Occupants of naval special warfare (NSW) high speed planing boats experience repeated impacts with amplitudes reaching 10 to 15 g in the vertical direction. Ensign, et al. (2000) found compelling evidence of a significant injury problem in a self-reported study of NSW crewmen subjected to repeated impacts on high speed planing boats. Two sea trials were performed in a Mk V Special Operations Craft (SOC) using shock isolated and non-isolated seats. The sea trials provided data with which the relative performance of conventional injury assessment methods used by the naval architecture community can be evaluated. This evaluation includes the assessment of alternative methods for assessing impact injury from dynamic measurement. There is no current injury criterion that is fully acceptable for predicting injury from impact exposure during high speed craft operations. ISO 2631 Part 5 is currently the best injury criterion available. However, the existing injury reference values may result in a substantial limitation of high speed planing craft operations. This paper also presents a numerical model, based on a Madymo (TNO, Inc.) lumped mass seated human mode, to simulate the response of a human under typical high speed planing boat impacts. Using this model, a simplified dynamics model was developed for seated humans to allow evaluation of different impact conditions. The results of the simplified model were compared with the results of the neural net dynamics model specified in ISO 2631 Part 5. For the acceleration impacts above 4 g, the Madymo model produced substantially higher accelerations in the lumbar spine than those calculated using ISO 2631 Part 5. This suggests that the use of the neural net dynamics model of ISO 2631 Part 5 may not be warranted outside of the validated region of +/- 4 g.