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

24 Dossier | XOcean XO-450 can be pulled by a pick-up truck, and it can be launched from and recovered to the trailer on a slipway, a process that takes about 20 minutes. It also has lifting/ towing eyes that allow it to be launched and recovered by crane from a ship or towed into an operating area. The whole system also fits into a 20 ft shipping container for international transport. Stable platform Needing a very stable platform for the sensors, XOcean designed two slender hulls that allow the boat to cut through small waves. As the slender hulls lack room for carrying bulky equipment, the XO-450 has a central pod for this purpose. The primary sensor, such as a bathymetric sonar or an acoustic fisheries echo sounder, is carried on the end of a rigid vertical post that emerges from the pod. Different sensor heads are mounted on the bottom of the post using aluminium adapters. There is no requirement for quick sensor changes in the field, as missions are planned months in advance. Any changes that are needed are carried out between missions as part of a full commissioning procedure, because the installations have to be assembled and measured very accurately. CTO Andrew Carlisle explains, “For instance, you have to measure the position of the sonar head relative to the motion reference unit and the GPS receivers, and you have to know those to sub-millimetre accuracy.” A secondary sensor can be deployed and recovered on a cable through an aperture in the rear of the pod by an electric winch, and there are also mounts along the pod’s keel for additional sensors and other devices. The undersides of the catamaran hulls are left clean, to avoid any imbalance in hydrodynamic drag that might have to be compensated for and because the hulls are points of contact with the trailer. The hulls, centre body and sonar post are joined by cross-beams and covered by the solar deck, which consists of four removable panels that sit on a skeleton frame that is semi-permanently bolted to the hulls and cross-beams. The deck takes no structural loads, and removing the panels provides access to the hulls, which also have hatches to provide maintenance access to internal components. The deck panels carry solar arrays, while a tubular aluminium superstructure supports the gantry that holds the navigation, comms, weather monitoring, lighting and camera equipment. Not wanting to experiment with structures and materials, the company drew on well-established and regulated practices from the boat-building industry. The structure is made from glass fibre- reinforced plastic (GRP), which consists of a polyester resin reinforced with unidirectional E-glass fibre, while the most critical structural elements are the cross- beams that join the hulls together and support the sonar post. Widely used in glass fibre reinforcement for plastics, E-glass provides high strength and stiffness with low weight. Before it was first spun into fibres, E-glass was used in electrical insulators, hence its name. The design work was carried out by the XOcean team, led by Carlisle and including Kevin Harnett and Emma Ives, using a combination of manual calculation and Fusion 360 3D CAD software. First-principles engineering Carlisle emphasises that they took what he describes as a first-principles engineering approach to the structure, explaining that there was no need at that stage to finesse the details with techniques such as finite element (FE) modelling, instead looking at whether critical components such as the cross-beams had enough strength. “A standard engineering calculation is sufficient,” he says. “An example of what you might use FE for is to precisely understand the stresses through the joints at the edges of the cross-beams, and there’s been no need for us to go into that level of detail.” The reason for this apparently casual approach is that the boat’s small size is very much on the designer’s side. “By the time you’ve made it practical to build – the walls need to be a certain thickness just so that you can make them – everything ends up much stiffer and stronger than you need. For example, our cross-beams have a factor of safety of 10 in terms of strength,” Carlisle explains. “You could make the vessel lighter, but it would not be practical to build it.” Nor did the company have to October/November 2018 | Unmanned Systems Technology Narrow, wave-piercing bows are designed to cut through rough water. Panels on the deck support solar panels that are part of the XO-450’s hybrid power system. (Courtesy of XOcean)