The lift of the hydrofoil subjected to a developed travelling bubble cavitation tends to decrease at constant a provided the number of vapour bubbles and consequently the concentration of active nuclei increases. Irrespective of the state of the boundary layer, inception of cavitation depends essentially on the minimum pressure on the profile and the quantum of nuclei, characterised in this case by the susceptibility of the liquid-critical pressure of the weakest nuclei. Discrepancy among the experimental results usually observed when one makes a comparison of well-conducted tests in similitude may be partially influenced by the precise definition of the threshold of cavitation. The concentrations of active nuclei corresponding to which it is possible to obtain spectra obeying similitude are uniquely related to the dimension of the profile. The spectra of nuclei have thus to be adapted to the change in flow velocity and dimension of the profile.