Cycloidal Rudder and Screw Propeller for Very Manoeuvrable Combatant

SCHIFFSTECHNIK Paper to the International Symposium WARSHIP 2001 FUTURE SURFACE WARSHIPS The Royal Institution of Naval Architects Institute of Directors, London 20th to 21st of June 2001 Cycloidal Rudder and Screw Propeller for Very Manoeuvrable Combatant Authors Dr.-Ing. Dirk Jürgens, Naval Architect, Head of Research Department, Voith Schiffstechnik GmbH & Co. KG, Heidenheim, Germany Dipl.-Ing. Torsten Moltrecht, Naval Architect SNAME, Project Department, Voith Schiffstechnik GmbH & Co. KG, Heidenheim, Germany Authors' Biographies r Dirk Jürgens studied Naval Architecture and made a PhD-work on numerical hydrodynamic. Afterwards he took responsibility of special manoeuvring and propulsion concepts at Blohm + Voss / Jafo Technology located in Hamburg. Since 1999 he is Head of Research Department at Voith Schiffstechnik GmbH & Co. KG. r Torsten Moltrecht is a Naval Architect at Voith Schiffstechnik GmbH & Co. KG. He is responsible for projecting VOITH SCHNEIDER Propellers and VOITH CYCLOIDAL Rudders. In 1995 he concluded his Naval Architecture and Ocean Engineering education at the University of Berlin. sv Moltrecht - 2002-04-09 62005/M Page 1 SCHIFFSTECHNIK SUMMARY Based on the VOITH SCHNEIDER Propeller (VSP), which has been a hallmark of maximum manoeuvrability, minimum magnetism, least waterborne noise and best shock resistance for special mine counter measure vessels for decades, the VOITH CYCLOIDAL Rudder (VCR) is under development. It is a new propulsion and manoeuvring system for all ships requiring maximum manoeuvrability over the entire speed range. During slow speed operation and manoeuvring the VOITH CYCLOIDAL Rudders operate in active mode similar to two bladed VOITH SCHNEIDER Propellers. This enables precise, quick and safe manoeuvres and best fuel economy. At higher speeds, the two blades of the VOITH CYCLOIDAL Rudder operate like a conventional tw rudder while the conventional propeller drives the vessel. The lower drag resistance of the in VOITH CYCLOIDAL Rudder increases the cruising efficiency of the vessel. In outline, the advantages of the VOITH CYCLOIDAL Rudder for warships: r Low resistance rudder for high speed operation. r Improved manoeuvrability in comparison to conventional propulsion arrangement. r As VCR is main propulsion for low speeds, CP-propellers may be replaced by FP-propellers. r Redundancy of propulsion and steering (take home capability) r Roll stabilisation even during stand-still of vessel is possible. r High shock resistance, low magnetic signature, low radiated noise levels r Ideal complement to advances propulsion systems sv Moltrecht - 2002-04-09 62005/M Page 2 SCHIFFSTECHNIK 1 HYDRODYNAMIC PRINCIPALS OF CYCLOIDAL PROPULSION SYSTEMS The idea of this unique propulsion and manoeuvring system was born by the Austrian engineer Mr. Schneider in 1926. In the following a short explanation of the hydrodynamic principle will be given. The physical principle of the thrust generation by a VSP is comparable to a fish’ fin or a bird’ wing s s action. They are also producing simultaneously thrust and steering forces. Animals with such movements have the optimal adoption to their living environment. Fig 1. On a cycloidal propulsor (VSP and VCR) the blades project below the ship's hull and rotate on a rotor casing about a vertical axis, having an oscillatory motion about its own axis superimposed on this uniform motion. The blade’ oscillating movement - a non-stationary process in hydrodynamic s theory - determines the magnitude of thrust through variation of the amplitude, the phase correlation determines the thrust between 0 and 360 degrees. Therefore an identical thrust can be generated in any direction. Both variables - thrust magnitude and thrust direction - are controlled by the hydraulically activated kinematics of the propeller, with a minimum of power consumption. Consideration of the processes on each blade position during one revolution provides the simplest explanation of the blades velocities and the resultant hydrodynamic forces. 1.1 ACTUAL PATH OF ONE CYCLOIDAL PROPULSOR BLADE (CYCLOID) Fig 1.1. Cycloidal path sv Moltrecht - 2002-04-09 62005/M Page 3 SCHIFFSTECHNIK By superimposing the rotary movement of the rotor casing on a straight line perpendicular to the rotational axis (to represent the movement of the vessel), the blade of the cycloidal propulsor follows a cycloid. The rolling radius of the cycloid is equal to ? x D/2 and the forward motion of the propeller during one revolution is therefore ? x D x p. 1.2 VELOCITIES ON CYCLOIDAL PROPULSOR BLADE O propeller centre of rotation N steering centre Pn oscillating centre of the blade u circumferential velocity ve speed of advance w resultant velocity ? = ve / u advance coefficient D blade orbit diameter Fig 1.2. Velocities on cycloidal propulsor blade for no thrust condition For the no thrust condition of the propulsor (the hydrodynamic lift is zero) the blades are set in such a manner that at each point the velocity w, resulting from the circumferential velocity u and the forward velocity ve, is directed towards the profile axis (zero lift). This basic law governs the motion of the blades: The geometric triangle NOPn is similar to the velocity triangle uve w for all blade positions. The perpendiculars to the profile axes for all blade positions during one revolution must meet at one point, “ the steering centre N” During thrust . generation the steering centre N is always displaced at the right angles to the resultant thrust direction by the dimension ON from the centre of rotation O (eccentricity). For the no thrust condition N’ coincides with N. (See Fig. 1.3.) The ratio of the distance ON to D/2 corresponds to the ratio of forward velocity ve to the circumferential velocity u, “ advance coefficient ?” As long as the propulsor generates no thrust the . the advance coefficient is identical to the pitch ratio. sv Moltrecht - 2002-04-09 62005/M Page 4 SCHIFFSTECHNIK 1.3 FORCES ON THE CYCLOIDAL PROPULSOR BLADE u circumferential velocity ve speed of advance w resultant velocity α angle of attack O propeller centre N steering centre NN´ displacement of steering centre A hydrodynamic lift W induced and profile drag R resultant hydro. force Fig 1.3. To generate thrust the propulsor blade profile has to be turned against the blade path by the angle a by moving the steering centre from N to N’ The ratio ON’to D/2= ?o is the pitch ratio of a cycloidal . propulsor. Through this angle of attack a hydrodynamic lift will be generated at right angles to the resultant velocity w, i.e. perpendicular to the cycloidal path. The magnitude of the hydrodynamic lift depends on the angle of attack a and the resultant velocity w. 1.4 THRUST GENERATION BY THE CYCLOIDAL PROPULSOR O propeller centre N´ steering centre Fig 1.4. The hydrodynamic lift varies during the blade’ revolution due to the “ s non-stationary”condition of the blades. Integration of the components of the lift forces created over the entire propulsor circumference shows: - the lift components acting in the direction of motion result in the propulsor thrust - the lift components acting at right angles to the direction of motion cancel each other out. Consequently only the lift forces acting in the direction of motion generate thrust.