Turbine reduction gears versus electric propulsion for ships

Abstract

The question of whether electric or geared propulsion should be used depends entirely upon the type and operating conditions of the vessel. The Navy Department after years of comparative study, and having actual operating data on electrically propelled vessels available, has decided to equip all the larger Navy vessels with electric drive. In this type of ship, electric drive has a decided advantage, as economical cruising speed (which is so important for this type) can be obtained, the machinery can be located in the most convenient place farthest removed from disturbances from gun fire or torpedoes, plurality of generating units can be used so that damage to one or more parts will not disable the vessel, and maneuvering can be accomplished more quickly and accurately, etc. Geared turbine drive has an advantage in fast destroyers and light cruisers. In this type of vessel, the power is generally large and the propeller speed very high — factors which make ideal gear conditions. Geared turbines can here be built very much lighter than electric drive and, as the weight of propelling machinery in this type of ressel is of utmost importance, electric drive being heavier cannot be considered. In merchant ships each individual case must be studied, and here the questions of service, economy, reliability, weight and cost must be taken into consideration. For fast passenger liners, electric drive has an advantage. The transmission efficiency can be made practically equal to a conservatively proportioned double-reduction gearing. It has, also, the advantage that the more efficient turbine can be built. If two generating units are used, one unit will propel the vessel at about three-quarters speed with only slight sacrifice of economy. Noise, so objectionable in passenger vessels, is practically eliminated with electric drive. In moderate-power twin-screw ships of about 6000 to 8000 h. p., electric drive has a decided advantage both in economy, weight and price, if one turbo-generating unit is used to drive both propellers. In case this unit, or any of its auxiliaries, should become disabled, a simple arrangement can easily be made by which the ship could be run at low speed from the auxiliary generating unit which normally drives the electrically driven auxiliaries. On the other hand, if two generating units are used, the equipment becomes slightly less efficient, the weight and cost also go up considerably, and the comparison then becomes similar to a 3000-h. p. single-screw freighter. Low-speed single-screw freighters, 2500–3000 h. p.: Here the geared turbine is somewhat lighter. The actual transmission efficiency of the gears is also better. However, when the losses of the reversing turbine, power taken by the oiling system, packing losses, as well as leakage of steam due to larger clearances, are taken into consideration, the transmission efficiency is practically equal to — if anything in favor of electric drive. If, however, the motor is located aft, doing away with long shafting with its expensive bearings, supports and shaft alleys, the electric drive is the lightest, cheapest and most economical, as well as the most reliable method to use. With the main auxiliaries driven electrically, the ship can by very simple arrangement be run home by power supplied from these auxiliary units, should the main propelling machinery or its auxiliaries become disabled. In freighters of 1500 h. p. or less, geared turbines are lighter and cheaper. As the power is small, a gear can be designed which is less affected by sudden load variation or disalignment caused by springing of the hull or inaccurate workmanship.