Ship propellers - how do they work?
Propellers are the driving force behind most ships and planes around the world. They can drive large commercial ships through the water at 15 knots and commercial aircraft at 550 miles per hour. But how do they actually work? Today we are going to take a look at how ship propellers work, some of their designs, and how they differ from aircraft propellers. We’ll also take a look at an alternative that might make ship propellers obsolete.
Four bladed ship propellers. Image from Metallisation.
Propeller design affects all aspects of a vessel from “handling, riding, comfort, speed, acceleration, engine life, fuel economy and safety. In determining boat performance, propellers are second in importance only to the power available from the engine itself”, according to Killcare Marina. A seemingly simple device can have drastic effects depending on the shape, size, angle, and number of blade tips. First, the basics: do propellers push or pull through the water? The answer is both - similar to how a household fan draws air in the back and blows it out the front, as propellers rotate, they push water backward which creates space in the front water must rush in to fill. It both pulls water in and pushes it out to create forward thrust. Each blade has a pressure differential where the bottom of the blade creates positive pushing pressure and the top of the blade creates negative pulling pressure. Most propellers have three or four blades (although they can have many more) each of which create these forces simultaneously. Pulling water in and pushing it out at a higher speed creates thrust that moves a vessel through the water.
The whole process is more easily visualized with an azimuth thruster pod, which combines the functions of a propeller and rudder in order to provide 360 degree motion. Water is sucked into the front of the thruster and pushed out the back, and the entire unit can be turned to provide thrust in any direction. Thrusters are basically a propeller inside a rudder for greater agility. The downside to this setup is it is not cost effective for ships that spend most of their time cruising at slow speeds on the open ocean, like a container ship. Those ships are built to maximize efficiency and are usually built with one large propeller that is stationary and only provides motion in one direction, which can then be modified by the rudder. For its massive size, this makes for a relatively fuel efficient vessel, but one propeller and a small rudder make for poor maneuverability.
Azimuth thrusters installed on a vessel. Image from openPR.
What are the parts of a propeller?
Each part of a propeller has a name, here is some terminology for the geeks (geeks are awesome!) who really want to know their propellers:
The front of the blade where water is pulled in is called the leading edge, the back of the blade where water leaves the propeller is called trailing edge, and the outermost part where these two meet is called the blade tip. The cup is the small curve on the leading edge that helps the blade to hold water. The blade face is the side facing away from the vessel that creates positive pressure, while the blade back faces the vessel and creates negative pressure. The blade root is the bottom of the blade where it attaches to the outer hub. The inner hub is attached to the shaft that turns the propeller. There are a few other special parts of propellers, and if you’re interested in a more in-depth look at these, check out Killcare Marina’s excellent article that goes into a lot more detail.
So how does the design of the propeller affect its operation?
The easiest way to categorize a propeller is by its size. The diameter of a propeller is the distance between the edges of a circle drawn around the tips of the blades. This can be as small as 1.25 inches for a Traxxas Blast radio controlled boat to as big as 9.6 meters (31.5 feet!) for one of the largest containerships, Emma Maersk. The Emma Maersk propeller also weighs 131.4 tons, or (according to Survival Tech Shop) more than the weight of a small house! The right propeller diameter has many determining factors, including RPM, amount of power provided, required speed of the vessel, and if the propeller will be partially surfaced. Generally, larger propellers are used on slower boats and smaller propellers on faster boats, and more power will usually mean a bigger propeller.
Emma Maersk propeller. Image from gCaptain.
Pitch is similar to the pitch on a screw - it measures how far a propeller would move through a soft solid, according to Killcare Marina. A 16 x 22 propeller would have a 16 inch diameter and move forward 22 inches with a full revolution. Poor manufacturing and damage can affect pitch. There are usually two types of pitch: flat pitch means the blade is flat and at a constant angle from the leading to the trailing edge. Progressive pitch means the blade starts with a lower pitch at the leading edge and increases to the trailing edge. Progressive pitch is used to improve performance in higher horsepower applications or when the propeller may break the water surface. In effect, it acts like another set of gears with more pitch creating higher gearing - pitch has to be properly matched to the application.
Constant pitch and progressive pitch of propeller blades. Image from Killcare Marina.
The number of blades also has an effect on the performance of a boat. One blade would be the most efficient setup, but would create a lot of vibration. Three or four blades are most common as they are the best compromise between efficiency and reduced vibration. Most large vessels and thrusters use four blades, but five blades are also used in some cases. There are many additional factors to consider when selecting a propeller such as blade thickness, angle of attack, and even blade shapes. If you want to learn more, Killcare Marina’s article has lots of information!
What’s the difference between boat and plane propellers?
Ship propellers usually have broad, thin blades that spin relatively slowly. Airplane propellers, or airscrews as they are sometimes known in Britain, use narrow, thick blades that spin very fast. Wider, slower spinning blades work well in water, while thinner, faster spinning blades work better in air. Why is this? According to Explain That Stuff!, the easiest way to think about it is that a propeller moves an object forward through a fluid. Since seawater has approximately 1000 times the density of air, you need to move a lot more air to produce the same amount of thrust that you would in water. Airplanes also travel much faster than ships, and rely on that speed to stay in flight, while ships float on water and move much more slowly. The average speed of a containership is about 14.75 knots, or about 17 mph, while the passenger aircraft you fly on for vacation travels at around 550 mph. These vastly different requirements call for different sizes, shapes, and angles of propellers.
“The propellers of a C-130J Super Hercules military transport aircraft”. “Photographed by Adrian Pingstone, Public Domain, https://commons.wikimedia.org/w/index.php?curid=439643”. Image from Wikipedia.
Propeller materials are also different between the skies and the ocean. Ship propellers have to deal with saltwater, which can be very corrosive to many materials, and commonly use alloys like brass. Airplanes can use all kinds of exotic materials and manufacturing methods like magnesium alloys, hollow blades, and wood composites. Ships also have to deal with cavitation, which is when a propeller working under a heavy load can create vapor bubbles that form and burst next to propeller blades, making little pits and wearing away the surface.
A future without propellers?
According to Explain That Stuff!, Archimedes first came up with the idea of using screws to move water up a cylinder in the 3rd century BC. This method is still used today in combines and factories. Leonardo da Vinci designed a screw-style propeller for a helicopter in the 16th century that was never built. In 1796, John Fitch, an American inventor made the first basic propeller for a steamboat, with the modern propeller design coming not long after in 1836 from Francis Petit-Smith and John Ericsson, who both independently invented the new design. In 1903, twisted propellers would power the Wright Brothers in the first powered flight. The idea of a screw or propeller to move an object is over 2000 years old, with the modern propeller being almost 200 years old.
Is it time for a change? An interesting point was made recently in an article by the Economist: “NO KNOWN SEA-CREATURE uses propellers.” Interesting observation, but what other solutions are there for ships? Most vessels use propellers, and bigger propellers are more efficient, but then you have to worry about packaging that propeller and hitting underwater obstacles. Fins and flippers are evolution’s choice for water propulsion, used by almost every type of fish and mammal in the ocean. Benjamin Pietro Filardo is working on using flexible materials to change the way we move through the water. His company, Pliant Energy Systems has developed a prototype based on a cuttlefish - flexible fins on each side that allow movement underwater, on top of water, and even on mud or ice. The results are already impressive as the initial prototype already generates “around three times as much thrust per unit of energy expended as a typical small boat’s propeller can manage”, according to The Economist. The new follow-up vehicle, called the c-Ray, will be autonomous with initial uses in mine detection, reconnaissance, and patrols. This undulatory propulsion does not create cavitation like a propeller and is much quieter during operation. It has the potential to even be scaled up to full size submarines and other large vessels as a more efficient, quieter method of propulsion that could even provide recharging abilities if moored in place while passing waves move the fins.
Pliant Energy Systems cuttlefish-shaped c-Ray prototype. Image from The Economist.
Propellers are used in some form in everything from small boats to fighter jet engines. While they’re not going anywhere soon, we’re excited to see new technologies that could make vessels more efficient. For now, different materials and designs are the only changes we are likely to see in the near future. New thrusters are especially interesting as they are used on dynamically positioned vessels and OneStep Power tests DP2 and DP3 vessels for fault ride through in the event of a short circuit.
Happy Fun Fact Friday!