It has been reported that not a few ships were fatally damaged at their fore bodies due to heavy slamming among rough seas. A series of model tests has been conducted to study the mechanism of failure and to evaluate the ultimate longitudinal strength of the fore bodies of hulls by applying forces simulating impact load due to slamming. Three types of structural models of fore bodies were tested they were designed to be similar to a 50, 000 DWT bulk carrier, a 120, 000 DWT ore carrier and a 1, 400 TEU containership with scale ratios from one sixth to one eighth. In all cases, their side shells were firstly observed to yield due to heavy shear stress. Then the yield zone spread throughout the side shells and resulted in a progressive rise of the longitudinal stresses in the deck plating, which might eventually cause buckling. In the case of bulk carriers and ore carriers with U-form sections, the collapse was brought about immediately after buckling occurred in a whole section of the deck structure. In the case of containerships with V-form sections, the spread of yielding throughout the side shells led to the collapse. This paper also presents an analytical method to predict the ultimate strength of the fore bodies of hulls subjected not only to bending moment but also to heavy shearing force. Compared with the experiments, the calculation method is found to be effective to estimate the final stage at which a section of the hull collapses along the whole circumference and it does not carry any more load. The yielding throughout the side shell plays a fatal role because it gives rapid increase to the deck stresses as well as brings about the loss of shear rigidity.