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

26 resort to computational fluid dynamics (CFD). “We did first-principles calculations on properties like drag to determine the requirements for power,” Ives says. “CFD is great for showing incremental improvements to a design, but it’s very hard to get it to give you an absolute value for drag,” Carlisle adds. “At this scale, actually building the full-scale USV and trialling it is faster and more economical.” However, they did consult a naval architect and their chosen boat builder, MB Yachts, which is based in south-west England, to make sure they were sticking to sound design principles. Seeking stiffness One critical area for the XO-450 to be able to gather good quality data is the sensor mounting post, which must be very stiff to prevent the sensor on the end from moving in relation to the rest of the boat. Here, XOcean did carry out exacting calculations and worked with the naval architect to ensure a very rigid and robust structure. The sensor post itself is a large diameter, thick-walled GRP tube. “You could confidently mount 100 kg of sonar off the sonar post,” Carlisle notes. “It’s really belt-and-braces engineering.” The choice of details such as which resins and grades of glass fibre to use was made in consultation with the boat builder, he says. “We provided the shapes and the hull forms we wanted to achieve, and then the boat builder helped transform that into how many layers to put in certain locations – and how to, for instance, actually construct the joints between the cross-beams and the hulls, using standard boat-building techniques,” he says. Ives adds, “We wanted to be able to leverage expertise. Boat building is a classic example: we didn’t want to become experts in boat building.” Podded simplicity It was from another such consultation that the centre pod emerged. With a catamaran, the designer must decide which hull will house which components, Carlisle explains, citing the fuel tank as an example. “If you put that in one hull or the other, you then have to manage weight distribution for the duration of the mission as the fuel goes down,” he says. “So maybe you put in two fuel tanks, one in each hull, but then you have to plumb them together and you need a way of transferring fuel from one to the other. “A far simpler solution is to have a central fuel tank, and it became the third hull you see now.” Many other details were decided in this way, Ives says, such as how to attach the tubular aluminium superstructure to the deck. He emphasises that they were keen to implement the boat builder’s suggestion, which was the bolted flange arrangement eventually used. The topside equipment gantry the superstructure supports is a combination of GRP and carbon fibre reinforced plastic, the latter being chosen for weight reduction at the vessel’s highest point. Stability is important, both to data collection in normal operating conditions and to the craft’s survival in extreme weather. To prove the boat’s stability, XOcean has carried out extensive wave tank testing on a 3D-printed 1:7.5 scale model at the Lir National Ocean Test Facility at University College Cork. “We have completed a programme of subjecting the vessel to the worst wave conditions we can create, also putting the vessel in its worst orientation, which would be side-on as for any boat,” says Ives. “We were delighted with the vessel’s response. It remained stable in the maximum wave heights that could be generated in the test tank, equivalent to sea state 6.” The 1:7.5 scale of the model was a compromise, he explains. “The smaller we made the model, the larger the relative waves would be, but also then October/November 2018 | Unmanned Systems Technology The sensor mounting post is a thick-walled GRP tube built to to eliminate movement of the sensor head (Author’s image)