An unsteady actuator disk model is constructed in order to understand unsteadiness in crashback operation of a marine propeller as a competition between two flows of opposite direction: reversed flow through the propeller and the ambient flow due to motion of the vessel. The large eddy simulation methodology is applied to predict the flow corresponding to forward and crashback modes of operation using a non-dissipative, robust numerical algorithm developed by Mahesh et al. (2004, J. Comput. Phys., 197: 215-240) for unstructured grids. Flow visualization shows creation and shedding of unsteady ring vortices which are correlated to the fluctuation of the thrust coefficient. Results of the model are compared to the LES of the full propeller geometry as well as to experimental data.