Editing Steamship (section)

==Triple expansion engines==
Throughout the 1870s, compound-engined steamships and sailing vessels coexisted in an economic equilibrium: the operating costs of steamships were still too high in certain trades, so sail was the only commercial option in many situations. The compound engine, where steam was expanded twice in two separate cylinders, still had inefficiencies. The solution was the triple expansion engine, in which steam was successively expanded in a high pressure, intermediate pressure and a low pressure cylinder.{{r|Gardiner and Greenhill 1993|p=89}}{{r|Griffiths|p=106-111}}

The theory of this was established in the 1850s by [[John Elder (shipbuilder)|John Elder]], but it was clear that triple expansion engines needed steam at, by the standards of the day, very high pressures. The existing boiler technology could not deliver this. Wrought iron could not provide the strength for the higher pressures. Steel became available in larger quantities in the 1870s, but the quality was variable. The overall design of boilers was improved in the early 1860s, with the [[Scotch marine boiler|Scotch-type]] boilers – but at that date these still ran at the lower pressures that were then current.{{r|Griffiths|p=106-111}}

The first ship fitted with triple expansion engines was ''Propontis'' (launched in 1874). She was fitted with boilers that operated at {{convert|150|psi}} – but these had technical problems and had to be replaced with ones that ran at {{convert|90|psi}}. This substantially degraded performance.{{r|Griffiths|p=106-111}}[[File:StateLibQld 1 132949 Aberdeen (ship).jpg|thumb|''Aberdeen'', the first successful commercial use of triple expansion engines]]

There were a few further experiments  until {{ship|SS|Aberdeen|1881}} went into service on the route from Britain to Australia. Her triple expansion engine was designed by Dr A C Kirk, the engineer who had developed the machinery for ''Propontis''.  The difference was the use of two double ended Scotch type steel boilers, running at {{convert|125|psi}}. These boilers had patent corrugated furnaces that overcame the competing problems of heat transfer and sufficient strength to deal with the boiler pressure. ''Aberdeen'' was a marked success, achieving in trials, at 1,800 [[indicated horsepower]], a fuel consumption of {{convert|1.28|lb|kg}} of coal per indicated horsepower. This was a reduction in fuel consumption of about 60%, compared to a typical steamer built ten years earlier. In service, this translated into less than 40 tons of coal a day when travelling at {{convert|13|kn}}.{{efn|If this fuel consumption is compared to {{ship|SS|Agamemnon|1865}} (previous section), the relative sizes and the cruising speeds of the two ships should be taken into account: ''Aberdeen'' 3,616[[Gross register tonnage|GRT]], {{convert|13|kn}}, ''Agamemnon'' 2,270[[Gross register tonnage|GRT]], {{convert|10|kn}}.}}  Her maiden outward voyage to [[Melbourne]] took 42 days, with one coaling stop, carrying 4,000 tons of cargo.{{r|Griffiths|p=106-111}}{{r|Gardiner and Greenhill 1993|p=89}}

Other similar ships were rapidly brought into service over the next few years. By 1885 the usual boiler pressure was {{convert|150|psi}} and virtually all ocean-going steamships being built were ordered with triple expansion engines. Within a few years, new installations were running at {{convert|200|psi}}. The tramp steamers that operated at the end of the 1880s could sail at {{convert|9|kn}} with a fuel consumption of {{convert|0.5|oz}} of coal per ton mile travelled. This level of efficiency meant that steamships could now operate as the primary method of maritime transport in the vast majority of commercial situations.<ref name="Griffiths">{{cite book |last=Griffiths|first=Denis|editor1-first=Robert|editor1-last= Gardiner|editor2-first=Dr. Basil|editor2-last= Greenhill
|title=The Advent of Steam - The Merchant Steamship before 1900 |publisher=Conway Maritime Press Ltd |date=1993 |pages=106–126|chapter=Chapter 5: Triple Expansion and the First Shipping Revolution |isbn=0-85177-563-2}}</ref>{{rp|106–111}}<ref name="Gardiner and Greenhill 1993">{{cite book |last1=Gardiner |first1=Robert J |last2=Greenhill |first2=Basil |title=Sail's Last Century : the Merchant Sailing Ship 1830-1930 |date=1993 |publisher=Conway Maritime Press |location=London |isbn=0-85177-565-9}}</ref>{{rp|89}} In 1890, steamers constituted 57% of world's tonnage, and by World War I their share raised to 93%.<ref name=blus/>