In contrast to the cargoes of earlier times, which consisted largely of sumptuary and priority goods, mid-19th-century maritime trade increasingly involved bulk cargoes of commodities, with steam propulsion and iron construction becoming more important as alternative technologies to sail propulsion and wood construction. The mid-to-late-19th century was an important period of transition, during which expanding economies of scale and these new technologies evolved together in a complex manner within the social context of the Western mercantile system. Underwater archaeology is proving a potent and effective tool for understanding this transition. Archaeologists studying this period cannot always expect to find preserved cargoes, even with ships known to have been used as freighters or transports. With cargoless wrecks in which all or part of the cargo (often perishable) may have been lost after the ship sank, however, analysis of the ship's structure can be as informative about the conduct of trade as it is about ship technology. Two areas discussed earlier as notorious ship traps, the Dry Tortugas and Bermuda Islands, have provided exceptional opportunities for studying maritime commerce of this period.

The Killean

Underwater surveys and documentation sponsored by the NPS in the Dry Tortugas in Florida have enabled maritime archaeologists to identify two basic categories of mid-19th-century shipwrecks there: construction wrecks, the remains of ships that were engaged in bringing building materials to Fort Jefferson between the 1840s and late 1860s, and en-route wrecks, the remains of vessels transporting cargoes through the Straits of Florida.