Wolff had business interests outside Harland and Wolff, including the Belfast Ropeworks, which he founded in the early 1870s with W.H. Smiles, who was the son of Samuel Smiles, a Scottish author. With Wolff as chairman, the firm became one of the largest ropeworks in the world, challenging the Gourock Ropework Company, who were based on the River Clyde in Scotland. Wolff also bought shares in the Union Steamship Company, and became a director; with his influence, he ensured Harland and Wolff received regular orders from the Union Steamship Company. After Wolff's negotiation, the Union Steamship Company merged in 1900 with the Castle Line, which was owned by Donald Currie; the new company formed was the Union-Castle Line.