It is well known that the flow abaft of ships is both spatially and temporally varying. This variability arises from the “prior” or upstream history of the flow produced by the action of viscous stresses and hull-pressure distribution acting on the fluid particles as they pass around the ship from the bow to the stern. Thus the blade sections “see” gust patterns which over long term have mean amplitudes but from instant-to-instant change rapidly with time because of the inherent unsteadiness of the turbulent boundary layer.

Our knowledge of the distribution of flow in the propeller disc is almost entirely based on pitot-tube surveys conducted on big (≈ 6 m) models in large towing tanks and in the absence of the propeller. These are termed nominal wake flows. As is well appreciated, pitot-tube measurements provide only long-term averages of the velocity components at various angular and radial locations in the midplane of the propeller. These measurements depend upon the calibration of the pitot-tube in uniform flow whereas the wake flow radially and tangentially has the effect of shifting the stagnation point on the pitot-tube head, a mechanism not operating in the calibration mode. Thus there is a systematic error which is, to the authors' knowledge, not generally corrected. Moreover, wake-fraction (and thrust-deduction) calculations based upon tests with the same model in several large model basins and upon repetitive tests with the same model in the same large model basin, have shown remarkably different results. A similar scatter was also found in results of wake surveys.