[Illustration: Fig. 10. Diagrammatic section of a steamship which has been 'converted' from the ordinary method of propulsion to the 'Paragon' system of electric main marine propulsion. The reciprocating engine has been replaced by a steam turbine, coupled direct to an electric generator which supplies current to a motor attached to the propeller shaft. The tests carried out with this vessel will indicate the advantages of the electric method of propulsion even with the usual long length of shaft. The vessel has a gross tonnage of 1241, and its speed is 9 knots. The engines replaced ran at 78 revolutions per minute and gave 500 brake horse power. The turbine now installed runs at 2500 r.p.m., and develops 630 brake horse power. (Illustration reproduced by courtesy of _The Electrician_.)] The gulf between the two has been narrowed by the improved design of propellers. Some engineers assert that continued improvements will bridge the gulf completely. Others have sought the solution in the same way as the motor engineer--by the use of mechanical change-speed gears. The suggestion has also been made to employ hydraulic gear as an intermediary; and in some recent vessels reciprocating engines with comparatively low-speed turbines driven by exhaust steam have been adopted. In the electric system the turbine is coupled direct to an electric generator and may run continuously at the highest economical speed. The propeller shaft may be quite short and is driven by a slow speed motor connected by cables to the generator. Various arrangements for controlling the supply of current to the motor (with appropriate design of generator and motor) have been devised by Mr Durtnall, Mr Mavor, and other workers in this field; but whatever the details of these arrangements may be, they all give a wide range of speed both ahead and astern. The direct drive with the steam turbine has really only one speed--full speed ahead; and as the turbine is irreversible, 'astern' turbines have to be installed in addition. These limitations and complications are removed entirely when electrical transmission is adopted. Moreover, the electric system can be so arranged that the control gear may be operated from the bridge itself. The facility in manoeuvring is, in fact, so marked that it would recommend electric marine propulsion even if that system offered no advantages on the score of economy in weight, space, and steam consumption over the existing systems. The steam turbine, it may be noted, has been adopted so far only in high-speed vessels; and it is generally recognised that its extension to vessels which run at 12 or 16 knots depends upon its adaptation to slow-speed propellers. Advocates of electric marine propulsion claim that they hold the most efficient solution of this problem.