A simple closed analytical model (20) of the spectrum of the fluctuating thrust component of a non-cavitating marine screw propeller operating in a non-uniform field of water flow velocity was constructed. The model provides the necessary data for the evaluation of the far-field spectrum of the rotational noise of an acoustically free screw modeled by a dipole, according to (1) and (2). A number of assumptions were built into model (20) and more details were omitted during construction; it makes him naive. The goal of this approach was a mathematical model, which, even without numerical realization, will enable the assessment of the influence of individual parameters of the screw and the flow velocity field. Hence the purpose of the model: use in the early stages of screw design, not exact calculations of rotational noise. In the important special case of a radially similar inflow velocity field, the model takes the form (23), according to which the spectrum of impulse fluctuations has the same shape as the spectrum of the inflow velocity field, with the fact that only harmonics whose orders are multiples of the number of wings appear in the former. In this case, the contribution to the amplitude from the wing elements at various radii is proportional to the radius, and to the length of the profile and the amplitude of the inflow velocity at that radius.