Experimental Assessment of Entanglement for an Unmanned Underwater Vehicle with an Open, Three-Bladed Propeller

May 1, 2022

Naval Postgraduate School, Monterey, California

This research evaluates the entanglement of an unmanned underwater vehicle (UUV) operating in marine vegetation common to littoral environments. Entanglement was assessed for a traditional UUV with an open, three-bladed propeller transiting a vegetation field at a constant heading and depth. Factors such as the vegetation density, vegetation placement and configuration, propeller revolutions per minute (RPM), and vehicle speed were varied to determine their impact on vehicle entanglement. Results provide insight to the mechanism of entanglement and operating conditions that result in a high or low likelihood of entanglement. These results are of particular interest to the Department of Defense as the military's use of UUVs in littoral environments becomes more prevalent.

Experimental tests were conducted in a tow tank located at the Naval Postgraduate School. Synthetic eelgrass and synthetic giant kelp plants anchored to a plate located at the bottom of the tow tank were used to simulate marine vegetation common to the littorals. The UUV tested in this work was an early variant of the Remote Environmental Measurement UnitS (REMUS) 100 manufactured by Hydroid. The REMUS 100 was attached to a movable carriage located on top of the tow tank using a vertical sting and custom-made adapter. This configuration allowed the REMUS 100 to self-propel itself, and the attached movable carriage, down the tank at a constant depth and heading. Vehicle speed and direction of travel (forward versus astern) was adjusted by changing the propeller RPM of the REMUS 100. A position sensor was used to correlate propeller RPM to the vehicle's steady state speed and to determine the vehicle's speed as a function of position (accelerating speed).

Three different entanglement tests were conducted: a vegetation density and constant speed test, a lateral placement test, and an accelerating speed test. In the vegetation density and constant speed test, the density of giant kelp and eelgrass was varied, and the vehicle steady state speed and propulsion direction (forward versus astern) was varied by changing the propeller RPM. In the lateral placement test, the eelgrass and giant kelp position was offset different distances from the center line of the vehicle. In the accelerating speed test, the propeller entrance speed to an eel-grass field was varied by giving the vehicle a limited distance to accelerate up to speed and thus resulted in the vehicle not being at its steady state speed when it passed through the vegetation field.

The different entanglement conditions identified for each test run were no entanglement, an entanglement event, and an entanglement kill; these conditions were assessed by visually observing the speed of the vehicle as it entered, transited, and exited the vegetation field. During a no entanglement condition, vehicle speed remained constant throughout the entirety of the run. An entanglement event condition was characterized by a decrease in vehicle speed caused by vegetation wrapping around the hub of the vehicle's propeller, but not actually stopping the vehicle, so that eventually the vehicle was able to pass through the vegetation field. An entanglement kill condition was characterized by a decrease in the vehicle speed to zero caused by extensive vegetation wrapping of the vehicle's propeller.

A 2 (marine vegetation type) × 4 (marine density configurations) × 3 (vehicle speeds) × 2 (propulsion directions) × 3 (trials) design was used for the vegetation density and constant speed test. The two marine vegetation types used were giant kelp and eelgrass. The four different marine vegetation density configurations considered were high, medium, low, and single. Three vehicle steady state speeds corresponding to three different propeller RPMs (600, 900, and 1200) were tested.

This work was done by Lieutenant Katherine E. Irgens for the Naval Postgraduate School. For more information, download the Technical Support Package (free white paper) below. NPS-0022

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Experimental Assessment of Entanglement for an Unmanned Underwater Vehicle with an Open, Three-Bladed Propeller

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