We have proposed a practical method for calculating the main parameters of a hydroturbine (diameter and range of changes in the rotational speed of the blade runner) which ensure a maximum output of electric power at the hydroelectric power station due to a variable frequency of rotation at specified operating conditions. We have proved the principal possibility of replacing a Kaplan double regulated turbine with a propeller turbine which, when operating at a variable speed at actual operating conditions, is not inferior to a hydro turbine by its energy parameters. Propeller hydro turbines with a variable frequency of rotation are proposed for reconstructing the Irkutsk HPP. The optimal blade angle of the blade runner and the variation range of rotation frequencies for operating with a variable rotation speed were calculated. The operational characteristic of the proposed propeller turbine was determined.