Water jet propulsion systems have been around for thousands of years. For example, the chambered nautilus zips around underwater by sucking up water and expelling it. Thanks to Newton’s third law of motion, the creature moves forward as it squirts water out.

The same principle applies to the user of water jet propulsion systems in watercraft. Sir William Hamilton designed the first successful jetboats using this kind of propulsion system. Twenty-first century lakeside dwellers are probably familiar with jet skis, which scoot around the water using water jet propulsion. Other small craft, including Coast Guard ships and littoral combat ships, use water jets. These ships do not have propellers that can get snarled up in marine growth and they are also highly maneuverable.

How water jet systems work

Marine water jet propulsion systems, also known as pump jet or hydrojet systems, are more complex and sophisticated than that of the chambered nautilus. Regardless of specific design, these systems require a water inlet in the ship’s hull that feeds water into a pump. The pump increases pressure on the water and forces the water backward, toward the ship’s stern, where it exits the ship through a nozzle. A gas turbine or piston engine drives the pump. Then Newton’s Third Law takes over, and the ship moves forward. The crew steers the ship by changing the orientation of the water exhaust (see Figure 1) or by covering the exhaust with a reversing bucket, which can either stop the ship or reverse its forward movement.

Figure 1. Left to right, the drawing illustrates watercraft forward movement, stop or reverse, movement to one side and turning. Source: Wikipedia/CC By-SA 3.0

Water jet propulsion systems use three different types of pumps, optimized for the type of craft in which it is installed. Axial flow pumps increase water pressure by diffusing the flow as a high volume of water passes through the pump impeller blades and stator vanes. These systems are suited to low- and medium-speed watercraft. Centrifugal flow pumps, which use radial flow to pressurize water, are not commonly used. Mixed flow pumps, as the name implies, use both diffusion and radial flow to pressurize water. They use lower water volume and produce higher velocities than axial flow pumps and are well-suited to smaller craft that operate at higher speeds.

Pluses and minuses of water jet propulsion

Water jet propulsion systems offer designers multiple advantages.

• Quieter operation includes a low sonar signature, a significant advantage for military craft.
• Shallower draft allows the craft to operate in shallower water — a design feature that Hamilton took great advantage of.
• Greater speed is possible, depending on other design features, since the drives are in the ship’s stern.
• Excellent maneuverability comes from the ability to aim the water jets in different directions, eliminating the need for a rudder.
• Smoother ride comes from the lack of hull vibration, torque effects and high-speed cavitation.
• Internal equipment means no exposure to hazards like marine growth and submerged rocks and reefs.
• Safer operation around humans and marine life since there is no propeller.

This type of propulsion does have some drawbacks, including higher maintenance costs and less maneuverability at low speeds. According to Dataware Marine Solutions, the most efficient operating speed range for water jet propulsion is between 25 and 50 knots. Below 20 knots, propeller-driven craft are more efficient. Water jet systems cost more than propeller systems and they are heavier and less fuel-efficient than propeller systems.

Not just for small craft anymore

Use of water jet propulsion systems is expanding beyond personal watercraft and jet boats. For example, the U.S. Navy’s Virginia-class fast-attack submarines use water jet propulsion, taking advantage of the quieter system to help evade enemy attackers. Other navies and coast guards are on board with water jet systems, including Norway (Skjold Class patrol boats), India (fast attack craft) and Korea (patrol boats).

An interesting new application for water jet systems is dynamic positioning (DP). A vessel with a DP system can maintain its position regardless of environmental conditions — and without an anchor to help. It uses a combination of position reference and motion sensors, a gyroscope, a computer that processes information relayed by sensors and operates thrusters as needed to maintain position. Typical DP applications include offshore drilling vessels and support vessels and offshore construction vessels. Water jets can be used instead of azimuth thrusters in DP systems. Ships that do not need anchors to maintain position are particularly useful where anchors or mooring lines cannot be used, such as areas with subsea structures like pipelines and communications cables.

Maneuverability is an advantage in several situations. For example, the water jets simplify the process of weathervaning (turning a ship into the prevailing wind to prevent damage). Tugboats that need to dock quickly and accurately also benefit from water jet propulsion.

As a sign of active interest in water jet propulsion, the Royal Institute of Naval Architects is sponsoring its third international workshop on the subject in Shanghai in December 2019. The comprehensive list of subjects covered ranges from numerical methods to experimental technology.