Slave ships carried goods even before they carried slaves in any particular journey. In Europe a slave ship would bring goods or a particular shell, used by Africans, in preparation to trade for slaves in Africa. These goods could either be used to trade with European trade posts within Africa or with Africans directly so the type of goods within the ship's cargo could vary drastically. Usually this was the case due to preventing as much profit loss as possible instead of having a ship sail with an empty hull. In each port the ship would then be refitted for its purpose, whether carrying goods or slaves. The same concept was used when shipping between Africa and the Americas and the Americas and Europe. Slaves would be shipped through the Atlantic and upon arrival in America, the ship would usually be refitted to carry goods to another destination, could be sugar from S.America or Caribbean into N. America or N. American raw goods back to Europe.

"This is important because it paints a fuller picture of the economic motivation for the slave trade for me, potentially. " I'm not certain what you mean here but if you think that the shipment of cargo was profitable for slave ships, it was almost never the case. The goods shipped were to reduce the losses of a ship as much as possible when not transporting slaves. In most instances the ship would not even break even transporting goods, rather they would work at a loss. The profit of transporting slaves was so lucrative though that it was extremely profitable to participate in the slave trade for slavers and their backers in Europe. The only purpose of trading goods for slavers was to reduce the amount of money they lost when they didn't have slaves on board.

Empire at the Periphery: British Colonists, Anglo-Dutch Trade, and the Development of the British Atlantic, 1621-1713 Book by Christian J. Koot

Our Savage Neighbors: How Indian War Transformed Early America Book by Peter Silver

The many-headed hydra Book by Peter Linebaugh

The shortest answer is -- nothing.

The "triangle trade" metaphor that u/I_Tory_I mentions is useful for understanding the basic outlines of the general trade, but not for understanding how a given ship would have moved.

The majority of American crops -- sugar, cotton, indigo, tobacco, etc. -- reached European markets in much larger, more slowly moving vessels than those which were used for slaving.

Due to the need for security, the smaller, faster slaving ships carried at least twice the number of seamen as were needed to man the vessel. Time spent waiting for seasonal crops added to the overhead cost of the voyages, at twice the rate of a regular ship. Slaving ships were also the first to invest in copper sheathing for the ship, which was expensive, but prolonged the life of the ship, and allowed it to move faster.

Size, speed, crew, cost -- all of these were factors that helped to ensure that many slaving ships were focused on the act of shipping those kidnapping victims.

Those slaving ships would also have carried gold and ivory, and to a far lesser degree, silver -- items with a high value-to-weight ratio that they could pick up in Africa, and be able to easily ferry all the way back, through two trans-Atlantic legs (west to the Americas, and then east to Europe). Along with those items, the ships would carry remittances (bills of exchange) back to Europe in order to collect.

Two key things to realize about the ships headed *to* Africa:

First, the most important commodity used to pay for humans was East Indian textiles. That's a key reason why merchants in Nantes and Liverpool -- two leading importers of Asian textiles -- moved into, and were able to take a leading role in, the slaving trade. Rum, guns, and other manufactured goods were all part of the package as well, but the quality, and colorfastness, of Asian textiles made them the premier product of exchange.

So, imagine the crops of the West Indies flowing to a variety of ports -- London, Glasgow, Amsterdam, etc., on slow-moving ships, that are necessarily tied to the harvest times of crops they are shipping. Meanwhile, the slave circuit is tied to the different rhythm of ships picking up quality goods in Europe, and to loading up with slaves in Africa -- slavers may not arrive in the Americas when the plantation owners need their products moved out, and they need not wait for those products to be harvested, because those bills of exchange allowed for credit, to grease those wheels.

Second key thing to realize about the ships headed to Africa -- many came from Brazil. The trade between Brazil and Africa was the biggest slave trade of all, and it doesn't quite fit into the "triangle trade" metaphor.

Still, for explaining the basic nature of important trade circuits, I think it's a great metaphor, and I made a slam poem about it.

Sources:

Herbert S. Klein, "Economic Aspects of the Eighteenth Century Slave Trade," in The Rise of Merchant Empires: Long Distance Trade in the Early Modern World, ed. by James D. Tracy.

Francis E. Hyde et al. "The Nature and Profitability of the Liverpool Slave Trade." Economic History Review, (1953) pp. 368-377

R. B. Sheridan, "The Commercial and Financial Organization of the British Slave Trade, 1750-1807." Economic History Review, (1958), pp. 249-263