This thesis details the design process that led to the fabrication of a machine that is capable of reproducing large-amplitude motions and freefall slam impacts that are similar to those experienced by the seat deck of a high-speed craft in a wide range of operational conditions. The machine features computer-controlled motor-driven motion of a suspension seat in two directions; emulating the heave and sway of a watercraft. A freefall slam impact can also be simulated as the seat can be dropped onto electrorheological dampers, which are electronically controlled to provide a tunable response force. In this document, first, a background and the driving motivation are presented. Then, defining steps of the conceptual development are discussed. The assemblies and major parts are described in detail. Finally, structural analysis is presented as a combination of conventional stress calculations and finite element analysis (FEA).