How Personal Watercraft Work

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.

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.

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.

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

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.

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.

The steering mechanism of a personal watercraft is very simple.

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.

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.

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:

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:

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.

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

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.