A lightweight, ergonomically beneficial, hydrodynamic, and volumetrically efficient hybrid pressure vessel having at least two longitudinally extending, semi-cylindrical sections with flattened rib portions at a common interface between the sections. Additional longitudinally extending sections may be employed to provide additional internal volume. One or more apertures extend through the ribs to provide communication between sections. The pressure vessel comprises a cast metal material, optionally including exterior reinforcing structure for containing internal pressure.