Steamboat Hill

Copyright 2009 by Paul Niquette. All rights reserved.
Derivation of the name for this puzzle.

he Robert E. Lee was built in 1866 for the princely sum of $200,000. Nicknamed the "Monarch of the Mississippi," this luxurious side-wheeler is most widely remembered for winning the famous steamboat race in 1870 against the Natchez VI, traveling down-river from St. Louis, Missouri to New Orleans, Louisiana, a distance of 1,003 nautical miles in a bit over 90 hours (3 days, 18 hours and 14 minutes).  That indicates an average of about 11 knots, allowing for stops to refuel alongside pre-arranged barges in mid-river -- delays which would have been partially offset by the 'down hill' current.

As depicted in this painting by August Norieri, one might surmise the vessel to be commanded "All Ahead Full" making good its fastest speed on the return trip 'up hill' against the river's current. 

Fourteen decades ago, "carbon footprint" was not considered an issue.  If it were, 'sustainability' would have taken precedence over performance. 
Suppose that the river was flowing 'down-hill' at four knots.  Applying 'green' criteria, the steamboat would have been commanded to paddle its way 'up-hill' at what speed?
[a] five knots
[b] six knots
[c] seven knots
[d] eight knots


For solvers who care, "Steamboat Hill" is merely a rhyming pun on the title Steamboat Bill, a 1928 silent film, featuring Buster Keaton and presently available for viewing in its entirety at the Internet Archives.