Jet Ski® and Sea-Doo®
Last updated: December 12, 2006.
A Personal Water Craft (PWC) during combat training in Malaysia, July 2006.
Picture courtesy of the US Navy.
A motorbike that can ride on water—how cool is that? Jet-Skis and
Sea-Doos (collectively called Personal Water Craft or PWCs)
are among the fastest and most maneuverable water craft. That's why
lifeguards and marines use them. Unlike a motorbike, where the petrol
engine turns the back wheel, a PWC moves along by squirting a
high-powered jet of water behind it. The power of the water squirting
backward pushes the PWC forward. That's the power of science for
you—but how exactly does it work?
Newton's laws
The science behind PWCs was first figured out nearly 350 years ago
by a brilliant Englishman named Isaac Newton (1643-1727). You might not
have thought about PWCs before, but you'll already know about Newton
and his science from party balloons. Everyone's done that trick where
you blow a balloon up till it's almost ready to burst... then release
it so it whizzes round the room. It's always good for a laugh at
Christmas time—but did you know there was solid science behind it? The
science is called Newton's third law of motion. His... what?
Around 1666, Isaac Newton set out his three laws of motion—three
simple rules that explain how things move:
- Things stay still or travel at the same speed unless something (a
force) pushes or pulls them. Pretty obvious really: a ball stays on the
ground until you kick it.
- When a force pushes or pulls an object that's moving, it makes the
object speed up or slow down. The more the force pushes or pulls, the
move the object speeds up or slows down. This is another pretty obvious
one: the harder you kick the ball, the faster it flies through the air.
- When a force pushes or pulls something, another force just as big
pushes or pulls in the opposite direction. This is the most confusing
of Newton's laws. It means that if you kick a ball, the ball kicks you
back!
Action and reaction
Newton's third law is also called "action and reaction" and you
sometimes see it written like this: for every action (or force), there
is always an equal and opposite reaction (a force of the same size
going the opposite way). It sounds counter-intuitive, but it's
perfectly true. Think about it. If you're on a skateboard and you want
to go forward, you kick backward. The backward kick (the
"action") makes you go forward (the equal and opposite "reaction"). If
you're in the sea and you want to swim forward using freestyle (crawl),
you pull backward with your arms. The backward pulling force of your
arms (the "action") makes you go forward (the equal and opposite
"reaction"). Space rocket engines and airplane jet engines also work by
action and reaction. In each case, the force of the hot gas rushing
backward from the engine hurls the rocket or airplane forward through
the air.
How a PWC works
That's also how a PWC works. It has a tiny pump inside it called an
impeller. When you crank the throttle, the impeller sucks in water and
blasts it out of the back of the craft. The force of the jet pushing
backward drives the whole craft forward. Since the PWC weighs so much
more than the water, the water jet has to fire backward at immense
speed. Steering a PWC is as easy as steering a motorbike: you just turn
the handlebars to go one way or the other. But instead of turning a
wheel, as on a motorbike, the handlebars swivel the water jet to one
side or the other and that steers the whole craft to one side.
Further Information
You can read more on these websites:
- Jet Ski®: A registered trademark of Kawasaki Motors Corp., U.S.A.
- Sea-Doo®: A registered trademark of Bombardier Recreational Products Inc.
