Smalls chose this hour deliberately. He had been on Charleston Harbor long enough to know when the sentries were most alert and when they were not. He was not guessing at the rhythms of the harbor’s defensive routines; he had observed them from inside those routines for months. The timing of the escape was part of the competence.
The darkness also mattered to the specific method of the crossing. A figure on the deck of a vessel, wearing a coat and hat associated with the vessel’s captain, is less clearly identifiable from a fort’s walls in darkness than in daylight. The signals that identified the vessel as the Planter were auditory — steam-whistle sequences — not visual. The system of identification had been designed for a harbor that operated at all hours, and Smalls was exploiting the specific characteristics of that system: the signals worked in the dark; the visual inspection of the crew did not, or at least not from a distance.
The interval between leaving Southern Wharf and arriving at North Atlantic Wharf to pick up the families — the time of maximum exposure before the crossing even began — was the period during which the plan could most easily have unraveled from an accidental encounter with a Confederate patrol vessel or a night watchman who asked the wrong question. That it did not unravel is a function of the timing, the familiarity of the vessel, and whatever decisions Smalls made in real time about speed and course that are not fully documented in the historical record.