79 at sea mid-mission because there are no crew or parts inventory on the vessel for fixing the engine. Fortunately, a new generation of USVs is being engineered that overcomes these problems through astute design and component choices, which are bringing a variety of additional benefits across sustainability, efficiency and more to push the state of USV tech forwards. SWATH vessels UK-based ACUA Ocean is eschewing engines for the use of hydrogen fuel cells to power its H-USV. The first prototype of this vessel is under construction (and near completion) at the AMC yard on the Isle of Wight. It is set to be 13.5 m long and 9 m wide, giving it a notably more rectangular profile than most USVs when viewed from above. “Distributing its weight and buoyancy like that delivers a significantly more stable platform than most. As a SWATH [small, waterplane area, twin-hull] vessel, the H-USV’s size and 25 t weight helps it to significantly outperform the stability of a conventional, 39 m monohull,” says Mike Tinmouth, COO at ACUA Ocean. “Meanwhile, smaller, below 6 m-length, autonomous vessels just aren’t capable of long-endurance, open-ocean operations that industry requires.” A SWATH vessel is a form of catamaran in which the hull cross-section area at the surface of the water (where wave energy exists) has been minimised, with most of the boat or ship’s displacement sitting below the waves instead. This approach greatly reduces the impact of sea state on the vessel’s stability, including at high sea states and vessel speeds. “We wanted a platform that could operate in open ocean conditions, and function as a stable survey ‘hub’ for launching and recovering whatever payloads or smaller, uncrewed vehicles an end-user might like. Too many USVs nowadays pitch and roll about in any sea state of significance, which is a nightmare for consistent comms, data quality, payload deployment – so for that we targeted high stability, as well as designing for much space, weight capacity and available power onboard the H-USV as possible,” Tinmouth explains. “Its configuration was developed by Ad Hoc Marine Designs, a global leader in SWATH vessels, in partnership with the University of Southampton, and the Southampton Marine and Maritime Institute for wave-tank testing and other key analyses.” SWATH vessels have been in active deployment since the late 1960s, with their inherent stability putting them in high regard among naval architects and particular usage as crew transfer vessels (CTVs). However, their cost and engineering complexity has hindered widespread adoption, so much of ACUA and Ad Hoc’s focus over the last two years has gone into design refinement to lower such costs and make for a practical, profitable production and business proposition. On certification, Tinmouth says: “While there is talk of a future mini-MASS code for smaller USVs, currently these are hard to get past muster from a regulatory perspective. Compliance and approvals are an important objective for ACUA, and we are working in line with the UK MCA’s Workboat Code Edition 3, as well as the Lloyd’s Register UMS Code. Very few vessels have been able to achieve certification to those standards; Sea-KIT and XOcean being the industry leaders in that regard. Meanwhile, the vast majority of USVs continue to struggle to win approval for full, open ocean operations, because they just haven’t got the stability or redundancy built in.” While one might presume, as a newer technology than ICEs, that adopting hydrogen as a power source would risk certifiability, Tinmouth says fuel cells have been through multiple cycles of development and improvement over the last decade. Several manufacturers are working towards or have achieved approval in principle for marine fuel cells, and the number of supply chain uncertainties hindering longer-term benefits of hydrogen-electric powertrains is shrinking. “Those benefits include the reliability of fuel cells, the high electrical power they generate for payloads, the low vibratory output from being a solid-state, electrical system, and chemical benefits in that they only vent water and a small amount of H2, compared to diesel engines, which give off CO2, NOx and other things,” he adds. “They also have a lower acoustic and thermal signature, compared with diesel engines, which you can imagine is very appealing to end-users who don’t want to be detected.” The hydrogen-electric powertrain was developed under Innovate UK’s Clean Maritime Demonstration Competition, while the H-USV’s thrust comes from RAD Propulsion’s drive-by-wire, 40 kW electric twin drives. “This is a prototype vessel, so there will certainly be learnings and iterations as we go forwards, but with our large, modular and payload-agnostic design, we are looking at multiple uses across the offshore sector, including industrial surveys, marine conservation, biodiversity data collection, and defence and security applications,” Tinmouth concludes. USVs | Insight We targeted high stability, as well as designing for much space, weight capacity and available power onboard the H-USV as possible Uncrewed Systems Technology | June/July 2024
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