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The setting level of a hydraulic machine, specially for low head machines, is decided with respect to the possible alteration of the efficiency due to the cavity development. This alteration can easily be noticed by plotting the evolution of the efficiency 'eta' as a function of the Thoma number 'sigma' leading to the so-called 'eta'-'sigma' cavitation curves. ln order to protect the efficiency at the best operating point against the travelling bubbles effects, the IEC norms suggest some measures relating to turbines standards cavitation tests. A prediction method of the efficiency drop due to the outlet travelling bubble cavitation is then so needed to guide the runner geometry design in the first state of the project. The main objective of this paper is the presentation of a cavitating pressure field prediction method in the runner in order to calculate the corresponding efficiency alteration.