An airship ballast system includes an engine driven propeller and a cooling tube that is positioned coplanar with the propeller and outside its tip path. One end of the cooling tube is connected to receive exhaust gases from the engine while the other end is connected to a ballast tank. In operation, exhaust gases from the engine are cooled as they transit the cooling tube, and water condensed from the cooled exhaust gases is pumped to the ballast tank to maintain a ballast for the airship.