Foam core sandwich composite panels repeatedly slammed onto the body of calm water indicate greater accumulation and progression of damage accompanied by lower lifetimes as a function of increasing slamming energy or lower deadrise angle. E-N (slamming energy vs. lifetime) curves showed an exponentially decreasing trend with extensive scatter in the data. E-N curves also differed dramatically when compared with the conventional S-N fatigue life curve. A significant reduction observed in fatigue life of slammed specimens as compared with the non-slammed specimens was used as a basis for the development of a life assessment methodology. Catastrophic failure under slamming resulted from a major crack formation near the chine or the keel depending on the deadrise angle, however, during post-slamming fatigue, failure always occurred near the keel. Core tearing along the interface and core shear were observed to be the dominant modes of failure, while facesheet damage activity was largely absent prior to catastrophic failure.