What is "vampirates"

Vampirates is a series of books by British author Justin Somper, about a set of twin children, Connor and Grace Tempest, who get separated at sea and are picked up by two very different ships. The term Vampirate refers to a vampire living on the pirate ship, the Nocturne. Later changed to "Nocturnal" to differentiate themselves from rebellious and uncontrolled Vampirates.