The turbopropeller transfers mechanical energy from the flow of a working fluid to that of a momentum medium. The same transfer can be effected more directly by pressure exchange, that is, through the work of mutually exerted pressure forces at appropriately generated interfaces between the two flows. The main drawback of pressure exchangers is that they are of necessity nonsteady-flow machines, highly time-sensitive and not easily controlled. This difficulty is overcome in the “bladeless propeller,” a pressure exchanger whose flow processes, although nonsteady in the frame of reference in which they are utilized, admit a frame of reference in which they are steady. Available theories and experimental results indicate that this device promises to combine an attractive efficiency with advantages of compactness and mechanical simplicity.