The Josephus Puzzle. A medieval version

The medieval question was usually presented in the following form. A ship, carrying as passengers fifteen Turks and fifteen Christians, encountered a storm, and, in order to save the ship and crew, one-half of the passengers had to be thrown into the sea. Accordingly the passengers were placed in a circle, and every ninth man, reckoning from a certain point, was cast overboard. It is desired to find an arrangement by which all the Christians should be saved.