Actually, Proud Mary (in the river industry) was horse manure that had been mixed with other manures and baled, to be added to the coal or wood, in the days of steam. When the steam boats were going downstream, wood and coal would suffice to build enough steam to navigate the rivers. Going upstream, the boat needed more steam, so the nitrogen in the horse manure would burn much hotter and build much more steam. The baled manure was known as "Proud Mary". 'Big wheel, keep on turnin', Proud Mary keep on burnin'.'