Nathaniel Portlock was an American. He was born in Norfolk, Virginia, about 1749, into a family which had a shipping business. He and his half brother went to sea on one of the family's brigs. In 1771, when employed as sailing master on a ship in the West Indies trade, Portlock was threatened with being pressed on to HMS St. Albans. Portlock volunteered and within two months was rated as a midshipman. After two years on the St. Albans he transferred to spend time in the Ardent (19 months) and in the Ramillies (11 months). By choosing to stay with the Royal Navy, Portlock alienated his family in America and eventually lost land and property bequested to him. Like Bligh, Portlock had sailed on Captain Cook's third voyage.
There are two versions of Nathaniel Portlock's logbook of the Assistant and neither is complete. This one ends on March 5, 1793, and the other on January 23, 1793, which is one of the reasons I chose this one. But the main reason was that the other version, although it has hourly remarks, is nearly indecipherable, both because of the small handwriting and because the pages were reproduced at less than life size, so that it impossible to enlarge them to any degree without pixelization.
Transcribed from images of the original, held at the National Archives, Kew, England.