A barge, in general, is a flat-bottomed boat used for transporting heavy cargo. During the industrial boom in St. Thomas of the later 1800s, barges were common within the harbor and near the wharfs of the many coaling and shipping operations on Hassel Island.
Often, large vessels were not able to berth at the wharfs on Hassel Island or in Charlotte Amalie because of shallow water, and thus had to anchor in the harbor. Barges would then be utilized to lighter* coal, freight, and passengers between ship and shore.
*Lighter: a nautical word meaning to transport by barge
Take a look through these historic postcards, which illustrate coal barges in the St. Thomas Harbor: