Unmanned Systems Technology 022 | XOcean XO-450 l Radar systems l Space vehicles insight l Small Robot l BMPower FCPS l Prismatic HALE UAV l InterDrone 2018 show report l UpVision l Navigation systems

XOcean XO-450 | Dossier the lower the weight needs to be. It was a practical consideration; you can make the models too small.” Wave height and period combine to define the steepness of a wave, which is what is important in terms of risk to the vessel. Comparisons with records for the Celtic Sea (an area of the Atlantic Ocean off the south coast of Ireland) showed that the waves they generated in the Lir tank have only been equalled in the area for two periods of an hour in the past decade, Ives says. Full-scale prototype Confident in the design’s basic stability, the team then decided to build the first prototype at full scale, Ives explains. “If the boat were going to cost millions to build to find out whether you’d got your design right, you’d do a lot of design analysis and a lot of model testing,” he says. “Because of the small size of the USV we could afford to prototype at 1:1 scale – and nothing’s going to give you better information than a full-scale test.” This applied in particular to a critical issue for all survey vessels, known as bubble sweep-down, in which bubbles forced down by the hull reach the sonar transducer and interfere with its signals. “That is something engineers try very hard to mitigate at the design stage, but they still have periods with new survey vessels of sea trialling and figuring out that they need to change aspects of the design,” Carlisle says. “So there’s nothing better than doing a real trial at 1:1 scale.” This approach seems to have paid off, and the team is pleased with the hull’s design. “It floated with a level trim, and where we expected it to, and it also behaved as we expected it to,” Ives says. Testing has also shown the XO-450 to be a good sensor platform, continuing to collect good data even in very poor conditions including force 6 gales. Ives comments that they haven’t yet found the wind limit, and the same applies to the maximum sea state it can survive in. “It turns out to be a very high number, but we don’t know what the exact number is just yet.” On the water, the XO-450 follows its programmed track very accurately, even in high winds, proving to be less affected by such conditions than the 9 m rigid hull inflatable boat (RHIB) XOcean uses as an escort boat in the harbour. “A big part of that is the lack of windage on the boat,” Carlisle adds. “It’s low to the water, and the gantry doesn’t present much in the way of wind drag. We do get some violent squalls in the lough here, and while the RHIB can be blown off track, the USV stays perfectly on course.” Energy and power For the kind of missions for which it is designed, the XO-450 needs an endurance of around 18 days while