Ferry eight across when the boat holds four
Asked at Akuna
A group of eight people must get across a river using a single rowboat that seats at most four at a time. Everyone can row. The one snag: the boat cannot come back on its own, whenever it needs to return to the near bank, at least one person must be aboard to row it back.
Count each time the boat travels from one bank to the other (in either direction) as one crossing.
What is the fewest number of crossings needed to get all eight across?