I obtained the graph above from William Tilley of the Independence Seaport Museum in Philadelphia. Special thanks to William for sending it. He used it as a slide in his lecture in Philadelphia on June 22, 2002. It is from John Kane's paper "The Speed of the SS United States," read before the Society of Naval Architects and Marine Engineers on November 3, 1977. Once declassified, performance data for the SS United States could be revealed publicly. John Kane was the Vice President of Newport News Shipbuilding and Drydock Company during that time.
The above graph is the horsepower curve for the SS United States observed during the Builder's Trial on May 15, 1952 and the Official Trial on June 9-10, 1952. The left Y axis of the graph and upper linear line illustrate propeller rpms of the SS United States. The right Y axis of the graph and curved line on the graph depict the shaft horsepower curve of the SS United States. The X axis illustrates the speed in knots of the SS United States at various propeller rpms and shaft horsepower outputs. As is evident, at approximately 135 shaft hp, the SS United States made roughly 34 knots. This approximates service speed. As speed increases, more and more power is required to make the ship go faster. On runs 5-6 of the Official Trial, the SS United States averaged 38.32 knots at 187.47 propeller rpms and made 241,785 shaft hp. From a mathematical standpoint, by following the curved line up further, it is clear that a substantial amount of additional horsepower would be required for the SS United States to exceed 40 knots.