Full-scale speed trials of a ship have been questioned for the uncertainty of speed and power measurements especially when the sea conditions differ from the ideal calm water conditions. Such uncertainty has been investigated by utilizing ITTC standard speed/powering trial analysis procedure through Monte Carlo simulations. A case study was conducted for a set of sea trials with 12 sister ships for which sea trial data were available for a range of displacement, water depth, water temperature, wind speed and wave height values. Precision errors were observed as the most influential error source for the whole speed range, even though their effects were more substantial at low speeds. Beaufort scale was observed as the most important elementary error source indicating the need for the best weather conditions for the most reliable sea trail predictions.