Answer and explanation:
Let's be careful with this question, because we cannot simply interpret the adjective "prevailing" in its original sense. Prevailing, by itself, refers to something existing at a particular time, or something that is influential. However, when referring to winds, prevailing has a more specific meaning. A prevailing wind is a wind in a certain area or region of our planet that blows predominantly from a particular direction. This type of wind can be permanent in some places while, in others, it may take place during a certain period of the year. Therefore, the word prevailing in the excerpt suggests that the wind that allowed Magellan's ships to sail with fewer crew members was constantly blowing from a particular direction. That would probably have made sailing easier and faster than usual.
