surface layer of the fluid. As the fluid slows and adds energy to the disks, it spirals into the Center exhaust. Since the rotor has no projections, it is very sturdy. Tesla wrote: "This turbine is an efficient self-starting prime mover which may be operated as a steam or mixed fluid turbine at will, without changes in construction and is on this account very convenient. Minor departures from the turbine, as may be dictated by the circumstances in each case, will suggest themselves but if it is carried out on these general lines it will be found highly profitable to the owners of the steam plant while permitting the use of their old installation. However, the best economic results in the development of power from steam by the Tesla turbine will be obtained in plants especially adapted for the purpose." Smooth rotor disks were originally proposed, but these gave poor starting torque. Tesla subsequently discovered that smooth rotor disks with small washers bridging the disks in about 12 to 24 places around the perimeter of a 10″ disk and a second ring of 6–12 washers at a sub-diameter made for a significant improvement in starting torque without compromising efficiency. Efficiency and calculations In Tesla's time, the efficiency of conventional turbines was low because turbines used a direct drive system that severely limited the potential speed of a turbine to whatever it was driving. At the time of introduction, modern ship turbines were massive and included dozens, or even hundreds of stages of turbines, yet produced extremely low efficiency due to their low speed. For example, the turbine on Olympic and Titanic weighed over 400 tons, ran at only 165 rpm, and used steam at a pressure of only 6 psi. This limited it to harvesting waste steam from the main power plants, a pair of reciprocating steam engines. The Tesla turbine also could run on higher-temperature gasses than bladed turbines of the time, which contributed to its greater efficiency. Eventually, axial turbines were given gearing to allow them to operate at higher speeds, but the efficiency of axial turbines remained very low in comparison to the Tesla turbine. As time went on, competing for axial turbines became dramatically more efficient and powerful, and the second stage of reduction gears was introduced in most cutting-edge U.S. naval ships of the 1930s. The improvement in steam technology gave the U.S. Navy aircraft