Gearing the steam turbine

The steam turbine was a major propulsion medium for much of the 20th century, but its success was dependent on the adoption of gearing.

When the steam turbine was first considered for shipping by the Hon Charles Parsons (knighted in 1911), it was directly coupled to the propeller; but as the output speed of the turbine was in the thousands of rev/min, the propeller was forced to be small in diameter to prevent tip cavitation that would reduce efficiency. For example, Parsons built the experimental vessel Turbinia in 1894, equipped with a single radial steam turbine driving a single propeller.

Further experimentation continued leading to the vessel being fitted with three propellers on the single shaft, running at 1,750 rev/min and achieving 19.75 knots. Finally, three axial turbines, each driving a shaft fitted with three propellers, were fitted - the total of nine propellers, and the 1,475kW at 2,200 rev/min, produced a trial speed of 32.76 knots and an unofficial figure of 34.5 knots.

Early gearing experiments

Parsons then turned his focus to what was then an insurmountable challenge - achieving a practical propulsion arrangement when reconciling a turbine revving at high speed and a propeller needing to turn at a much slower speed. The solution was to gear down the turbine's output speed.

The idea had been tried, on a small scale, as early as 1897 in the 6.7m long launch belonging to the yacht Charmian. The 10 horsepower turbine operated at 20,000 rev/min and was geared to a pair of propeller shafts by helical spur wheels with a reduction ratio of 14 to 1.

Following on from his Turbinia experience, in 1908 Parsons experimented with high speed and more compact, geared turbines in the 4,350-ton collier Vespasian, that had been built in Sunderland in 1887. Two turbines were installed that ran at 1,400 rev/min and drove the single propeller at 70 rev/min via single-reduction gearing. The results were excellent and by 1911 the vessel had covered some 50,000 miles in two years' service with around a 10 to 15% reduction in fuel consumption.

The gearing for the destroyer HMS Badger (Credit: C A Parsons Co)

Operating problem

There was, however, a problem with the thrust bearings when operating with geared turbines. On a direct drive turbine, the thrust of the screw could be partly counterbalanced by the thrust of the turbine, with the thrust bearings just accommodating the difference. But with gearing this was not possible, and both the turbine shaft and the drive shaft had to absorb the full force. More advanced thrust block technology was needed. It was only during World War One that these problems were successfully solved for large scale applications, allowing smaller diameter turbines rotating much faster and more efficiently.

The Royal Navy adopts gearing

Geared turbines were more highly regarded following the successful performance of the Vespasian and the Royal Navy was at the forefront of this development, progressively adopting the geared turbine.

In 1911 the destroyers HMS Badger and HMS Beaver (contracted by Parsons but with hulls built by Denny Brothers) entered service with so-called semi-geared turbines in which the low-pressure turbines were geared to fast-running, small diameter high-pressure and cruising turbines.

That same year, US naval architects incorporated geared turbines for the cruising turbines, while retaining direct drive main turbines, into the design of the battleships Nevada and Oklahoma.

The British destroyers were followed into service in 1914 by two more units, the Leonidas and Lucifer with what might be termed fully geared turbines. Here each of the two screws was driven by a pair of geared turbine installations - the high-pressure units turning at 3,000 rev/min, the low-pressure ones at 1,800 rev/min and the propellers at 380 rev/min.

The gearing for the destroyer HMS Leonidas (Credit: C A Parsons Co)

In 1915 the light cruisers HMS Calliope and HMS Champion were completed, being the first cruisers so equipped. It was reported that this arrangement resulted in a noticeable absence of vibration and machinery spaces were roomier and access much improved compared with direct drive ships. Efficiency was also greater. Later this cruiser machinery was adopted, albeit with twice as many sets on four rather than two shafts, for HMS Courageous, HMS Glorious and HMS Furious, all large light cruisers completed in 1917, and intended for Fisher’s planned Baltic expedition.  

Merchant adoption

In 1915 the San Francisco-built Pacific was fitted with double reduction gearing and in 1918 the UK’s first example of the technique involved the 9,777 gross ton Somerset which was equipped with three turbines producing a total of 4,500 horsepower and driving a single screw at 85 rev/min.

Over the next few years geared turbines would gradually replace reciprocating machinery and become the norm until challenged by diesel.

The steamer Somerset, first merchant ship in the UK to be fitted with geared turbines, shown in a World War One dazzle paint scheme (Credit: www.wrecksite.eu)

Footnote

The Imperial German Navy adopted an alternative to gearing to reduce propeller speed. This was the Föttinger converter, a form of fluid coupling that was successfully fitted in the light cruiser Wiesbaden of 1915, but that is another story.

John Barnes is a journalist and author and former editor of Marine Engineers Review.