Transplant — D.B. Reynolds-Moreton

A generation-ship bound for a new Earth plunges through space for hundreds, perhaps thousands of years. Within, the teaching device broken, the true nature of the ship long forgotten and the Captain unseen, Glyn stumbles upon the ancient journal of a young crewman of the third generation of travellers.
What he finds tells of a time on board so much different to the lives that he and his fellow travellers live.

Soon afterwards, a series of malfunctions brings about a revelation as to the true nature of their ship, and only then can their journey truly begin in a bewildering and dangerous new land.