needs.

It's easy to see why there's a difference if we go back to Newton's third law. The simplest

way to think of a propeller is as a device that moves a vehicle forward by pushing air or

water backward. The force on the backward-moving fluid is equal to the force on the

forward-moving vehicle. Now force is also the rate at which something'smomentum changes,

so we can also see a propeller as a device that gives a ship or a plane forward momentum

by giving air or water an equal amount of backward momentum. Sea water is about 1000

times more dense than air (at sea level), so you need to move much more air than water to

produce a similar change in momentum.

That's one reason why airplane propellers turn much faster than ship propellers. Another

reason is that airplanes generally need to go fast to fly (lift produced by the movement of fast

air over the wings is what balances the force of gravity and holds them in the sky), whereas

ships don't: buoyancy lets them float whether they move or not. While planes travel entirely

through air, remember that ships operate at the tricky interface between the oceans and the

atmosphere where waves make life complicated; submarines, which operate mostly

underwater, have an easier time in calmer water. Ships have powerful diesel engines that

rotate at high speed, so their propellers could easily turn as fast as airplane propellers if that

were what we wanted. In practice, propellers work most efficiently in water at slower speeds,

so a ship has a gearbox that transforms power from the fast-turning engine down to much

lower speeds in the propeller.

Propeller materials

Once laboriously carved from wood, propellers are now more likely to be made from more

predictable materials. Airplane propellers are typically made from lightweightaluminum or

magnesium alloys, hollow steel, wooden laminates or composites. Ship propellers have to

withstand the corrosive effects of saltwater, so they're typically made from copper alloys

such as brass. They range in diameter from about 15cm (6in) on smaller outboard motors to

as much as 9m (30ft) on the world's biggest container ships.

Ship propellers are also designed to minimize a problem called cavitation, which happens

when a propeller working under heavy load (turning too quickly, for example, or operating

too near the surface) creates a region of low pressure. Bubbles of water vapor form

suddenly and then burst next to the propeller blades, blasting little pits into the surface and

wearing it away.

Photo: Ship propellers are made from alloys such as brass, but don't stay this color for long! This new

propeller was fitted to the aircraft carrier USS George Washington in 2005. It's 6.7m (22ft) in diameter and

weighs about 30 tonnes (33 tons). Photo by Glen M. Dennis courtesy of US Navy.

Who invented propellers?

Here's a quick summary of a few key moments in propeller history:

3rd century BC: The idea of using screws to move things dates back to Greek

scientist Archimedes, who figured out how to enclose a long spiral screw inside a

cylinder so it could lift water. Archimedes screws, as these are known, are still widely

used in factories today for moving things like powders and pellets. They're also a key

feature of agricultural machines such as combine harvesters.

16th century CE: Leonardo da Vinci (1452–1519) sketched an upward-facing screw

propeller on his design for ahelicopter, which he never built.

1796: American inventor John Fitch made the first basic propeller, shaped like a

screw, for a steamboat.

1836: Englishman Francis Petit-Smith and Swedish-American John Ericsson

independently developed modern-style propellers with blades for ships.

1903: The brothers Wilbur and Orville Wright used twisted propellers shaped like

airfoils to make the first powered flight, ushering in the modern age of air travel.

 

Photo: Developing effective propellers was a major part of the Wright Brothers' success in taking to the air

in 1903. By 1908, their plane was advanced enough to offer to the US military for use in war. Left: Here's

the Wright Flyer pictured at a military test that fall. Catastrophically, one of the propellers split during flight,

causing a crash that injured Orville seriously and killed his passenger. Right: Here's a closeup of one of the

propellers and the mechanism that powered it. Note how the propeller twists along its length. You can also

see how it's driven from the engine at the center by a chain drive similar to that used on a bicycle. No

wonder, really: the Wright brothers were originally bicycle makers! Photo by courtesy of Great Images in

NASA.