Waterjet propulsion moves a ship or submarine by taking in water, speeding it up inside the hull, and ejecting it out the stern as a fast jet. This creates thrust that pushes the craft forward, making the system useful for fast patrol boats, ferries, rescue craft, and some compact underwater vehicles. Because the main moving parts are inside the hull, waterjets can operate in shallow water with less risk of striking rocks, debris, or the seafloor.
They also allow quick changes in speed and direction, which is valuable for maneuvering at high speed.
A typical waterjet system has an intake, duct, pump, nozzle, and steering or reversing bucket. The pump adds energy to the water, increasing its momentum before the nozzle shapes it into a high-speed stream. By Newton's third law, the sternward jet pushes on the surrounding water, and the water pushes the vessel forward with an equal and opposite force.
Steering is done by deflecting the jet left or right, while reversing is done by redirecting the jet forward instead of simply spinning a propeller backward.
Key Facts
- Thrust comes from changing water momentum: F = Δp/Δt.
- For steady flow, thrust can be estimated by F = ṁ(vout - vin), where ṁ is mass flow rate.
- Mass flow rate is ṁ = ρQ, where ρ is water density and Q is volume flow rate.
- A nozzle increases jet speed by forcing pumped water through a smaller exit area.
- Waterjets are best suited for high-speed and shallow-draft craft because no large propeller hangs below the hull.
- Steering and reverse thrust are produced by redirecting the jet, not by turning a rudder behind a propeller.
Vocabulary
- Waterjet propulsion
- A propulsion method that draws water into a vessel, accelerates it with a pump, and ejects it as a jet to produce thrust.
- Thrust
- The forward force that moves a vessel by pushing water backward.
- Intake
- The opening in the hull where water enters the waterjet system.
- Impeller
- A rotating pump blade assembly that transfers energy to the water and increases its speed and pressure.
- Nozzle
- A shaped outlet that converts pumped water into a directed high-speed jet.
Common Mistakes to Avoid
- Thinking the boat is pulled forward by suction at the intake. The main forward force comes from ejecting water backward and changing its momentum.
- Forgetting to include mass flow rate when comparing thrust. A fast jet with very little water may produce less thrust than a slower jet with much more water.
- Assuming waterjets are always more efficient than propellers. Waterjets are often excellent at high speeds, but propellers can be more efficient at low speeds or for heavy towing.
- Drawing the steering system as a normal rudder behind a propeller. A waterjet usually steers by swiveling or deflecting the jet itself.
Practice Questions
- 1 A waterjet takes in water at 4 m/s and ejects it at 24 m/s. If the mass flow rate is 180 kg/s, estimate the thrust using F = ṁ(vout - vin).
- 2 A pump moves 0.35 m3/s of seawater with density 1025 kg/m3. If the water speeds up by 18 m/s through the waterjet, find the approximate thrust.
- 3 Explain why a waterjet-powered rescue boat can be safer in shallow, debris-filled water than a similar boat with an exposed propeller.