From a SS United States site: Speed: The Big U was capable of attaining 44.7 knots, which equates to over 50 mph. Cruising speed was between 30 and 33 knots. The SS United States was the fastest ship afloat in her day, and is the fastest object of her size ever built. It is interesting to note that the SS United States was capable of traveling at 25 knots in reverse, roughly Titanic's forward top speed.