Propulsion systems are currently classified as either Electro-hydraulic or Electric. Generally, the weight and relatively lower efficiency of an electro-hydraulic system effectively eliminates this system from consideration in vehicles weighing much less than 500 Ib (227 kg). In larger vehicles, however, it has the advantage of simplicity, ease of packaging, versatility, reliability, and low electrical noise. Although not a practical limitation in commercial operations, the higher acoustic noise inherent in the electro-hydraulic system may be important when considering the mission of the ROV, especially in the military mission of mine countermeasures.

Typical direct drive electric propulsion systems use a separate electric motor for each propellor, although a multiple output gearbox can be driven by a single motor. Electrical propulsion has weight advantages in small ROVs.

Propulsion unit styles include:

  • Continuous pitch propellors with constant speed motors (50/60 Hz)
  • Variable frequency AC driven
  • Universal motors with double reduction gear
  • Brushless DC motors
  • Permanent magnet brush type motors

The ROV can be characterized as a small tugboat, with the consequence that the thrusters must be pitched to obtain good bollard pull—essentially the thruster’s maximum static thrust. But one must be careful using bollard pull to determine thruster requirements. System efficiency must be taken into consideration along with the fact that most thruster output will decrease as velocity increases. The optimum pitch is also a function of vehicle speed. Therefore, the wise engineer will use the design curves available on the candidate thrusters to determine the proper size and location of thrusters based on expected vehicle speed.

Since the velocity of the water surrounding the thruster, essentially the inlet velocity, effects the output of the thruster, the location of the thruster is very important. Accordingly, the location of the thruster in the vehicle frame or body is not just a matter of strapping on a thruster. Thruster size and location should be considered within the overall system, including the balance of power used by the thruster and other subsystems, ensuring that one doesn’t rob the other of needed output in a critical situation.

Thrusters come in several sizes and configurations and may be powered electrically or hydraulically, through direct or gear drives, with or without shrouds or ducts. Generally, most thrusters will have a ducted shroud or a Kort nozzle to increase the output efficiency such as the Innerspace high performance thrusters shown below.

Marketing brochures advertise output thrust ranging from 30 to 100 pounds per shaft horsepower input. Obviously, the results will be put in the best light for the company, so the thruster’s design curves should be reviewed and appropriate adjustments made based on system integration. A rule of thumb that has been used for some time to estimate thrust or power requirements is 35 to 40 pounds of thrust per horsepower, however, it appears that technology is making the "modern thumb" a bit larger.