Trying to identify a novel I read - probably in the 1950s or 1960s - about a cabin boy on board a sea-going freighter who battles with an alien called "The Ancient" who has taken over the ship. The alien materializes by de-materializing the substance of an ordinary object (such as a carpet).
Almost certainly written for children or younger readers - but with some fairly ahead of their time technical concepts - for example: the implication that the alien entity had been on Earth an extremely long time and the careful adherence to conservation of mass/energy, rather than making its existence and materializations/dematerializations "magical" or "unexplained".
Since I read this book probably 60 or more years ago, I have very little recollection of fine detail - or even how the defeat of The Ancient came about.
I do remember that it was one of the first Sci-Fi books I ever read and it left me with a permanent attachment to the genre.
Originally purchased in UK.
Hardback cover was a picture of "The Ancient" (human form, shoulder length grey /white hair (center parted), dressed in a sweater and slacks, surrounded by some members of the freighter crew, pointing angrily at the story hero - the cabin boy).
Noting that the name I remember is that of the alien (The Ancient) I cannot say for certain whether this was any part of the book title.