Achievements and challenges in cavitation research on ship propellers

Cavitation occurs on nearly all ship propellers. It may lead to expensive problems if not acknowledged in an early design stage. The two most frequently occurring problems are noise and vibration in the afterbody due to cavitation-induced pressure fluctuations on the hull, and cavitation erosion on propeller blades and appendages. Early recognition of these adverse effects is important, not only to ensure compliance with contract requirements, but also because often cavitation has to be controlled at the cost of propeller efficiency. To ensure that the propulsor meets the requirements relating to comfort (vibration and noise) and safe and economic operation (erosion), model scale experiments or computations addressing cavitation are conducted prior to construction. As reliable computational techniques that incorporate all aspects of cavitation that relate to noise, vibration and erosion are lacking, scale model experiments are still the preferred choice.
From experiments in the first cavitation tunnel, built by Parsons in 1895, insights were built up as to how one should model cavitation phenomena to allow for full scale predictions. This has led to the use of medium sized cavitation tunnels in the mid nineteen hundreds and eventually to the large cavitation tunnels of today which allow for complete ship models to be installed in front of the propeller. At the Maritime Research Institute Netherlands (MARIN) propeller cavitation behavior is investigated in the Depressurized Towing Tank (DTT) which allows for the testing of large scale ship models in the presence of a free surface. The DTT is in operation since 1972 and has been modernized in 2001. A medium sized cavitation tunnel and a small high-speed cavitation tunnel are also available. They are primarily used for background research on cavitating propellers and wing type geometries.
Reliable predictions of full scale cavitation behavior have become more important with the ever increasing powers that must be absorbed by a single propeller. This necessitated the careful validation of predictions made by cavitation laboratories through full scale measurements and observations. This paper addresses a number of such validation studies and argues that despite the undeniable success of cavitation facilities around the world in qualitatively aiding in the design process, the reliability of predictions is not yet such that problems are avoided at all times. It must be understood that propeller designers, in order to meet the often contradicting requirements of high comfort and propulsive efficiency, are in need of a more quantitative improvement of prediction capabilities.
Whilst showing areas where successes are scored, the paper also indicates topics where further research is necessary. In brief, cavitation research has allowed for today’s greatly improved design of propellers and afterbodies, resulting in much reduced levels of noise and vibration. Also our understanding of the phenomenological nature of cavitation erosion has greatly improved. Nevertheless, the state-of-the-art is such that several research issues are still outstanding and hamper our capability to predict cavitation behavior. In this respect one should think of hull-pressure fluctuations at orders of blade rate, which are observed to be sensitive with respect to environmental conditions, leading to varying accuracy in predictions. A similar statement holds for low frequency broadband pressure fluctuations. Also worth mentioning are the practical prediction of cavitation erosion due to a lack of detailed understanding of the erosion mechanism and the delay in inception of certain types of cavitation on model scale. Each of the main topics identified above is treated separately, starting with inception, then followed by hull-pressure fluctuations, and finally erosion.