Celtic Ring

In the 1990s, a sailor hands his logbook to a Swede named Ulf living on his sloop in Denmark, then disappears. Log entries hint of a Druid revival, with allusions to Scotland's Caledonian Canal and a grisly cult. Ulf wants answers and asks friend Torben, a Celt/IRA buff, to join him on a Scotland cruise to get them, but doesn't reveal his motive. Sound familiar? Translated from Swedish, this is a fine cruising yarn in which The Riddle of the Sands is a clue.