Feature 1 | GEARS AND PROPELLERS


The integrated propulsion manoeuvring system


Propulsion option to reduce fuel cost without compromising manoeuvrability, noise, and vibration performance


Rolls-Royce Hydrodynamic Research Centre; Kåre Krøvel Nerland, Research & Technology Department; and Jörgen Persson, commercial product manager PROMAS.* The last year’s raised fuel prices and


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environmental interest have increased the focus on total propulsive performance and fuel economy of today’s ships. The potential for improving the propulsive efficiency of ships is limited to a couple of percent if only the propeller design is considered, but if the propeller, rudder, and the hull are considered as one propulsion system the potential becomes much larger, for some cases even up to 10%. Over the years a large number of


‘energy-saving devices’ have been invented and tested both in model and full-scale on ships, however none of them are commonly used in ship design of today. The main reason is that most of these devices suffer from different types of technical problems. Some of them are unreliable, some have cavitation erosion problems, and some are too space demanding.


The system The development of


the Integrated


Propulsion Manoeuvring System (IPMS) started in 2004 and included a large number of systematic model self-propulsion tests as well as cavitation tests. During these three years of development a total of 28 single-screw and seven twin-screw configurations were tested and contributed to the optimisation. The IPMS consists of a twisted


full-spade rudder with bulb, a hubcap, and an adapted propeller design.


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new propulsion option is explored in this paper written by Göran Pettersson,


The rudder bulb is welded to the


rudder blade and moves with the rudder. The hubcap is mounted on the propeller hub and rotates with the propeller. Blunt single-screw vessels such as chemical tankers with block coefficients of >0.80 and design speeds of ~15knots give the largest improvement in efficiency. Such ships can gain 4%-6% compared corresponding conventional


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alternative. Compared to more simple standard type rudders the gain can be as much as 6%-9%. Faster and slenderer single-screw vessels such as car carriers can have an efficiency gain of 2%-5% and twin-screw vessels 1%-3%.


Twisted rudder Twisted rudders are normally used at speeds above 22knots-23knots for applications with high propeller loading to avoid or reduce erosive sheet cavitation on the rudder plating. For lower speeds the margin against sheet cavitation can be increased by a twisted rudder design and this will result in an increased cavitation free steering angle range of the rudder. A twisted rudder will also improve the total propulsive efficiency. A well-designed twist adapts the rudder


to the rotation of the propeller slipstream and reduces the local angle of attack on the leading edge of the rudder. This gives a more efficient rudder profile with lower drag and better recovery of rotational energy from the propeller slipstream. A twisted rudder with a modified


leading edge that reduces the angle of attack to the leading edge will be able to handle larger steering angles before the flow separates from the rudder surface and lift is reduced. This is especially important in harbour manoeuvring at low speeds when the rotation of the propeller slipstream may be large.


The PROMAS propulsion manoeuvring system.


Hubcap A hubcap that makes a streamlined transition between the propeller hub and the rudder bulb will further reduce hub vortex losses. The hubcap is mounted outside of the standard hub cylinder and acts purely as a hydrodynamic fairing cover that has nothing to do with the mechanics or the hydraulics of the hub. This means that a special hub design is not needed and therefore cost and technical complexity is kept kept to a minimum.


Integrated hydrodynamic design As mentioned previously, the potential for improvements by considering only the propeller is rather limited. If the complete propulsion system is considered and the propeller and rudder is optimised as one propulsion unit the potential for improvements can be substantial. A great deal of attention has to be


made to the interaction between the propeller and the rudder during design of


the integrated propeller rudder Ship & Boat International September/October 2008


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