Voyage in time to an 18th-century pirate ship; The Crime of Martin Coverley, by Leonard Wibberley. New York: Farrar, Straus & Giroux. $8.95.

Time travel is a popular device in fantasy novels. Sometimes people return to yesteryear to meet their ancestors or famous personages or just to see a physical setting in some earlier form. And their journey into the past may vary from an act as simple as walking through a door to the conjuring up of a mystic force with a magic talisman.

"The Crime of Martin Coverley" is a tale that falls within this tradition. The story opens in Key West, Fla., with narrator Nick Ormsby describing his arrival at his uncle's house, where he has gone to live after his father's death.

His uncle is kindly, but strange, witha preoccupied air that makes him appear aloof. The house, too, is strange, almost spooky, with a strong sense of the past, and the Ormsby heritage is briny, for many members of the family have been sailors. An uncomfortably large number have been lost at sea. The hint of a curse lingers in the house, something Nick wishes to ferret out, and his uncle wishes to let lie.

Nick soon gets the beginning of an answer. On a night when his uncle is away , he maintains a vigil in the study, thinking to confront a visitor whose appearance he has earlier suspected. His visitor turns out to be none other than Martin Coverley, the black sheep of the family, who died in 1721. As if this were not surprise enough, Coverley summons him and his uncle to a rendezvous with him on Shark Island.

When the meeting takes place, Nick finds that his uncle has vanished and that Nick himself has been somehow transported back 2 1/2 centuries, to a ship captained by Martin Coverley himself. Eventually his own identity is lost in that of Nicholas Coverley, Martin's nephew, an apprentice seaman.

Their adventures take Martin and Nicholas from merchant ship to pirate vessel , and finally to a search for the King Solomon's fabled mines.

It is Martin's actions as a pirate that offer the key to his reappearance in our time, and his desire to seek out Nick Ormsby in particular. Coverley has a crucial choice to make, and Nick's life hangs on his balance. The narrative is full of authentic nautical and historical detail that sparks our interest and imaginations. In all of this, the time traveling itself is never really explained. It is a tribute to Wibberley's skill as a storyteller that no explanation is needed.