Hydrodynamic interactions

Rolls-Royce is studying the complex hydrodynamic interactions between hull, propeller and rudder from several different angles. The accuracy of numerical methods is being improved progressively, and these are expected to contribute more and more to the design progress over the next few years.

An important hydrodynamics research area is the verifying of numerical methods by selective model testing and analysis of full-scale performance. The company says that a new university technology centre that has been set up at the Chalmers University of Technology in Gothenburg, Sweden, is expected to contribute strongly to this in the longer term.

One area of interest is propeller and rudder interactions, and this is of general concern in the marine industry at present. The design of propellers themselves has improved considerably as the various types of cavitation have become better understood. At the same time, blade production methods have improved, so that the shape laid down by the designer is accurately carried through to the full-size blades.

Propeller/rudder interaction leading to cavitation damage over parts of the rudder blade tends to occur on fast vessels with high propeller loadings, for example, some passenger ships, ropax vessels, and so on. This can occur even when the propeller itself is cavitation-free.

The main cause is the amount of energy in the slipstream of the propeller. Flow approaches the rudder with a swirling motion having both an axial and a rotational component, which can induce cavitation on the rudder itself. Cavitation can occur with the rudder centred or when manoeuvring. The phenomenon has been studied and solutions developed, including changes to rudder shape in three dimensions to avoid self-induced cavitation, and a better integration of hull, propeller and rudder hydrodynamics to avoid damaging situations.

The large amount of energy in propeller slipstreams can cause other problems, particularly the energy in tip vortices shed by the propeller. Flow conditions downstream of propellers and rudders are now the subject of close scrutiny, since noise from this source can be fed into the ship?s structure. The frequency is much higher than that caused by pressure fluctuations at blade passing frequency and can be of concern in passenger vessels.

Rolls Royce says that overall, propeller efficiency is now approaching that which is theoretically possible. Considerable strides have been made in developing more efficient propellers in the last 30 years, but the trend is now flattening out. However, the company says that there are large efficiency improvements to be had when hull/propeller/rudder interaction is considered as a whole.

Tunnel bow thrusters were originally developed to make vessels more independent of tugs when berthing. They have since been used in many other applications with success. They do, on the other hand, have well-known limitations. For example, flow across the tunnel mouth reduces the available thrust as the vessel?s forward speed increases. Tunnel thrusters applied to offshore production (FPSO) vessels are coming up against related hydrodynamic limits. These vessels are expected to use dynamic positioning when wave heights are large, and it is found that strong cross flows and aeration can occur under these conditions, degrading the position-keeping ability. Azimuth thrusters are not affected nearly as much, being typically much deeper immersed in rough seas.

There is currently intensive research and development activity in the area of water jets to perfect the next generation of very large units, which will be needed as fast ferries become larger and the high-speed freight market emerges. Good hydrodynamic and mechanical design is vital here. Rolls-Royce, through Kamewa, believes it has the tools and skills to optimise the inlet tract, the pump impeller itself and the nozzle bucket steering and reverse system to give an efficient product that is also tolerant of the extreme load changes which occur when air is drawn into the jet inlet.