Sandbags on the monitors’ decks
The ironclads withdrew at quarter past four in the morning after 185 shots, having been seen to be struck many times. In the last twenty-four hours the Confederate batteries had fired 983 rounds and the Federals 691. The rest of the day the fleet was quiet, its crews laying sandbags on the monitors’ decks — the fort was dropping mortar shells on them from above.
Journal of Operations in Charleston Harbor, OR I, 28, Pt. 1