This paper presents a study of the nature of the unsteady forces in the field of a propeller rotating in the vicinity of an appendage. The propeller is assumed to be one of a high aspect ratio while the appendage, although of finite width, is assumed to be infinitely long, and thus the problem is reduced to a study of the unsteady flow field around two flat plates. An essential feature of the analysis is that the mutual interference effects of propeller blade and the appendage are taken into account. The method of solution employs the technique of the substitution vortex which yields explicit analytic expressions for the quasisteady, apparent-mass and wake forces for both the propeller and appendage. These equations provide the magnitude and variation of the total forces as functions of tip clearance, distance, and relative size of appendage and propeller.