Pumpjets

Pumpjets are a type of propulsion system for watercraft that generate thrust by drawing in water, accelerating it with an onboard pump, and expelling it through a nozzle. The core component is typically a centrifugal impeller housed in a ducted casing, driven by an engine or electric motor. Water enters through an intake, is accelerated by the impeller, and leaves via a narrow outlet where the increased velocity and pressure produce forward thrust. In many designs a stationary diffuser or stator follows the impeller to straighten flow and improve efficiency; some configurations use multiple stages or devices to mitigate cavitation at high power.

Compared with open propellers, pumpjets are generally enclosed, providing protection against debris and reducing the risk of injury or entanglement. They are often quieter and produce smoother thrust, which can enhance maneuverability and comfort in certain vessels. Pumpjets can deliver strong low- to mid-range thrust and are well suited to shallow or congested waters where a propeller might strike the bottom or be damaged. Throttle response is governed by pump speed, enabling precise control of propulsion.

Limitations include potential lower peak efficiency at very high speeds relative to high-performance propellers, and greater mechanical complexity and weight. Maintenance focuses on the pump housing, seals, bearings, and intake screens to prevent foreign-object ingestion and cavitation.

Applications span submarines and other vessels where quiet operation and safety are valued, as well as some small to medium-sized boats and research or specialized craft that benefit from a compact, enclosed propulsion unit.