byChris Woodford.Last updated: June 7, 2023.
Until engines were invented, the only way to power asmallboat was with oars or sails. Calm and elegant it might havebeen, but it took ages to get anywhere quickly—and you had to relyon there being wind or muscle power available.Outboard motors havechanged all that. Invented in the early years of the 20th century,outboards brought the same freedom to small boats that gasoline enginesbrought to cars. Let's take a closer look at these handy machines andfind out how they work!
Photo: Outboard motors are perfect for powering a RIB (rigid-hull inflatable boat) like this.Unlike a car engine, which is often at the front, outboards are always at the back (just below the flag on this picture). That's because they need to createa backward-pushing draft of water to push a boat forward (an example ofNewton's third law of motion). Photo by Maci Sternod courtesy of US Navy andDVIDS.
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Photo: A typical outboard motor on the back of a scuba diver's RIB (rigid inflatable boat). Thepropeller at the bottom of the motor "screws" through the water to push you along (that'swhy propellers are sometimes called screws).
If you've read our article oncar engines, you'll know that theyproduce motion by burning gasoline with oxygen in metal cylinders.The cylinders have sliding pistons that push acrank around and thecrank drives a shaft that (eventually) powers thewheels. Much thesame happens in an outboard motor. The main difference is that there are usually fewer cylinders, operating in either a two-stage orfour-stage cycle. Instead of driving agearbox, the motor powers apropeller. To steer a boat with an outboard motor, you simply tiltthe whole motor casing so the propeller pushes the water away from it at an angle.(Some outboards you can tilt by hand; others are steered by turninga steering wheel that tilts the motor usinghydraulic cables.)You can go faster by opening up the throttle so the outboard burnsmore fuel and turns over more quickly.
Open up an outboard and this—hugely simplified—is what you'll find inside:
The very simplified illustration up above is designed to show you the basic operating principle of an outboard motor;real motors are somewhat more complex than this! Here's a very clear cutaway illustration prepared by Suzuki Motor Corporation fora patent application they were granted in 1999 for a new design (US Patent #5,980,341: Outboard Motor, courtesy of US Patent and Trademark Office). I've colored it and greatly simplified the numbering so you can make sense of it more easily; if you want to know all the details, check out the patent, where you'll find more drawings of the same engine. Here a few of the parts that are worth noting:
Here's the same motor pictured from above and using the same colors to show the same parts. Now visible are the cams (green) opening and closing the cylinder valves (dark blue), the electrical equipment box (purple), and the fuel injection system (orange). In the middle, you can see a cylinder (light blue), piston/crank (yellow), and crankshaft (red). Artwork prepared by Suzuki Motor Corporation courtesy of US Patent and Trademark Office.
Here's what a real outboard looks like inside:
Photo by Gary Ward courtesy ofUS Navy.
FrenchmanGustave Trouvé designed thefirst electric-powered outboard motor around 1870 and made his first voyage in it on May 26, 1881.
Illustration: Gustave Trouvé's experimental electric outboard motor. Notice the batteries on the right, the motor on the left, and the long strings of wire between them. Artwork courtesy ofWikimedia Commons
It wasn't the most practical invention at a time when batteries were huge and heavy. You can see how low the boat is sitting in the water, which might be a bit of artistic flair—or a reflection of how perilious this pioneering voyage really was! And what about "mixing" electricity and water with all those wires strung around you? Gasoline-powered outboards followed about 20 years later, developed by Ole Evinrude of Milwaukee.He patented what he called a"marine propulsion mechanism," with a very obvious resemblance to the modern outboard, in 1911,and developed numerous other innovations, including amethod of water cooling outboards in 1928.
Artwork: Ole Evinrude's original motor looks much like a modern one. From top to bottom, the key parts are the flywheel (blue), fuel tank (orange), cylinder (red) and piston (green), drive shaft (yellow), and propeller (purple). Artwork fromUS Patent 1001260A: Marine propulsion mechanism by Ole Evinrude courtesy of US Patent and Trademark Office.
Other pioneers included Swedish brothers Carl and Oscar Hult, who studied and improved on Evinrude's designs(and gained Swiss patentCH63928A: Propulsion device for small boats in 1912).
Artwork: "Summer vacations that never end" was the promise with which Ole Evinrude advertised his outboards in LIFE magazine in 1914. According to the copy, an Evinrude outboard was the perfect way to convert your basic rowboat into an eight-mile-an-hour motorboat that anyone could use: it didn't need a rudder, so you could steer "without the exertion of strength." Artwork courtesy ofUS Library of Congress
Chris Woodford is the author and editor of dozens of science and technology books for adults and children, including DK's worldwide bestsellingCool Stuff series andAtoms Under the Floorboards, which won the American Institute of Physics Science Writing award in 2016. You can hire him to write books, articles, scripts, corporate copy, and more via his websitechriswoodford.com.
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Woodford, Chris. (2008/2023) Outboard motors. Retrieved from https://www.explainthatstuff.com/outboardmotors.html. [Accessed (Insert date here)]
@misc{woodford_outboard, author = "Woodford, Chris", title = "Outboard motors", publisher = "Explain that Stuff", year = "2008", url = "https://www.explainthatstuff.com/outboardmotors.html", urldate = "2023-06-07" }
