Significant 19th-Century Shipbuilders

These shipyards were pioneers of the industry in the second half of the 19th century, as steel and steam replaced wood, iron and sail.  (In this context, "the second half of the 19th century" includes the pre-WWI years of the 20th century.)  Some of these yards, such as that of William H. Webb, are well documented, others hardly at all.  In addition, there are many more 19th-century yards that probably should be documented.