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Polish engineer Sebastian Oledzki has invented a UUV with a variable-geometry internally pressurised hull that enables it to submerge, change depth and surface by changing buoyancy using an ingenious length-change mechanism (writes Peter Donaldson). That eliminates the need for ballast tanks, which are pressure vessels that have to be strong and therefore heavy, as well as all the associated pumps, plumbing and auxiliaries. This allows the UUV to be structurally simple and relatively cheap while providing far more payload capacity than similar conventional UUVs, according to Oledzki. Internally pressurised with a gas, typically CO 2 , the hull is flexible longitudinally to allow it to change its length and consequently its internal volume and buoyancy. The gas pressure is adjusted to slightly exceed the pressure of water at the UUV’s depth to prevent it from being crushed. To achieve that, a system of valves allows the gas to be transferred between the hull’s interior and an ambient space. The CO 2 supply is contained in liquid form in a cylinder attached to the hull’s underside. Structurally, the hull consists of a flexible plastic pipe enclosed at each end by a roughly hemispherical cupola to create a watertight vessel. Transverse rings that encircle the flexible pipe at intervals can be moved along a set of longitudinal guide rods by electric, pneumatic or hydraulic actuators to change the length of the pipe. The energy source is intended to be either a battery or hydraulic accumulator to propel the vehicle and power its length-change actuators. It also has a small propeller on a shaft that protrudes from the rear cupola. The concept has been granted patent number US9776694 B2. Variable geometry controls the UUV’s buoyancy by changing the length of its flexible plastic hull (11), which is supported by an external backbone (12) of longitudinal guides (121) and moveable transverse rings (122) (Courtesy of US Patent Office) This UUV is a shape-shifter Unmanned vessels