Possible explanation: a navy ship runs on rotation of three watches, each in turn taking control of the ship for 8 hours. Thus a ship ought to have a crew of 3x

Possible explanation: a navy ship runs on rotation of three watches, each in turn taking control of the ship for 8 hours. Thus a ship ought to have a crew of 3x the manpower needed to run it. So by "crewed with about 1.5X the number of sailors that were actually needed", GP means a very understaffed vessel.