In accordance with the present invention a start/stop system is provided for a machine that drives an electric generator by moving buoyant modules on a duty-cycle pathway through a bi-level water tank. Included is a valve mechanism that maintains different water surface levels in the bi-level tank, and also establishes different spaces within the bi-level tank. Additionally, grips are included for selectively holding individual modules in respective spaces. To stop the machine, water is drained from the bi-level tank, and grips are activated, to hold modules in their respective spaces. To restart, water is introduced into the spaces and the grips are deactivated to release the modules for operation.