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 ships and 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!
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 airplanes were. 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.
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.
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).
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’s momentum 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.
Once laboriously carved from wood, propellers are now more likely to be made from more predictable materials. Airplane propellers are typically made from lightweight aluminum 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.
Who invented propellers?
Here’s a quick summary of a few key moments in propeller history: