Turboelectric Drive in American Capital Ships
Between 1913 and 1919 the United States Navy designed battleships with a unique propulsion system to meet its operational demands for great range and survivability. This highly successful system, turboelectric drive, has gained an undeservedly checkered reputation in recent years and many of its benefits have been forgotten.
The direct drive steam turbine was introduced into capital ship propulsion by the HMS Dreadnought, the first ship completed with a uniform heavy caliber armament and the namesake of the type. In a direct drive system, the boilers create steam which is routed to turbines acting directly upon the head of the propeller shaft. Waste steam is then run through condensers and returned to the boilers as feed water to complete the cycle. Direct drive turbines transfer mechanical energy very efficiently, but mate the high fuel-efficient rotation rate of a turbine with the low fuel-efficient rotation rate of a propeller very poorly. Running the turbine slowly either wastes fuel or energy is lost through cavitation when the propeller turns too quickly.
Direct drive turbines were fitted with separate high and low-pressure stages in an effort to provide both fuel-efficient cruising and maximum speed tactical capability. This necessarily expands the footprint of a direct drive turbine system. To make matters worse, the need for backing power requires yet another turbine stage. The extensive steam piping to serve three turbines and the multiple combinations of valves to enable cross connection do much to destroy the simplicity of the system.
Besides the conflicting efficient rotation speeds at either end of the propeller shaft, the direct drive turbines developed weak backing power and risked damage to the turbine rotors if backing steam was introduced too abruptly. The problems of conflicting efficiency were not solved until the introduction of complex single and then double reduction gearing between the turbine and the propeller shaft. Unfortunately, reduction gearing added further to the footprint of the system and presented yet another mechanical item that could be deranged by physical shock effects.
As Norman Friedman reports in his seminal design history of US battleships, the General Electric company proposed an alternative to direct drive turbines. Like direct drive steam turbine plants, the turboelectric drive plant uses boilers to generate steam and turn a turbine, then returns waste steam via a condenser for reuse. There the similarity ends. There is only a single turbine, and rather than driving the propeller shaft, it turns one or two electric generators. The electricity created is then routed via a bus bar system to electric motors mounted to the propeller shaft heads. The turbine spins at a single constant, highly efficient rotation rate, while the electric motors, mechanically divorced from the turbines, turn at the rate most efficient for the propellers.
To achieve full backing power, the electric motors are simply reversed, there being no physical need for a separate reverse stage. This eliminates several redundant pieces of equipment and much steam piping.
Turboelectric drive offers several advantages:
- There is no mechanical connection between the turbogenerator shaft and the propeller shaft, allowing both to turn at their disparate efficient speeds. This reduces propeller rotation speeds and increases fuel efficiency.
- The motor rooms can be placed nearer the stern than can reduction-geared turbines, eliminating the need to lead the propeller shafts farther forward in the ship.
- The machinery components are more easily segregated into multiple compartments, and require fewer steam line penetrations of watertight bulkheads.
- The turbo-electric drive consumes less beam, allowing more hull breadth to be devoted to the torpedo defense system.
- The propeller shafts can be immediately reversed by simply switching the direction of the electric motors without the need to reroute steam to a separate reversing turbine.
- Equal power (but not speed) is available for ahead or astern steaming. Astern steaming can also be maintained indefinitely.
- The machinery is more easily cross-connected in the event of battle damage through the switching of electrical loads between different turbogenerators and motors, and the elimination of propulsive steam lines.
- More steam is available at all power levels for the ship's service turbogenerators (SSTGs), making more power available for ancillary systems (including main battery elevation and training) and electronics.
- Most major electrical components are reparable by the ship's company at sea.
The turboelectric drive also has several inherent negatives:
- It is heavier and more expensive than a direct drive or reduction geared turbine installation.
- It is susceptible to turbogenerator room damage.
- It is susceptible to damage to the main control compartment containing the bus bar system.
- It is susceptible to shorting out from shock damage to the bus bar system.
Turboelectric drive ships realized fuel economies of as much as 20% compared to comparable turbine ships, according to Freedman's report of the difference in fuel consumption between USS New Mexico (BB-40) and her two direct drive turbine sister ships USS Mississippi (BB-41) and USS Idaho (BB-42).
In compartmentation, the turboelectric drive was typically twice as segregated as a direct drive plant and four times as segregated as later reduction geared turbine plants in US service. The machinery in the direct drive turbine USS Idaho (BB-42) was divided into eight main spaces, while the machinery in the turboelectric USS Tennessee (BB-43) was divided into fifteen main spaces. This increase in compartmentalization meant that there would be less flooding in the ship in case of battle damage such as from a torpedo. The later reduction geared USS North Carolina (BB-55) had only four main spaces and required each propeller shaft to be led progressively farther forward in the hull.
Turboelectric machinery also permitted more rapid development of accelerating and decelerating power on the shafts. It made the last ditch maneuver of "twisting" a ship out of a torpedo's path by backing down one side's shafts while running the opposite side full ahead and applying full rudder toward the backing side more effective. It also permitted extended periods of backing. After suffering a torpedo hit in the extreme bow while at anchor off Saipan in 1944, USS Maryland (BB-46) backed to Pearl Harbor at 10 knots so as not to strain the collision bulkhead forward.
The same ship also escaped two collisions in a matter of minutes during a close order fleet maneuvering exercise between the wars. When USS Oklahoma (BB-37) sheered out of column to avoid running down an errant destroyer, she intruded on the next column of ships, crossing the Maryland's bow. The Maryland performed an immediate "crash back" to avoid the Oklahoma, decelerating and letting the other battleship pass ahead, only to be confronted with the direct drive turbine USS Arizona (BB-39) vainly trying to back down behind her. Maryland's electric motors were immediately thrown back to flank speed ahead and the turboelectric ship accelerated ahead of the less responsive Arizona.
The Maryland also escaped an aerial torpedo at Leyte Gulf by "twisting" the ship out of the torpedo's path. When the order was given to put the helm over to evade the torpedo, the steering gear shorted out, leaving the rudder amidships. The captain then directed maneuvering by the motors while a damage control team attempted to restore the helm. The ability to maneuver effectively prior to the restoration of helm control saved the ship from being hit. Helm control was restored prior to the weapon crossing Maryland's track and placing the helm hard over near the end of the maneuver may have assisted in moving the ship's stern out of the path of the weapon which passed close aboard. Had the ship not commenced its maneuver under motor control prior to recovering helm control the ship would have been hit.
The matters of cost and weight led to the demise of the turboelectric drive under Washington Treaty tonnage limitations and Depression Era fiscal stringency. Reduction geared turbines were lighter and less expensive for the horsepower generated.
The vulnerability of the turbogenerator rooms and main control space was problematic at best. Shielded by the boiler rooms, the torpedo defense system and the vertical armored belt to outboard and by the armored deck, splinter deck and armored uptakes above and by a deep triple bottom beneath, the turbogenerators were nestled protectively in the very center of the ship. If weapons could reach the turbogenerators, then the magazines were also similarly vulnerable, making the point largely moot. However, since the turboelectric system provided large reserves of electric power, virtually all systems, including main battery training, elevation, stabilization and loading gear were run electrically. So, the potential did exist for a ship to be crippled through a total power loss in the event of damage to these compartments. However, no turbogenerator room or main control space was ever penetrated by an enemy weapon during WWII.
Shorting out of the bus bar system could knock out the turboelectric drive. This occurred only once, as a result of a torpedo hit exactly between the frames of the main control space housing the bus bar system in USS Saratoga (CV-3). The serendipitous location of the hit transmitted the physical shock of the hit in sufficient force to defeat the vibration damping shock mounts of the busbar system which produced a short that took the turboelectric system off-line. Power was restored within minutes and although several more shorts and brief power losses (all lasting less than five minutes) took place, the Saratoga was able to proceed under her own power for three hours. At that time, the engines were deliberately stopped to allow the shorted turbogenerator and a second one that the first one had damaged to be electrically isolated. A third generator was also isolated as it kept overloading the first generator. This shutdown for two and a half hours under tow was only necessary because the fourth, undamaged turbogenerator was not available due to an unrelated condenser casualty. From this single incident the turboelectric system derives its reputation for being dangerous and unreliable. One author goes so far as to describe this as "typical" of the system's vulnerability to battle damage. For a single incident out of 21 to be described as typical is highly questionable.
US turboelectric ships were battle damaged in 21 separate cases by 16 torpedoes, 13 bombs, 13 Kamikazes and more than 26 medium and light caliber shells. Of these incidents, only seven had the potential to effect the turboelectric drive in any way, and only three actually did. The torpedo hit on the Saratoga on 31 August 1942 succeeded in knocking the system off-line for less than five minutes before damage control measures restored power. The two torpedo hits on USS California (BB-44) at Pearl Harbor on 7 December 1941 contaminated the fuel lines, causing the boiler fires to go out, thus producing a power loss. This would have produced a power loss in any steam-propelled ship and cannot be counted against the turboelectric drive components. The nine torpedo hits on USS West Virginia (BB-48), also at Pearl Harbor on 7 December 1941, so overwhelmed the ship that immediate counterflooding was necessary to prevent capsizing. Between flooding from the torpedo damage and counterflooding, the machinery plant was knocked off line. As this would also have crippled any other steam-propelled ship, this incident, too, cannot be counted against the turboelectric powerplant.
Four other cases produced sufficiently violent shocks to have potentially effected the turboelectric drive, but all failed to do so. The two torpedo hits on USS Lexington (CV-2) at the Coral Sea battle, 8 May 1942, the torpedo hit on Saratoga on 11 January 1942, the torpedo hit in the extreme bow on Maryland off Saipan on 14 June 1944 and the kamikaze hit on Maryland off Leyte on 29 November 1944, all produced violent shocks, whipping of the hull and/or flooding. However, none of these hits caused any disruption to the turboelectric drive.
Thus, the system, while repeatedly proven reliable, has been damned for a five-minute failure due to a very lucky torpedo hit on Saratoga on 31 August 1942.
The US Navy ordered turboelectric drive for the USS New Mexico (BB-40), but not for her sister ships USS Mississippi (BB-41) and USS Idaho (BB-42). This plant was installed in the existing geared turbine subdivision shared with the other two ships, consisting of three boiler rooms from fore to aft, an auxiliary machinery room next aft, and four engine rooms positioned abreast each other. The turbogenerators were probably mounted in the two inboard engine rooms and additional SSTGs in the outboard engine rooms. Electric motors were attached to the propeller shaft heads at the rear of each engine room. When New Mexico was rebuilt in the 1930s, her turboelectric plant was replaced with a geared turbine installation of greater power. This was felt necessary to offset speed lost due to additional weights added during reconstruction, and buying three identical plants saved $300,000 over providing a separate turboelectric plant for New Mexico.
The next two US Navy battleship classes shared a common plant. USS Tennessee (BB-43) and USS California (BB-44) and their near sisters USS Colorado (BB-45), USS Maryland (BB-46) and USS West Virginia (BB-48), subdivided their machinery spaces into fifteen compartments. Two turbogenerator rooms occupied the centerline, each containing one turbine, two generators and three SSTGs. The main control space was also on the centerline immediately aft of the second turbogenerator room and immediately forward of the centerline motor room. Outboard on either side of these compartments were, fore to aft, an evaporator room (to port) or an auxiliary machinery room (to starboard) and four boiler rooms. Immediately aft of the outboard boiler rooms were the outboard motor rooms, driving the outboard shafts. The inboard shafts shared the centerline motor room.
The two Lexington Class ships were converted from planned battlecruisers (CC-1 and CC-3) and completed as carriers. USS Lexington (CV-2) and USS Saratoga (CV-3) had the largest turboelectric plants ever built. The installations were very similar to the Tennessee and Colorado Classes. However, each turbogenerator room contained two turbines, each driving a single generator, versus one turbine driving two generators as in the battleships, and additional SSTGs were fitted. There were sixteen boiler rooms, eight on either side, buffering the two turbogenerator rooms and the main control space. There were also three motor rooms, arranged similarly to the battleship classes, but each shaft had two drive motors versus one. Maximum speed was thus 33 knots, as compared to 21 knots in the battleships.
The aborted South Dakota Class (BB-49 through BB-54) was also planned for turboelectric drive. The plant would have repeated the Tennessee arrangement, but with six boiler rooms on either side versus four.
Turboelectric drive was a unique and elegant solution to the propulsion and range problems faced by the US Navy in the 1920s and 1930s. It performed well, and permitted a minute form of subdivision that rendered ships fitted with it highly resistant to torpedo damage. On the whole, it was a success and a good investment, abandoned only because of the cost in weight and money in an environment of Treaty limitations and later in an environment in which no limits mattered.
|Ship||Date||Cause of damage||Notes|
|BB-40 New Mexico||6 Jan 45||Kamikaze|
|12 May 45||Kamikaze|
|BB-43 Tennessee||7 Dec 41||2 bombs|
|14 Jun 44||3 coastal artillery shells|
|12 Apr 45||Kamikaze|
|BB-44 California||7 Dec 41||2 torpedoes; 2 bombs||Sunk|
|14 Jun 44||1 coastal artillery shell|
|06 Jan 45||Kamikaze|
|BB-45 Colorado||24 Jul 44||22 coastal artillery shells|
|27 Nov 44||2 Kamikazes|
|9 Jan 45||Multiple friendly AA shells|
|BB-46 Maryland||7 Dec 41||2 bombs|
|14 Jun 44||1 torpedo|
|29 Nov 44||Kamikaze|
|07 April 45||Kamikaze|
|BB-48 West Virginia||7 Dec 41||9 torpedoes; 2 bombs||Sunk|
|01 Apr 45||Kamikaze|
|CV-2 Lexington||8 May 42||2 torpedoes; 2 bombs||Sunk|
|CV-3 Saratoga||11 Jan 42||1 torpedo|
|31 Aug 42||1 torpedo|
|21 Feb 45||3 bombs; 4 Kamikazes|
|BB-40 New Mexico||Plant replaced by geared turbines during reconstruction.|
|BB-43 Tennessee||First installation with refined subdivision.|
|BB-45 Colorado||Completed under Treaty in exchange for Mutsu.|
|BB-47 Washington||Cancelled incomplete by Treaty.|
|BB-48 West Virginia||Completed under Treaty in exchange for Mutsu.|
|BB-49 South Dakota||Cancelled incomplete by Treaty.|
|BB-52 North Carolina|
|CC-1 Lexington||Completed as carrier (CV-2) under Treaty.|
|CC-2 Constellation||Cancelled incomplete by Treaty.|
|CC-3 Saratoga||Completed as carrier (CV-3) under Treaty.|
|CC-4 Ranger||Cancelled incomplete by Treaty.|
|CC-6 United States|
- 31 January 2001