Outboard motors

Last updated: July 4, 2008.

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's usually only one cylinder, 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 fires its jet 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.