A detailed description of a series of experiments conducted by Yarrow and Co., Poplar, in order to determine the most suitable screw propeller for a torpedo boat built by them for the Admiralty. Experiments were conducted with twenty-five propellers, varying in diameter, pitch and blade area. Each trial was conducted progressively in order to obtain curves of efficiency for each propeller at different speeds. Thin elastic blades gave better results than rigid blades, it was also noted that the most efficient propellers had the least variation in slip at different speeds. The speed eventually obtained by the trial vessel was the highest ever recorded by a boat of this type, namely 21.9 knots and the trials were considered to be a valuable contribution to the general knowledge on the subject.