Outboard motors
by Chris Woodford. Last updated: May 16, 2011.
Until engines were invented, the only way to power a
boat was with oars or sails. Calm and elegant it might have
been, but it took ages to get anywhere quickly—and you had to rely
on there being wind or muscle power available. Outboard motors have
changed all that. Invented in the early years of the 20th century,
outboards brought the same freedom to small boats that gasoline engines
brought to cars. Let's take a closer look at these handy machines and
find out how they work!
Photo: Left: Outboard motors are perfect for powering a RIB (rigid inflatable boat) like this.
Unlike a car engine, which is often at the front, outboards are always at the back. That's because they need to create
a backward-pushing draft of water to push a boat forward (an example of Newton's third law of motion).
Photo: Right: A typical outboard motor on the back of a scuba diver's RIB (rigid inflatable boat). The propeller at the bottom of the motor "screws" through the water to push you along (that's
why propellers are sometimes called screws).
Using an outboard
If you've read our article on car engines, you'll know that they
produce motion by burning gasoline with oxygen in metal cylinders.
The cylinders have sliding pistons that push a crank around and the
crank drives a shaft that (eventually) powers the wheels. Much the
same happens in an outboard motor. The main difference is that
there are fewer cylinders (usually only one or two), operating in either a two-stage or
four-stage cycle. Instead of driving a gearbox, the motor powers a
propeller. To steer a boat with an outboard motor, you simply tilt
the 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 turning
a steering wheel that tilts the motor using hydraulic cables.)
You can go faster by opening up the throttle so the outboard burns
fuel and turns over more quickly.
How does an outboard motor work?
In theory
Open up an outboard and this—hugely simplified—is what you'll find inside:
- Fuel burns in the cylinder (or cylinders) to make power. There's a fuel tank (not shown) inside the case of
the motor at the top big enough to hold perhaps 23 liters (6 gallons) of gas. The heavier your boat, the faster you drive it, the choppier the water, the more heavily loaded, or the lower in the water it sits, the more fuel you'll burn.
- Powered by the burning and expanding fuel gases, a piston moves back and forth in the cylinder. This is just like the
piston in a car-engine cylinder and often works through the same four-step process (four-stroke cycle), although
some outboards do use a simpler two-stroke cycle.
- The piston rod turns the crankshaft, converting the back-and-forth (reciprocating) motion of
the piston into round-and-round (rotary) motion.
- The crankshaft turns the main driveshaft running down the long spine of the motor.
- A small gearbox at the bottom of the driveshaft converts vertical spinning motion into horizontal spinning motion.
- The propeller powered by horizontally spinning gears powers the boat through the water.
In practice
The very simplified illustration up above is designed to show you the basic operating principle of an outboard motor;
real motors are much more complex than this! What do they look like? Here's a photo of a three-cylinder Yamaha outboard
with the the outer casing removed for maintenance. I've labeled a few of the parts that are visible:
- Three cylinders arranged horizontally. A medium-sized, three-cylinder outboard like this produces something like 40–50 horsepower. It's a fairly hefty machine, weighing in at 86kg (190lb)—almost the exact average weight of an American adult male!
- Sparking plugs (spark plugs) ignite the fuel in the cylinders.
- Anti-ventilation/cavitation plate: Cavitation is what happens when a spinning propeller churns up air or engine exhaust gas in the water. Bubbles form and burst, which, over time, wear away the propeller's surface. The anti-cavitation plate is designed to reduce that problem, but cavitation can still be caused by floating debris disrupting the smooth flow of water around the propeller blades.
- Lower unit: The propeller and gears attach to the base of the motor. They're not attached to this particular motor, which was photographed during maintenance and repair.
- Mounting bracket: Where the motor attaches to the back of the boat and swivels up and down.
- Drive shaft: Not actually visible, but you can imagine it for yourself—a kind of "spinning spine," running straight down through the center of the motor from the cylinders to the propeller.
- Three carburetors: Combine fuel with air to make an explosive mixture—there's one for each cylinder.
- Flywheel: A heavy wheel that builds up momentum as the engine accelerates, helping to maintain a smooth and steady engine speed.
Normally you'd start an outboard motor like this electrically, just as you'd start a car. If that's not possible, you can attach a pull cord to the flywheel and tug it vigorously to "crank" the motor into life. There's a special notch in the flywheel where you attach the cord. (Find out more about flywheels.)
- Top cowling: A removable, protective covering on top of the flywheel.
Photo: Adapted from a photo by David Hoffman courtesy of US Navy with annotations by explainthatstuff.com.
