Abstract A new algorithm to predict partial sheet cavity behavior on hydrofoils is proposed. The proposed algorithm models the unsteady partial cavitation using Boundary Element Method (BEM). In the proposed method the spatial iterative scheme is removed by means of a new approach determining the instantaneous cavity length. This iterative scheme is required in conventional algorithms to obtain the cavity length at each time step. Performance of the new algorithm for various unsteady cavitating flows with different reduced frequencies, cavitation numbers, hydrofoil geometries and inflow conditions are investigated. Comparison between the obtained results using the proposed method and those of conventional ones indicates that the present algorithm works well with sufficient accuracy. Moreover, it is shown that the proposed method is computationally more efficient than the conventional one for unsteady sheet cavitation analysis on hydrofoils.