pumpjet

A pumpjet is a marine propulsion device that uses an enclosed pump to move water and generate thrust. Unlike a conventional shaft-driven propeller, the machinery is housed in a watertight duct behind the intake. The system relies on an impeller to accelerate water, with a proximal propulsor often located in the duct.

Water is drawn in through an intake and passed to an internal impeller driven by an engine or electric motor. The impeller increases water velocity and pressure, which is then directed out through a nozzle. In many designs, a propeller sits behind the pump within the duct and works with stator vanes to straighten flow and improve efficiency.

Variants include fully ducted pumpjets in which the pump and propeller are enclosed, and open configurations where the propeller is less shielded. Forward and reverse thrust are achieved by reversing flow or changing the impeller’s rotation. Some designs use multiple stages or counter-rotating elements to boost efficiency.

Advantages include higher efficiency at certain speeds, reduced draft and exposed propellers, quieter operation, and protection from debris. They can be compact and integrate into hull lines. Limitations include higher cost, more complex maintenance, and sensitivity to debris clogging the intake.

Pumpjets are used on various vessels, including small and high-speed boats, patrol and rescue craft, ferries, and some submarines. The technology competes with conventional propellers and waterjets, offering a balance of protection and efficiency in specific operating regimes. The term pumpjet is sometimes used interchangeably with ducted waterjet, though designs vary by manufacturer.