80 Insight | USVs Trimarans Monohull vessels are a common sight among oceangoing USVs for the stability that such designs possess inherently, based on their weight distribution. Multihulled vessels, by contrast, leverage their width for stability, but experience a critical, angle weakness tipping point and they can roll over, requiring at-sea rescue. San Diego-based SubSeaSail designed its first autonomous undersea and surface vehicle (AUSV), HORUS, which was featured as the ‘Gen6 USSV’ in issue 33, with a monohull configuration. It is a semisubmersible that sits mostly submerged – a configuration that is stable and saves energy over traditional monohulls by not producing wave-making drag while sailing. SubSeaSail received funding from the US National Science Foundation (NSF) to conceive a second line of vehicles – multi-hull vessels that can self-right themselves when tipped over. This has motivated it to design and produce a new trimaran AUSV called HERMES, featuring new capabilities over its monohull sibling, while still including much of SubSeaSail’s established technological IP. “We’ve initially created a functioning, 4 ft-long [1.2 m] scale prototype. For ocean operations, HERMES will likely be scaled to a minimum of 6 ft [1.8 m] – a size needed to right itself at sea, but we suspect it will also be popular because AUSVs that small can be transported and launched by car or trailer. We believe the design can be scaled up to 100 ft [30.5 m] or more,” states Ryan Lorence, mechanical engineer at SubSeaSail. “At bigger lengths, HERMES will be able to generate significant electricity from its solar panels and be able to power a whole host of onboard systems. You could power payloads, recharge batteries for other vehicles, or generate hydrogen or fresh water from saltwater.” Lorence notes in particular that the trimaran might be used for transporting cargo with significant energy- and operational-efficiency. If a large trimaran AUSV should arrive at its harbour too early, or at a time with high loading traffic, insufficient free warehouse space or damage to the port, it can loiter out of the way, above or below surface, or sail up to the sand at the nearest beach. “The design doesn’t even need a full-sized port to deliver goods. Multi-hull vessels are very low draft – they don’t need a deep keel for stability, so this boat could sail right up onto the shore and be dragged up by workers.” While performance specifications are still being established, the trimaran is expected to achieve higher speeds than its monohull counterpart due to its shallower draft, resulting in less water resistance, and not needing a heavy keel for stability. Propulsion is achieved primarily by wind power via the wingsail and electric thrust as a backup. Solar panels will be located in the wingsail or on deck, depending on the vehicle and size. Wingsails are moulded from fibreglass with RF transparency, which results in a very low radar signature, to the appeal of SubSeaSail’s defence customer base. “New shapes and internal structures provide advantages to the support of the wingsail solar panels to keep them in a nice, curved shape,” Lorence notes. “Wingsail skins, including internal solar panels, are being designed to simplify manufacturing. The whole configuration will be much easier to replace and service than our past versions. “Maximising the surface area for solar collection utilising new solar-panel manufacturing techniques affords the greatest potential energy collection, as well as electrical system redundancy. This is a big improvement for all AUSV and USV operations.” Monohulls Arguably, the most aggressive litmus test of a USV’s reliability at sea is trialling whether it can perform a Transatlantic crossing autonomously, as explored in detail with the Mayflower USV in issue 42. The ambition to perform such a crossing was what inspired Dylan Rodriguez, CTO and cofounder of SeaSatellites Inc (Seasats), to create the Scout USV with his fellow students in 2013. June/July 2024 | Uncrewed Systems Technology SubSeaSail’s HERMES trimaran will be scalable, from 1.8 m to over 30.5 m long, to serve in applications such as maritime cargo (Image courtesy of SubSeaSail)
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