In this paper, we discuss design issues in buoyancy engines as the device to vary net buoyancy and to alternate the position of the center of gravity of a glider. The buoyancy engines arrangement contains two tanks located at the fore and end aft part of the hull. Buoyancy engines considered here are those of piston-type. Forces equations which model buoyancy, gravity, and hydrodynamic forces in gliding are derived. Performances of different sizes of buoyancy engines are compared. Operational constrains considering the power consumption of buoyancy engines are also specified. Gliders with rectangular wings of various shape and wing location are then examined in terms of the energy cost for gliding controlled by buoyancy engines.