Underwater propulsion technology is the core technology that uses various types of energy to generate propulsive force underwater to drive equipment such as torpedoes, submarines, and underwater robots. Its development is closely linked to the needs of marine military and marine development, spanning the iterative process from traditional mechanical drive to intelligent and efficient propulsion. As a core supporting technology in the field of marine engineering, advanced underwater propulsion technology must have good dynamic characteristics, safety and concealment. It must not only meet the concealed cruising needs of military equipment but also adapt to the efficient operation requirements of civilian marine exploration and resource development.
From the perspective of energy supply, underwater propulsion technology can be divided into four categories, each with clear application scenarios and development contexts. The mechanical energy storage system was the main form of early underwater propulsion, with a flywheel system as the core. It was applied to early torpedoes by virtue of its high energy density. A typical representative is the Howell torpedo produced around 1880, which could travel about 800 nautical miles (approximately 1481.6 kilometers) at a speed of 25 nautical miles per hour (46.3 kilometers per hour) through internally stored mechanical energy. However, limited by energy storage capacity, it has gradually withdrawn from mainstream applications.
The thermal energy storage system uses liquid metal or liquid metal salt as the energy storage medium, which releases heat through phase change to heat water vapor and drive the propeller to operate. Its core advantage is stable energy release, which is suitable for underwater equipment with high requirements for stable power output, but its application scenarios are relatively niche due to the characteristics of the energy storage medium. The chemical energy system is currently the mainstream energy form for underwater propulsion, divided into two categories: primary battery systems and adiabatic systems. The primary battery system has been applied since around World War II, evolving from early lead-acid batteries to silver oxide-zinc batteries, aluminum-silver oxide batteries, magnesium-seawater batteries, etc., with efficiency nearly 10 times higher than that of the early stage. The adiabatic system provides power through chemical reactions between fuel and oxidant, deriving various hydrocarbon fuels from early diesel. The later developed Otto fuel is widely used due to its outstanding safety and controllability. The water-based metal fuel emerging in the 21st century, with seawater as the oxidant and coolant, has the advantages of small volume and high energy density, becoming the optimal solution for high-speed propulsion (such as supercavitating torpedoes).
The cabled power system transmits power to underwater carriers through cables, first applied to torpedoes in the 1870s. Limited by the performance of early insulating materials, it could only achieve small-range and low-speed cruising. Currently, it is mainly used in remotely operated vehicles (ROVs), relying on mature power transmission technology to meet the power needs of near-shore exploration and underwater operations.
In terms of power mechanisms, underwater propulsors are mainly divided into three categories: propeller propulsion, water jet propulsion, and bionic propulsion. The propeller propulsion system has a simple principle and high reliability, generating thrust through pressure difference caused by propeller rotation. It is widely used in various underwater equipment, but its high operating noise limits its application in scenarios requiring concealment. The water jet propulsion system is composed of an inlet, a mixed-flow pump impeller, a stator, and an outlet nozzle, generating thrust by discharging water at high speed. It is suitable for equipment with high speed requirements such as large torpedoes and high-speed ships. The bionic propulsion system simulates the swimming mode of underwater organisms and is still in the research stage, with fish-like propulsion as the main direction. The "Robotuna" developed by the Massachusetts Institute of Technology (MIT) and the small robotic fish developed by the University of Essex in the United Kingdom have both achieved controllable swimming based on tail swing, and are expected to make breakthroughs in the field of low-noise and high-efficiency propulsion in the future.
Looking forward to the future, underwater propulsion technology will develop in the direction of diversification, high efficiency, and intelligence. The energy field will still be dominated by chemical fuel drive, but driven by the demand for submarine concealment, air-independent power devices such as Stirling engines, closed-cycle diesel engines, and fuel cells are gradually becoming popular, solving the pain point of short underwater continuous diving time of conventional submarines. Civilian submersibles will focus on energy conservation and high efficiency to extend underwater operation time. The continuous breakthrough of bionic propulsion technology will further enhance the low-noise and high-maneuverability advantages of underwater propulsors, providing more competitive power solutions for marine military and civilian development.
