Would a midshipman be reassigned to a different ship when than the one he was originally on after getting back with a captured ship.
I am reading the Hornblower books and at some point in the first book they capture a boat full of rice, Hornblower is assigned by the captain to sail it back to England, if he hadn’t sank it and if he had actually got back to port would he have to wait for the Indefatigable to return or would he be assigned to a new ship.
Yes, in that time period midshipmen formed part of a ship's company and were assigned to individual ships. IIRC the Indefagitable was operating out of an English port at the time, so Hornblower would rejoin the ship when it turned back up in England. Alternatively, if the ship were reassigned to a different station, he could also rejoin by sailing out to the ship as supercargo in another ship. For example, if his ship were reassigned to a blockade while he was in England, he could sail out with a transport or supply ship.
N.A.M. Rodger's history of the British navy covering this era spends quite a bit of time on social history, and also on manning/victualling requirements. This is the second part of his two part history: http://books.google.com/books/about/The_Command_of_the_Ocean.html?id=xh4aUiwxnW0C