The author formerly presented an efficiency calculation formula of water turbines. But, the formula involved somewhat complicated calculation. So a new simple formula for the case of propeller and Kaplan water turbine was deduced to avoid this complication. The theoretical difference between former and latter formulas lies in the calculation of wake energy. In the former, wake energy was estimated by inlet velocity to runner, and The latter case by outlet velocity. The assumption to deduce the former formula for Kaplan turbine was also investigated in detail, because there was no actual basis to justify this assuption. By investigation of actual example, it was found that the former assumption is closely equivalent to assume the incidence angle of aerofoil element at the representative radius is constant in the case of Kaplan turbine.