NOVELTY - HPC (102) has core support (108) at the center and coupled to each end plate (114) and drive axle (104). Vanes (110) are coupled to core support and end plates to promote low drag flow. A bucket area is defined by the core support, two of the vanes and end plates. A vane support (112) is coupled to the vanes at state parallel to the end plates. The vane support defines holes (113) through which fluid can pass through to equalize the pressure in the bucket area.    USE - Hydro-pneumatic cylinder (HPC) used in system for converting buoyancy of compressed fluid into mechanical energy (claimed). Can be used in driving industrial apparatus versus electrical alternator/generator, providing drive power to naval vessels, providing prime electrical power for naval vessels and or naval drilling platforms, providing prime electrical power for ocean/naval drilling platforms and in driving water pumps for hydro-electric dam.    ADVANTAGE - Converts buoyancy of compressed fluid into mechanical energy. Provides mechanical device that can reduce frictional hydrodynamic drag, balance buoyant forces along the vanes and capture the kinetic energy available during gas-to-liquid and liquid-to-gas transfers. Can be used in any environment as long as appreciable gravity is available. HPC uses either ultra-low-friction magnetic bearings or ultra-low-friction air bearings to support dive axle and reduce friction-based energy losses. HPC has operationally and neutrally buoyant design that reduces relative weight of HPC. Enables to reduce gravitationally-based frictional pressures on HPC's support bearings and reduce the effect of weight on the bearing support structures.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a system for converting buoyancy of compressed fluid into mechanical energy.    DESCRIPTION OF DRAWING(S) - The drawing shows the drive side orthogonal view of the HPC.    HPC (102)    Drive axle (104)    Core support (108)    Vanes (110)    Vane support (112)    Holes (113)    End plates (114)