The Zephyr is externally similar to smaller attack submarines of Earth. The particular case seems to be Project 877 Paltus (NATO classfifcation: Kilo) Soviet diesel-electric submarines, due to the turned-down aft section. The "sail" superstructure is slightly more similar to nuclear-powered designs, such as Projects 705 (Alpha) and 731 (Akula), bent as opposed to straight.