Cavitation testing in a free surface tunnel presents specific problems in terms of low content of undissolved micro air bubbles in the water owing to the high de-aerating capacity of the tunnel. Such air nuclei are necessary to provide realistic conditions for the inception and extension of cavitation, in particular bubble cavitation at intermediate propeller loading. This paper gives a brief discussion of the problem in relation to some test results obtained in the new free surface tunnel at the KMW Marine Laboratory. A special device built into the tunnel circuit to produce the necessary amount of micro bubbles and its effect on cavitation and propeller forces is described.

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