Propulsion Expediency

Steam powered warships are driven by two large turbines per shaft. Steam is admitted to the turbines by the throttle valve. Steam is coming from the main steam loop at 1,175 psi and 950 degrees. First, steam goes into the high-pressure turbine. This image shows two such turbines at a factory:


Steam is admitted into the center of the turbine and flows towards either end. Since as the steam flow drops in pressure as it expands through each stage of the turbine, the blades get bigger as you look towards either end of the turbine. The really large blades at the end are the astern elements used for backing down.

Then steam goes to the low-pressure turbine. The blades are of different design, designed to extract work from steam at (you guessed it) lower pressures. Steam that came out of the main steam loop at very high temperatures and pressures is exhausted into the main engine condenser at a near-perfect vacuum of 29" Hg and 110degF.



The turbines are connected to a set of double reduction gears:


The gears are a lot larger than this drawing implies. You can crawl into the oil sump of the gears. The big gear is the "bull gear," which is connected to the screw (propeller) shaft. They are double-helix gears to prevent gear lash and absorb the massive amount of horsepower being transmitted.

Reduction gears are very heavy, are very carefully machined and are very expensive. The access ports to the reduction gears are locked with the same type of locks used to secure weapons magazines. Lead anti-tamper seals are then affixed to each port. Both the sealing crimp and the lock keys are in the personal custody of the Chief Engineer, who must personally inspect the reduction gears prior to closing the access ports and then personally lock and seal the access ports.

During the Second World War, the companies that made steam turbines and reduction gears could not keep up with the number of warships being produced. The Navy decided that the use of steam turbines and reduction gears would be limited to fast destroyers, cruisers, battleships and carriers. The Casablanca class escort carriers used reciprocating steam engines, which why those carriers had a top speed of 20 knots. Many of the destroyer escorts were powered by large electric motors powered by diesel generators, in the same matter as a diesel locomotive.

The Navy went back to steam propulsion for destroyer escorts after World War II, with the exception of the four ships of the Claud Jones class, which were diesel-electric as a cost-saving measure, and which were gotten rid of by the Navy as soon as the Navy could justify doing so. Destroyer escorts were reclassified to frigates in 1975.

Steam ruled the destroyer escorts/frigates until 1974, when the last steam escort, USS Moinester (FF-1097), joined the Fleet. The Perry class, the only class of frigates built in the last thirty years, is powered by gas-turbines.