Recent field experiments have shown that inside the Kelvin wake of a moving ship there can be two soliton-like envelopes, which extend to a long distance behind the ship. The source mechanism of these solitons has so far been attributed to the steady advance of a ship. In this note we suggest that ship oscillations, induced by ambient sea waves, provide a more plausible explanation for the initiation. Reducing the two-dimensional nonlinear Schrodinger equation to a simple form, we use existing theories to predict the wave envelopes in the far wake of a travelling ship in heave or pitch oscillations. While available data are insufficient to give a conclusive check of the theory, qualitative relevance is discussed.