system. The propeller should be adapted to the wake field and the rudder with bulb, the rudders twist and bulb should be adapted to the velocity and swirl distribution of the propeller slipstream.
Propeller design To get the full advantage of the integrated propeller rudder system it is important that the propeller design is adapted to the special type of rudder. In conventional propeller design both the tip and root regions of the propeller blades are unloaded to avoid too strong tip and hub vortices. The tip load and the strength of the
tip vortex need to be controlled to keep pressure pulse amplitudes and broadband noise to acceptable levels. The root load and the hub vortex strength, need to be controlled to avoid cavitation in the hub vortex that could cause damages to the rudder and create additional noise. With an integrated propeller rudder
system where the propeller and the rudder is designed as one system, and where it is a bulb present on the rudder, a slightly different design philosophy can be applied. The bulb causes a potential effect
and affects the flow upstream of the bulb. The flow across the root sections of the propeller blades is decelerated at the same time as the contraction of the slipstream is reduced. The variations in the wake field are smoothed giving less variation of flow angles over the root sections of the blades. The hubcap and the rudder bulb will handle any hub vortex and reduce both losses and risk for damages on the rudder. By adapting the propeller design to
the affected flow field and optimising the load distribution of
the blades,
increased propeller efficiency and reduced levels of pressure pulse and noise can be achieved. The propeller design is
still a
compromise between efficiency and pressure pulses, but the potential of the propeller design is improved by the integrated system design giving possibilities for improving vibrations and noise characteristics without trading too much propulsive efficiency.
Rudder design Rudder design is a compromise between high propulsive efficiency and good cavitation performance at
operation and good low-speed harbour manoeuvrability. At service speed a ship normally uses the rudder for course corrections using small rudder angles typically max ±5deg for a conventional rudder. When a ship in transit wants to make a course
correction, it
needs a certain amount of side force (rudder lift). In terms of propulsive efficiency
it is of importance that this side force is created with the smallest possible rudder angle. Any rudder angle will increase the rudder drag and the drag will increase with increased rudder angle. In other words; the lift to drag ratio for small rudder angles should be as high as possible. To achieve good rudder performance,
the choice of rudder profile and its thickness ratio are important. A profile shape that has a high lift to drag ratio enables the rudder to create a large side force at a small rudder angle to a minimum of drag. At the same time the profile should not be sensitive to cavitation and have a large stall angle, which enables a large maximum side force at low speed manoeuvring.
Full-spade vs semi-spade rudder
The IPMS is based on a full-spade rudder, which means the whole rudder blade rotates when the rudder steers. The rudder blade is supported in a neck bearing and the lack of a lower bearing means that the rudder stock diameter needs to be slightly thicker than for a corresponding semi-spade rudder that have a lower pintle bearing. A semi-spade rudder will however
suffer from loss of movable rudder areas due to the fixed rudder horn. This means that a semi-spade rudder of the same area will need a larger rudder angle than a full-spade rudder to produce the same amount of side force. The smaller rudder angle gives a lower drag that compensates some of the losses due to a thicker rudderstock. The better manoeuvrability of
Ship & Boat International September/October 2008
transit
The Kamewa CP–A shows improved load distribution, improved effi ciency, and has increased bearing areas.
full-spade rudder also means that the rudder can be designed with a smaller area that compensates even more of the thickness loss. All together the overall performance difference between full-spade and semi-spade rudders will be small.
Hydrodynamic performance The main effect of the IPMS is increased propulsive efficiency, ie, the power consumption for a certain speed is reduced. The design has been optimised through model self-propulsion tests, CFD calculations, and cavitation tests. Different sizes of rudder twist and bulb diameters have been tested and compared for a number of hull types by means of self-propulsion tests. In total, 28 different single-screw and
seven twin-screw configurations have been tested and all configurations have been directly compared to a conventional rudder with the same dimensions of the rudder blade, but without any twist and bulb. In such way the effect of purely the twist, bulb hubcap, and propeller design have been separated. Generally the largest effects are
achieved on blunt single vessels with a block coefficient >0.8 and with a design speed in the range of 14knots-16knots. Such ships can gain 4%-6% compared to corresponding conventional alternatives
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