The C-GEN is a novel permanent magnet generator aimed at reducing overall system mass in direct drive power takeoff applications. The design of a C-GEN generator requires the combination of electromagnetic, structural and thermal models. Models used in the development of design tools applicable to both rotary and linear C-GEN generators are described in this study. The design tool is verified with the experiment results obtained from a 15 kW prototype. A genetic optimisation algorithm is developed combining the analytical model with economical issues to search for most suitable designs for specific applications. Designs are presented using the optimisation design tool for two marine renewable applications: a wave device called Oyster developed by Aquamarine Power and a tidal current device developed by Scotrenewables.