Abstract: Ship airwake modelling is essential for studying the safe operational envelopes and dynamic processes of shipborne helicopters’ taking off and landing. The carrier airwake modelling was established in the 1980s and incorporated into the widely used MIL-F-8785C military specification. However, the airwake of non-aviation ships is much more complicated than that of carriers. Even though extensive numerical simulations, wind tunnel tests and at-sea measurements have recently been conducted, a practical model for non-aviation ship airwake has yet to be developed. Spectrum analyses conducted by this paper reveal that the non-aviation ship airwake can be decomposed into steady, periodic, and random components, whose formation mechanisms have been discussed. Based on these findings, this paper proposes a practical mathematical model of the non-aviation ship airwake that has been verified through numerical simulations. The model is proven to significantly improve the efficiency of establishing SHOL (ship helicopter operational limitation). Therefore, it is expected to support the research on the shipboard helicopters' taking off and landing safety.