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