Jet Ski® and Sea-Doo®
by Chris Woodford. Last updated: April 23, 2018.
Motorbikes that can ride on water—how cool is that? Jet Skis and Sea-Doos (two popular brand names for what are collectively called Personal Water Craft or PWCs) are among the fastest and most maneuverable boats of all. That's why lifeguards and marines use them. A PWC isn't like a normal boat, powered by an outboard motor and a propeller. Nor is it like a motorbike, where the gasoline engine turns the back wheel. Instead, 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?
Photo: A Sea-Doo Personal Water Craft (PWC) sitting on a trailer waiting to be launched on the waves. Note the motorcycle handlebars and wing mirrors. Note also how much bigger a PWC looks when it's on land. The whole of the lower section (colored black) sits beneath the water.
The science of PWCs
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.
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!
Photo: Science in action: This Sea-Doo is using basic laws of physics (Newton's laws and the conservation of momentum) to propel itself through the water.
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.
People find the idea of action and reaction very confusing. Let's say you're swimming freestyle in the ocean and you pull backwards on the water with your arms. Now there's clearly an action force here (you pull backwards on the water), but if there's an equal and opposite reaction force, why don't these two forces simply cancel out? How come you go anywhere at all? The answer is that the action and the reaction act on different things. The action is you pulling back on the water. The reaction is your body moving through the water. The action is a force acting backwards on the water; the reaction is a force acting forwards on your body. The forces don't cancel out because they act on different things.
Action and reaction explains how a PWC works. The key to a PWC is a small pump with a rotating part called an impeller. When you crank the throttle, the pump sucks in water through a grate underneath the craft and the impeller blasts it out of a hole at the back, so the force of the jet pushing backward (action) drives the whole craft forward (reaction).
Conservation of momentum
Why does the jet need to exit at such high speed? A large PWC can weigh up to about 450kg (1000lbs—about as much as six adults), which is much more than the weight of the water shooting out of it. A law of physics called the conservation of momentum tells us that the momentum (mass × velocity) of the water jet firing backward must be equal to the momentum of the craft (and its passengers) going forward, so to get the PWC moving quickly, the water jet has to exit at immense speed. That's why PWCs need really powerful engines.