Turnock, Stephen and Molland, Anthony
Energy efficiency of ship propulsive systems: rudder-propeller interaction.
In, Ship Propulsion Systems Conference, London, UK,
27 - 28 Jan 2010.
Institute of Marine Engineering Science & Technology30pp.
The rudder of a ship is used to maintain its direction and for changing course. As such the goal of rudder design is to maximise sideforce for control of course while minimising resistive drag. Ship rudder’s are typically mounted downstream of the propeller to take advantage of the higher speed flow in the propeller race. This flow also has a swirl component that results in a complex flow regime arriving at the rudder. Likewise, the presence of the rudder has an influence on the flow speed and direction passing through the propeller. It is therefore recommended that design of the propulsive system incorporates the rudder design. This will ensure that appropriate decisions are made that maximise the net propulsive thrust for a given engine power.
We examine in this paper how the design of the combined propeller-rudder system can be best achieved for a goal of maximising propulsive efficiency without sacrificing the ability to manoeuvre. The design of the rudder requires careful thought as to its longitudinal, lateral and vertical position. The swirl component of propeller race results in a force distribution over the rudder that can give a net propulsive thrust by extracting energy from the flow rotation. Such effects can be used to compensate for the resistive drag of the form and surface area of the rudder itself. Design of rudders are often based on evolutionary principles that will not capture the subtleties necessary to enhance propulsive efficiency. We discuss a hierarchy of computational analysis tools that includes unsteady solution of the Navier Stokes equations, their coupling with propeller blade element momentum analysis and use of surface panel and lifting line theories to understand rudder forces. Such techniques are suitable for design optimization.
Conventionally it is the rudder structural strength and associated cost of construction that limits the types of rudders that can be designed. Recent work on a variety of twisted rudders has shown that there can be significant performance gains. For more radical shapes the use of suitable composite construction will give cost-effective performance while also reducing rudder mass as well. We show through three case studies related to: (1) twisted rudders, (2) cathodic protection and (3) the influence of the rudder on kite assisted ship propulsion; that to achieve improved propulsive performance all that is required is greater care and attention to the rudder design process. Such design detail will more than cover its cost in fuel savings and the resultant reduced engine emissions
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