A device with vertical buoyancy tanks fitted to its upper part is provided with rotors which generate horizontal and vertical propulsion, and which stirs the liquid uniformly as it moves freely through a liquid storage tank, thereby preventing the accumulation of sludge. The vertical buoyancy tanks stabilize...http://www.google.de/patents/US4786186?utm_source=gb-gplus-sharePatent US4786186 - Stirrer for use in liquid storage tanks, and a method for determining its position in the tank
