 On the western front , the Union built a formidable force of river ironclads , beginning with several converted riverboats and then contracting engineer James Eads of St. Louis , Missouri to build the City @-@ class ironclads . These excellent ships were built with twin engines and a central paddle wheel , all protected by an armored casement . They had a shallow draft , allowing them to journey up smaller tributaries , and were very well suited for river operations . Eads also produced monitors for use on the rivers , the first two of which differed from the ocean @-@ going monitors in that they contained a paddle wheel ( the USS Neosho and USS Osage ) . 
