Exploratory measurements of cavitation nuclei in the wake of a ship model

Cavitation is an important phenomenon influencing the performance of ship propellers and rudders. It is well known that if substantial cavitation develops on one of these components, it can lead to performance loss, noise and vibration hindrance or erosion of the surface material. Consequently, ships and propellers are designed in such a way to minimize the likelihood of excessive cavitation occurring on the propeller(s), rudder(s), etc. At MARIN, cavitation tests are performed regularly in the Depressurised Wave Basin (DWB). The DWB is a towing tank along which a ship model is towed at the appropriately scaled velocity, propeller loading, ambient pressure and wave conditions. However, in order to achieve the correct cavitation behaviour, it is also necessary to control the water quality [1,2]. Water quality in the context of cavitation experiments means: 1) dissolved gas and 2) free-gas content in the water (i.e. the size distribution and concentration of cavitation nuclei). The dissolved gas content is measured and controlled in the DWB. The cavitation nuclei are micrometer-sized bubbles present naturally in large numbers in the sea, but in insufficient numbers in a towing tank. To achieve adequate scaling, they need to be added to the water.