In Age of Empires, the Heavy Transport is an improved version of the Light Transport, able to carry more units, and with improved hitpoints and speed. It can be built at the dock once the player researches the "heavy transport" upgrade. The unit starts out with 200 hit points, no armor and 10 carrying capacity. Stats cannot be upgraded. This technology is not really needed in most games, but if played on sea maps with many island chains, researching this upgrade is strongly recommended. Most civilizations that can build triremes are able to build the heavy transport.
Yamato Heavy Transports have +30% hit points, totaling it up to 259.