Sophisticated they may be, but the designs of today's LNG, LPG and ethylene carriers are in reality based on the simple parameters of the ideal gas laws governing the relationships between pressure, temperature and volume. In addition, the materials in direct contact with the cargo must be matched with the properties and characteristics of the cargo. The key to the designs is the atmospheric boiling temperature of the cargo, i.e. ambient for fully pressurised LPG, -33 deg. C for anhydrous ammonia, -48 deg. C for propane, -104 deg. C for ethylene and -163 deg. C for LNG. In terms of fleet numbers the total number of gas ships is small compared with oil tankers, general cargo ships or passenger vessels.