Determining a ship's propulsive power is a critical stage in the design phase in which the evaluation of the stern plays a crucial role. Different flow regimes can be observed depending on the position and shape of the transom. This paper investigates the wetted-transom flow characteristics and their implications on the 1978 ITTC Performance Prediction Method. In the case of flow separation, such as the wetted-transom flow, the current ITTC-78 procedure does not provide an alternative method. Therefore, two alternative methods were proposed based on the investigations of CFD computations on seven hull forms. The firstly proposed method is a combined EFD&CFD method called the two form factor method. It requires CFD computations in model and full-scale, and it can handle any case of flow separation, including the wetted-transom flow. The second proposed method is an empirical correction formula for the hulls with a wetted-transom flow. Finally, the full-scale speed-power relations between the speed trials and the full-scale predictions from the two alternative methods and the standard ITTC-78 method were presented. It is observed that the two suggested methods considerably improve the correlation between the predictions and the speed trials. © 2022 The Author(s)