Howstuffworks "How Personal Watercraft Work"

A: All older models and some newer models of personal watercraft use two-stroke engines, which can vent oil and gasoline into the water with their exhaust. Other motorized boats also use two-stroke engines, but the sheer number of personal watercraft may make their environmental impact greater.

When snowmobile manufacturer Bombardier Inc. introduced the SEA-DOO® in 1968, the idea of a motor-driven consumer watercraft with no visible propeller was revolutionary. The idea of passengers standing on a watercraft instead of sitting in a conventional hull was also pretty out there. In fact, the design turned out to be too original. Bombardier's new product was not successful, and the company discontinued it in 1970.

A few years later, Kawasaki Motors introduced the JET-SKI® watercraft, which became so popular that many people now refer to all personal watercraft as "jet skis." In the mid-1980s, other manufacturers developed their own models, and Bombardier re-introduced its SEA-DOO design.

Since then, demand for personal watercraft has skyrocketed. There were 1.48 million personal watercraft registered with the U.S. Coast Guard in 2004. About 28.7 million people rode personal watercraft in U.S. waters in the same year [Source: USDA].

Personal Watercraft Image Gallery

Photo courtesy Yamaha Motor Corporation, U.S.A.
Yamaha Superjet. See more pictures of personal watercraft.

In this article, we will explore how these craft operate, and we'll examine the safety, environmental and legal concerns related to their use.

What's in a name? You might be more familiar with the terms "jet skis" and "jet skiing" than "personal watercraft." But like Kleenex and Xerox, "JET-SKI" is really a brand name. Technically and legally, "JET-SKI" means only Kawasaki-brand personal watercraft.

The Basic Principle
A personal watercraft moves through the water in the same sort of way a rocket moves through the atmosphere. But instead of using high-pressure gas to generate thrust, the craft uses a jet drive to create a powerful stream of water. In the jet drive, an impeller propels a large amount of water from underneath the craft through a steering nozzle at the rear of the craft.

A personal watercraft's engine and jet drive

The impeller is a rotor-like device that sits inside a cylindrical passageway in the body of the craft. The craft's engine rotates the impeller via a drive shaft. The impeller's curved blades spin rapidly, forcing water up through the passageway and out through the nozzle.

Photo courtesy Yamaha Motor Corporation, U.S.A.
Yamaha FX High Output impeller and steering

This moves the craft because of the principle described in Isaac Newton's third law of motion. According to Newton's third law, for every action, there is an equal and opposite reaction. In this case, the action is the expulsion of water through the nozzle. The reaction is the movement of the boat in the opposite direction. How Rocket Engines Work explains this principle in detail.

When you steer the craft, a cable linkage connected to the handles swivels the nozzle at the rear of the craft. This changes the direction of the "equal and opposite reaction." If the nozzle directs the water to the right side of the craft, the rear of the craft pushes to the left. That causes the front of the craft to turn to the right.

Click the handlebars to steer the watercraft.

That's the basic idea of a personal watercraft. Now let's examine the details of this system.

The Engine

Just like a lawn mower or a car, personal watercraft run on two-stroke or four-stroke engines. Two-stroke models have two or three cylinders. Four-stroke models usually have four cylinders. See How Two-stroke Engines Work and How Car Engines Work (describing the four-stroke engine) to learn all about these mechanisms.

A noticeable difference between marine and automotive engines is the cooling system. Instead of using a radiator, a marine engine uses the surrounding water to control temperature. Water flows through the engine or through a closed system that includes a chemical coolant.

The engine starts the same way a car engine does. The ignition button, usually found on the handlebars, sends a signal to an electric starting motor. The motor turns the engine to get it started. Then, the engine takes over powering the craft.

Photo courtesy Yamaha Motor Corporation, U.S.A.
Shutoff lanyard

All newer models of personal watercraft have a pin or key that inserts into a slot near the ignition. The pin is attached to a lanyard that connects to the operator's life vest or wraps around his or her wrist. The boat will not start or run without the pin in place. If the operator falls overboard, the lanyard pulls the pin out, and the craft coasts to a stop. This keeps the boat from traveling very far and reduces the likelihood of a collision.

Quieter Personal Watercraft Personal watercraft make a very distinctive, high-pitched sound, which some people feel disturbs residents, wildlife and other boaters. Recent technological advances have made personal watercraft significantly quieter. These include:

Noise-absorbing foam between the hull and the engine
Noise generators, such as pipes, which produce sound waves that cancel out engine noise
Resonators that pull air into the engine through a maze instead of a straight line, diffusing the sound

Steering and Acceleration

The steering mechanism of a personal watercraft is very simple.

Click the handlebars to steer the watercraft.

The handlebars are connected to a steering cable. When the operator turns the handlebars, the cable moves the steering nozzle, which changes the direction of the jet of water. Some models are also designed to allow the operator to lean into turns as they would on a motorcycle.

Photo courtesy Yamaha Motor Corporation, U.S.A.
Yamaha FX

A trigger on the handlebars controls the throttle, allowing the operator to accelerate. The throttle is as important to steering as it is to acceleration. Without water from the steering nozzle, the boat cannot change direction. Instinctively slowing down to avoid an accident is dangerous and can result in a complete loss of steering power. If this happens, the craft will float in a straight line until friction from the surrounding water brings it to a stop. Newer models use a computer to re-start the jet drive if the operator simultaneously releases the throttle and turns the handlebars hard in one direction.

Some models can go in reverse using a simple shroud that lowers over the steering nozzle. The shroud forces the water into a U-turn, sending it under the boat instead of behind it. Since the rear of the craft always moves in the opposite direction of the flow of water, this makes the craft go backward. This feature can help get the boat into and out of the water or get out of a tight space. It is not designed to slow or stop a craft that is moving at high speeds.

Now that we've covered the mechanical components, let's explore the environmental and legal concerns that surround personal watercraft.

Safety Issues It's important to understand a few safety concerns before getting on a personal watercraft.

First, keep in mind that passengers ride on a personal watercraft instead of in it (and they do so without restraints). If you combine that with speeds of up to 65 miles per hour and the absence of brakes, crashes are potentially deadly.

In accidents involving traditional boats, the most common cause of death is drowning, which you can prevent with the use of personal flotation devices. In accidents involving personal watercraft, the most common cause of death is impact trauma. Other than helmets, no technology exists to prevent physical trauma. And the reliability of helmets has not been proven in personal watercraft accidents.

Personal watercraft have no running lights, which means they're especially dangerous at night. The jet intake and steering nozzle can also be dangerous because of the force of the moving water.

The Personal Watercraft Industry Association has an online Personal Watercraft Safety Checklist that includes some basic safety guidelines.

Environmental Concerns

Personal watercraft are small and ride higher in the water than other boats, so they can get into very narrow, shallow spaces. This is particularly helpful for surveyors, search-and-rescue teams and U.S. Homeland Security officials, all of whom use the craft in their work. However, this ability to get into tight spaces can lead to:

Disturbance of sediment
Destruction of aquatic habitat and plant life
Disturbance of or injury to birds, fish and animals

Unlike motorboats, a jet drive has no external propeller to damage coral reefs or injure animals. However, the drive's suction and the force of the jet can still cause damage. Also, if the drive is not flushed thoroughly after every use, "exotic" species, like zebra mussels and invasive plants, can stow away inside the craft. They can then be transported to other bodies of water.

Another environmental concern is pollution. All older models and some newer models of personal watercraft use two-stroke engines, which can vent oil and gasoline into the water with their exhaust. Other motorized boats also use two-stroke engines, but the sheer number of personal watercraft may make their environmental impact greater.

The U.S. Environmental Protection Agency has outlined standards for marine engine emissions and efficiency that take effect in 2006. Virtually all current models of personal watercraft already meet or exceed these standards by incorporating technology such as:

Fuel-injection systems
Cleaner-running four-stroke engines

A number of government agencies have taken steps to reduce concerns about noise, safety and pollution. We'll look at these in the next section.

Laws and Regulations

Several national organizations monitor and regulate personal watercraft in the United States. They include:

The National Transportation Safety Board, which investigates accidents and recommends courses of action through other agencies
The Environmental Protection Agency, which regulates emissions
The United States Coast Guard, which provides boating safety regulations and maritime law enforcement

Photo courtesy Yamaha Motor Corporation, U.S.A.
Yamaha FX Cruiser High Output

The U.S. Coast Guard classifies personal watercraft as inboard boats, so operators and passengers must follow the same laws and guidelines as other boaters. Additional regulations vary by state. The National Association of State Boating Law Administrators has a complete state-by-state listing of personal-watercraft laws. It is up to all operators and passengers to make sure they use their watercraft safely and in compliance with local laws.

For more information on personal watercraft and related topics, check out the links on the next page.

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