How Does a Jet Ski Work®?

Each year, watersports enthusiasts around the country take their personal watercraft (PWC) like Jet Skis out on the water. This type of watercraft is one of the most popular, and for a good reason!

While PWC (commonly referred to as “Jet Skis”) are loved by many, few actually know how they work outside of personal watercraft mechanics. Personal watercraft, like Jet Skis, may appear to be a hybrid between a motorcycle and a boat at first glance, but they don’t fall into either category. They are unique in the way they function and must be maintained.

Whether you’re looking to purchase a Jet Ski or are a technician who works on them, it’s important to know the basics of how they work. Keep reading to learn all about Jet Skis, including what they are, the different parts they are made up of and how they run.

Key Points:
- Jet Skis, also known as personal watercraft (PWC), are popular watercraft that are neither motorcycles nor boats but function uniquely, using an internal impeller for propulsion instead of an external propeller, like traditional boats.

- The term "Jet Ski®" specifically refers to Kawasaki's brand of PWC, while "WaveRunner®" refers to Yamaha's PWC. PWC can be either seated, carrying up to four people, or stand-up models designed for one rider.

- Jet Skis move through the water by drawing it into a propulsion channel and then forcefully ejecting it through a cone-shaped jet nozzle, which offers advantages such as reduced injury risk, the ability to operate in shallow water, and decreased mechanical failure risk due to the lack of a transmission.

- Steering and braking on a Jet Ski are achieved through the redirection of the water stream; steering is controlled by pivoting the jet nozzle, and braking (for models equipped with brakes) is managed by a bucket that slows down the craft by redirecting the water flow.

- Jet Skis use either an open-loop cooling system, which cycles external water through the engine to absorb heat, or a closed-loop system, similar to an automotive cooling system, which uses a heat exchanger to cool the engine coolant. Yamaha WaveRunners feature a "rooster tail" for visibility and safety, which is a stream of water shot into the air during operation.

What Is a Jet Ski?

Before we dive into the specifics of Jet Skis and how they work, we should start with some definitions:

• “Jet Ski” is a Kawasaki-specific branded term used for their line of personal watercraft. While Jet Skis are Kawasaki-specific, the term is widely adopted to describe small watercraft in which the rider sits on top, similar to a motorcycle. Kawasaki sells both seated and stand-up Jet Skis.
• Similarly, WaveRunner is a Yamaha-branded term for their personal watercraft. Yamaha also manufactures stand-up WaveRunners, but for closed-course competition only.
• PWC, or personal watercraft, is a broad term used to describe all small craft that use an impeller vs. a propeller and the rider is on top of the hull, rather than sitting within it.
• Seated Jet Skis have a bench seat for the rider and can carry up to four people. Stand-up Jet Skis are for one rider only, and have a hinged section with the handlebar and other rider controls, allowing the rider to start in the kneeling position, and then stand up once underway.

How Does a Jet Ski Engine Work?

Seating format and size are not the only ways in which a Jet Ski, WaveRunner or other personal watercraft differ from boats. Propulsion through the water is another significant difference. PWC do not use a spinning external propeller to create force—rather, they use an internal impeller.

Jet Ski propeller

Traditional boats create driving force by spinning an external propeller. The propeller can spin forward, in reverse or it can remain still, even while the engine is running (neutral).

Boats need a method of raising and lowering the propeller, as it hangs quite low in relation to the boat hull. Without a method for raising it, a propeller risks damage in shallow water. Personal watercraft do not use an external propeller—rather, they use an internally mounted impeller.

Why do Jet Skis shoot water?

Jet Skis shoot water upwards through a process that is closely tied to the Jet Ski propulsion system. Jet Skis can move forward through the jet propulsion system, which is a powerful pump that draws water from underneath the vehicle and expels it at high speeds through a nozzle at the back. The visible stream of water shooting upwards is ejected from a separate outlet, designed as a safety feature to enhance visibility on the water, signaling the presence of the Jet Ski to nearby vessels.

How does a Jet Ski impeller work?

A Jet Ski generates forward thrust with a unique “corkscrew”-style impeller. It draws a large volume of water into a propulsion channel, and then forces the water out through a size-restricted cone-shaped jet nozzle. This setup has advantages over a traditional propeller:

• Significantly reduced risk of injury due to no external moving parts.
• Ability to operate in shallow water since no parts hang below the hull.
• Reduced risk of mechanical failure due to no transmission for neutral or reverse.

Steering, braking, neutral and reverse

As noted above, Jet Skis do not have a transmission to disengage the impeller from the engine. To achieve idling in place, or reverse, a physical barrier (bucket) redirects the exiting water stream back toward the front of the craft for reverse, or straight down, for neutral.

The image above shows the reverse bucket partially engaged, just starting its travel down to restrict or cover the jet nozzle.

To achieve left and right steering, the jet nozzle pivots from side to side, controlled via the hull-mounted handlebar. Unless the Jet Ski is under power, it cannot steer left or right. Many first-time riders have the scary experience of trying to steer the craft after letting off the throttle, only to continue to float straight ahead!

Do Jet Skis have brakes? Not all Jet Skis have brakes. For models that do have a braking system, the same bucket that controls reverse can also slow down the craft. Even without brakes, Jet Skis slow down very quickly when the rider releases the throttle. So, while it may be frightening to learn a personal watercraft doesn’t have dedicated ‘brakes,’ once underway, riders quickly discover that they can slow down easily and safely.

Jet Ski engine cooling

Modern high-powered engines generate a lot of heat, and Jet Skis are no different. To handle engine cooling, most Jet Skis use an open-loop cooling system, which takes advantage of cooler external ocean or lake water.

This external water cycles through the engine’s water jacket with the goal of absorbing the engine’s heat, and then the heated water is discharged back into the body of water where the craft is operating.

Sea-Doo® personal watercraft use a closed-loop system, similar to an automotive cooling system. These craft use a heat exchanger. The heat exchanger operates on the same principles as an automotive radiator, but instead of using external air to cool the engine coolant, it uses external ocean or lake water to cool the engine coolant.

Closed-loop system advantages:

• External water does not enter the engine, so there is no need to manually flush the craft to avoid corrosion.
• Debris such as plants, sticks and other water-borne material do not enter, which could potentially clog the cooling system.

Closed-loop system disadvantages:

• Heat exchanger setup is not as efficient as open-loop cooling.
• Additional components need maintenance and may possibly break.

Rooster Tail

Yamaha WaveRunners shoot a stream of water into the air during operation, which is known as a “rooster tail.”

Despite the commonly held belief that this stream of water is hot water exiting the cooling system, the rooster tail is primarily a safety measure. Personal watercraft are much smaller than boats, and riding them often involves sudden direction changes. The rooster tail helps other boaters spot the smaller craft, reducing the risk of collision.

This concept is similar to the use of tall flags on dirt bikes and ATVs when ridden in very hilly areas, such as sand dunes. To achieve the rooster tail, some of the water traveling through the propulsion channel exits the back deck of the WaveRunner, sometimes as high as 20 feet!

What Does This Mean for a Career?

Both motorcycle/powersports and marine technicians repair Jet Skis.

If your interests are more in line with the powersports industry, you may want to explore our Motorcycle Technician Training program with the Kawasaki K-Tech or Yamaha YamaPro® courses.^1.1 On the other hand, if you only want to work with watercraft, both large and small, our Marine Technician Specialist training program might be a great option to train for a career.^1.3

Start Your Training at MMI

Both the Motorcycle Technician Training and the Marine Technician Specialist programs start every six weeks, all year long. To learn more about these programs, visit our website and request information to get in touch with one of our Admissions Representatives today.