or take a page from spell jamemer. A wizard of a certain level depending on size, compulation, speed, etc... sits in a chair in a special room. he then expends raw magical energy to power the vessel. Like hes a battery, obviously a more higher powered wizard would be able to propel the vessel at greater speeds and for greater distances. Much like a Astropath in Warhammer 40K guides a vessel through the warp. The wizard sits in a near artifact magical chair that is hardwired into the vessel, maybe conected to the vessels propulsion unit (propellers, paddle wheel, sails, gunpowder ramjet, whathaveyou) via a magically conductive material (again rare and expensive, use your imagination what this could be, from a natural vine,root system to metals and minerals. Maybe a helmet that is linked to a device attached to the propulsion unit via a radio type system of magically conductive gems) theres all kinds of things that could work. As for the giant turtle dragons and severing their guy lines, well the guy lines used by the beasts to tow the ship would probablly be a special 6 inch thick metal chain or cable and dern near impossible to sever.