Some impact dynamics topics motivated by the ship-bridge collision process are analyzed in this paper. It is shown that (1) in order to reduce the impact force a flexible (i.e. with low structural dynamic generalized wave impedance) crashworthy device should be applied. (2) The work done by the impact force is transformed via wave propagation into the internal energy (deformation energy) and kinetic energy. The larger the irreversible part of the former is, as well as the greater the overall role played by the crashworthy device is, the better the buffer role played by the crashworthy device will be. Moreover, how to make the ship as soon as possible to end the collision and carry away as much as possible the remaining kinetic energy should be a key point. (3) Viscous dissipation of energy can buffer the collision process, prolong the dissipation duration, and help the crashworthy device to play an overall role, and create the condition for turning ship away to end the collision. Thus the scientific design idea of crashworthy device should be based on (i) impact flexibility in terms of low structural wave impedance, (ii) viscous dissipation in terms of damping collision process, (iii) let the crashworthy device plays an integral role as soon as possible so that the intense impact concentrated force could be changed into a weaker distributed load, and (iv) let ship turns away as soon as possible, and thus takes away the remaining kinetic energy as much as possible. As an engineering application example, such design idea is reflected in the new flexible crashworthy device consisting of hundreds of steel-wire-rope coils. Its validity has been confirmed by the engineering practice and the real ship collision tests.