Unmanned Systems Technology 027 l Hummingbird XRP l Gimbals l UAVs insight l AUVSI report part 2 l O’Neill Power Systems NorEaster l Kratos Defense ATMA l Performance Monitoring l Kongsberg Maritime Sounder

91 That would need to be balanced against the need to avoid adversely affecting the Sounder’s roll and directional stability, while keeping it lightweight and manoeuvrable. As Stian Michael Kristoffersen, product manager of USV solutions, notes, “The hull was gradually designed around cutting through the water without disturbing it too much. It needed a narrow bow, which would push away the water in front but also allow the water at the sides to glide along the hull as smoothly as possible. “Any imperfections or inconsistencies along the hull would generate microbubbles, leading to aeration in the water, causing noise and attenuation for the echo sounders or other hydro- acoustic sensors. So finding that optimal hull shape was crucial from the outset, and we spent a lot of time with Viking Norsafe to get it right.” However, developing the hull to give good hydrodynamic performance with respect to sensor aeration could have come at the expense of directional stability, since reducing hydrodynamic ‘disturbances’ along the hull might mean eliminating control surfaces. To that end the underside of the hull has two fixed fins, one at either end towards the rear, to provide directional stability far away from the payloads near the centre of the Sounder. Much of Viking Norsafe’s design analysis was based on CFD simulations, which were produced by fellow Norwegian company CFD Marine. “They have considerable experience and computer facilities geared towards modelling and analysing hydrodynamics,” Kristoffersen notes. “Now that the Sounder has been thoroughly tested in water, we’ve found it to perform very closely and accurately to CFD Marine’s software simulations.” In addition to hydrodynamic performance, the hull needed to be lightweight enough to enable manoeuvrability and easy handling, while remaining strong enough to support 20 days’ endurance at sea. “Viking Norsafe come from a background of building lifeboats, rescue boats and military RHIBs, which have very high demands for strength and reliability,” Kristoffersen explains. “That part of their experience went a long way towards informing the material composition and mechanical structure of the hull.” The hull consists of a two-layered ‘sandwich’ structure, with an outer hull, an inner liner hull and foam filling the space between them. The outer and inner hulls are made from fibreglass for a smooth and lightweight hydrodynamic finish, and the foam is polyurethane. “The foam adds buoyancy throughout the craft, and also helps with strength – the three layers are very stiff when combined together,” Kristoffersen notes. “The outer hull is just over a centimetre thick, and even if something penetrates that it would still have to go through the foam and inner liner for there to be any water ingress inside the USV.” The inner liner is also designed with extra foam in the bow, to provide extra protection in the Sounder’s forward direction, with a 40-50 cm foam-filled gap between the outer and inner hull about the tip of the bow. For the rest of the hull, the foam is roughly 1.1 cm thick. Overall, the hull’s dimensions come to 8 m in length and 2.1 m in width, with a draft of 0.7 m. In operation, the overall height of the Sounder is about 4.4 m. However, the mast near the back of the USV rests on a pair of hydraulic dampers that allow it to be pulled back and laid down on the aft deck, and that takes the vehicle’s height down to 2.3 m. This ability to make the Sounder more ‘compact’ allows it to be fitted in a standard shipping container for ease of transport to harbours or other bodies of water. After unpacking it, the mast Kongsberg Maritime Sounder | Digest The hull needed a narrow bow, which would push away the water in front yet allow the water at the sides to glide along the hull Unmanned Systems Technology | August/September 2019 The Sounder USV has been developed primarily with hydro-acoustic stability in mind