‘Semi-sub’ vehicle sails stealthily at water line

According to the researchers, the roughly 450mm long semi-sub prototype, built with off-the-shelf and 3D-printed parts, showed its seaworthiness in water tests, moving quickly with low drag and a low profile. Test results were published in the journal Unmanned Systems.

The team said that while authorities have discovered crudely made semi-subs used for illicit purposes in recent years, this project aims to demonstrate how engineer-developed semi-submersible vessels can serve military, commercial and research purposes.

“A semi-submersible vehicle is relatively inexpensive to build, difficult to detect, and it can go across oceans,” said Konstantin Matveev, the WSU engineering professor leading this work. “It's not so susceptible to waves in comparison to surface ships since most of the body is underwater, so there are some economic advantages as well.”

Since the semi-sub sails mostly at the water line, it does not need to be made of as strong materials as a submarine, which has to withstand the pressure of being underwater for long periods of time. The semi-sub also has the advantage of having a small platform in contact with the atmosphere, making it easier to receive and transmit data, researchers confirmed.

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Matveev and co-author Pascal Spino, a recent WSU graduate and former president of the WSU RoboSub club, piloted the semi-sub in Snake River’s Wawawai Bay in Washington state. They tested its stability and ability to manoeuvre.

The semi-sub reached a max speed of 1.5 metres per second (roughly 3.4 miles an hour), but at higher speeds, it rises above the water creating more of a wake and expending more energy. At lower speeds, it is almost fully immersed and is said to barely make a ripple.

Researchers also outfitted the semi-sub with sonar and mapped the bottom of a reservoir near Pullman, Washington to test its ability to collect and transmit data.

While not yet completely autonomous, the WSU team said its semi-sub can be pre-programmed to behave in certain ways, such as running a certain route by itself or responding to particular objects by pursuing them or running away.

It is relatively small at around at 450mm long with a 100mm diameter, but Matveev said it is possible for larger semi-subs to be built to carry significant cargo, such as to help refuel ships or stations at sea. Researchers believe they could even be scaled up to rival container ships and, since they experience less drag in the water, they would use less fuel creating an environmental and economic advantage.

Matveev’s lab is continuing work on optimising the shape of semi-submersible vehicle prototypes to fit specific purposes. He is currently collaborating with the U.S. Naval Academy in Annapolis, Maryland to work on the vehicle’s operational capabilities and compare numerical simulations with results from experiments.