Shock factors are conventionally used to designate the effect of an underwater explosion on a target. The concept of the effective shock factor was introduced by Rajendran and Narasimhan in a paper entitled ‘A shock factor based approach for the damage assessment of plane plates subjected to underwater explosion’, published in the Journal of Strain Analysis in 2006 (volume 41, issue 6, pages 417–425) for predicting the elastic and yield response of circular and rectangular plates; it was revealed that the coupling factor drastically influences the effect of shock factor on the response of plane plates. It will be of immense use to the designer if effective shock factors are introduced for predicting the inelastic damage of target plates, as shock factors notionally present a pessimistic picture about the damage scenario. The current investigation introduces the concept of effective shock factors to the existing prediction methodologies for estimating inelastic damage of plane plates. Water-backed plate is analysed along with air-backed plates to represent the water-filled side shell of a ship.