Liberty ship triple Expansion Steam Engine (MP 106)
One of the most dramatic stories in steamship history is that of the triple
expansion steam engine which powered Liberty Ships and several other classes of cargo ships during World War II.
Built in awesome numbers, the Liberty Ship engine was a massive machine. Weighing237,000 pounds, the engine was 21 feet long, and 19 feet tall. Imagine a piston and its rod weighing 4,900 pounds moving 4 feet up and down 76 times every minute. That's what the low pressure piston did when steaming full speed! The story becomes even more impressive when viewed in the context of the terrible struggles of the Allies' war at sea.
As World War II progressed into 1940, German U Boats sank Allied Merchant ships nearly fifty percent faster than new ships could replace them. Between December 1939and February 1940, German submarines sank 465,000 tons of Allied shipping while only300,000 was launched.
The British Admiralty knew that Great Britain's survival and the war effort depended upon rapidly replacing the lost ships, and devising strategies to limit the losses. Realising that British shipyards could not build enough ships, the Admiralty sought replacements from the United States and Canada. They established the Technical Merchant Shipping Commission, who's head was R.C.Thompson of Joseph L.Thompson & Sons Ltd., an old Sunderland shipbuilder.
Modern ship design at that time was capable of producing ships with speeds of 15©16knots with propulsion from diesels and steam turbines, and this was the choice of the U.S Maritime Commission. Convoy speed, however, was mostly limited to 11 knots because of the large number of older ships already in service from the 1930's. Powering the new emergency ships with high powered turbines would have created serious production problems. The lengthy and costly machining of the reduction gears and turbine blades for the more desired turbines limited their use to naval and high speed merchant ships. Equipment and factory trained personel were simply not available in sufficient numbers. American yards were already fully committed. Smaller machine shops, many located at a distance from shipbuilding centres, which had never previously made marine engines, could be equipped to build simpler reciprocating engines, and simplified production methods could be employed. The commission decided therefore that a number of compromises were essential under the circumstances.
The ship design finally approved was based on plans from Thompson's shipyards in Sunderland, England. They were general purpose, break bulk tramp freighters of 10,000tons deadweight (7,000 tons gross) with an overall length was 441 feet, breadth 57 feet and 27 feet draft when loaded. These plans were similar to those which had been used to build tramp steamers at Newcastle on Tyne since 1879. The engines decided upon were 2500 H.P triple expansion steam engines, originally designed by North East Marine Engineering Ltd. When the building programme got under way, ships were launched with an average building time of two months from keel laying to sea trials. One ship took 8 days to build, test and deliver.
It has been calculated that about 3,259 engines were built.
Twenty machine shops manufactured the engines in the USA, four in Canada, and twelve in the British Isles. These powered 2,710 Liberty Ships, 60 U.S. built Oceans, 354Canadian built North Sands, Victories and Canadians. An amazing production rate of one a day was achieved by one company alone in November 1943. Eventually with interchangable engine parts made by 35 manufacturers worldwide, spares were readily available. Repairers in many ports as well as several navies operating the engines kept plentiful stocks of the inter-changable parts on hand.
There are two surviving Liberty Ships. The Jeremiah O'Brien was launched by NewEngland Ship Building Co. in Portland (ME) in 1943. She is on display in the National Maritime Museum, San Francisco. Her plant steams and turns over monthly for preventive maintanance. A Hooven, Owens & Rentschler main engine powers the ship on a voyage around the harbour annually, rewarding benefactors of the ship and museum. She was present at the Spithead review on June 4, 1994 to commemorate the50th anniversary of the D Day landings. The other operating Liberty ship is the SS John W.Brown on public display in Baltimore.
A Liberty ship engine is on permanent display in the Mariners Museum of Newport News, Virginia.
The widespread and rapid construction of these engines was dramatic, but the basic design and subsequent quality control in building the engines emerges as the most outstanding part of the whole program. The engines were ruggedly dependable. They justified their choice of this obsolete power plant, which contributed to the ultimate success of the emergency wartime shipbuilding programs. These ships made victory possible for the Allies at a time when it was otherwise seriously in doubt.
Propulsion of the vessel is by a single screw driven by a direct acting, condensing, cylinder, triple expansion, reciprocating, double acting, steam engine, operating normally at 76 revolutions per minute. It was supplied with steam at 220 pounds gauge pressure and 440 degrees fahrenheit temperature at the throttle by two oil fired Scotch boilers. The engine is designed to exhaust at 26 inches vacuum to a surface condenser bolted to the back columns of the engine. The term 'triple expansion' refers to the fact that steam is fed in turn to the three cylinders, one after the other. The cylinders are named High Pressure (24 inches diameter), Intermediate Pressure (37 Inches diameter) and Low Pressure (70 inches diameter), and all have a stroke of 48 inches. The cylinders and their associated valve chests are bolted together forming a unit block. The cylinder arrangement from forward to aft is as follows: high Pressure, intermediate pressure, and low pressure. The direction of rotation of the engine, looking forward, is clockwise, with the crank sequence as follows: High pressure, low pressure, intermediate pressure.
Boiler pressure steam is supplied first to the high pressure cylinder via its valve, and is then exhausted through the same valve to the intermediate valve and cylinder. From there, the steam passes directly into the low pressure valve and cylinder. The cylinders become progressively larger, but even though there is a drop in steam pressure through each successive cylinder, the work done by each cylinder is therefore the same. Exhausting to a vacuum ensures that the maximum possible expansion takes place in the low pressure cylinder.
The valve gear is of the Stephensen link type. The eccentric rods are crossed and attached to the eccentric straps at the bottom, and to each end of the link bars on top. The valve stems bolted to the valves are attached to the link die block. The link bars are extended on one end to attach the activating drag rods which are moved by the reversing lever mechanism.
When the reversing lever is in the ahead position, the die block on the end of the valve spindle is adjacent to the upper end of the ahead eccentric rod. If the lever is moved to the opposite position, the top end of the astern eccentric rod is adjacent to the valve spindle, and the engine will then run backwards with maximum power. If the reversing lever is moved to the mid position, there is so little movement on the valve that the steam supply to the cylinder is cut off.
The engine bed plate is of cast iron. Cross girders provide flat bottom recesses for the main bearings. The columns are of box section, there being three front (right) and three back (left) columns. The lower ends of the columns are bolted to the bedplate and the upper ends to the cylinder block. Cross head guides are bolted to the back columns.
The crankshaft is of the built up type, made up in two sections with the high pressure and intermediate pressure forming the forward section, and the low pressure the after section. The crank pins and shafts are shrunk onto the crank webs. All eccentrics are bolted to the forward shaft section. A large turning gear is attached to the rear section, and this allows for very slow speed rotation of the crankshaft for repairs and service operations.
The piston sliding in the cylinder is fixed to the piston rod, which projects through the lower end of the cylinder, through a steam tight joint, the stuffing box. The lower end of the piston is attached to the crosshead, which works in slides that guide the piston rod and prevent any side strain being imposed on it by the angular thrust of the connecting rod. It will be realised that when the piston is half way down its stroke, the connecting rod is at an angle to the piston rod, and if it were not for the presence of the guides and slides, a severe bending strain would be put on the piston rod and its gland.
Reversing is accomplished through a small single cylinder reversing steam engine, to the crankshaft of which a reversing worm is attached. A hand reversing wheel is also fitted. A pin on the reversing gear connects to the reverse shaft through a drag rod.
The turning engine is a single cylinder steam driven engine, mounted on the after end of the bedplate. It operates through two sets of worm gears to the crankshaft of the engine. The engine is reversible so that it may be used for setting the valves or making repairs.
The other auxiliaries mounted on and driven off the main engine are the air pump, twobilge pumps and the evaporator feed pump. The pumps are driven together from a massive beam which is moved by beam links from the low pressure cylinder crosshead.
Mounted on brackets integral to the columns just forward of the pum when the engine develops full ahead power.
Water cooling service is provided for the eccentrics, main bearings, crankpin boxes and crosshead guides.
Forced lubrication is provided for the high pressure cylinder and packing of the piston rod, and also to the intermediate slide valve. Tallow cocks are used for the low pressure slide valve and the intermediate and low pressure valve stems. Oil boxes are provided for the main bearings, crank pins and many other moving parts.
The main steam pipe is 8 inches in diameter and the exhaust pipe is 25 inches in diameter. To permit admitting high pressure steam to the various cylinders, a bypass starting valve is bolted to the throttle valve, and pipe connections made to the intermediate and low pressure cylinders as well as the reversing engine.
The thrust bearing is bolted to the ship's tank tops, and transmits longitudinal forces derived from the propellor to the ship itself, and not the crankshaft of the engine.
The propellor shaft is 13 1/2 inches in diameter, and the single propellor is a right hand, four bladed manganese bronze or cast steel, 18 feet in diameter. The blades are of airfoil section.
Steam is supplied at 220 pounds per square inch pressure and 450 degrees fahrenheit by two water tube oil fired boilers. Power output from the engine was 2500 indicated horsepower. Fuel consumption was 170 barrels per day at 11 knots, giving a range of 19,000nautical miles.
Each engine cost approximately $100,000 to manufacture. Ships were to be seen all over the world after the war, and were in service for over 35 years.
This model is built almost entirely from standard 'Liverpool era' Meccano parts. It is a scale working model of the original Liberty ship steam engine. The machine can be separated into its main parts as follows: the bedplate with its main crankshaft bearings, the six columns and crosshead slides, each of which has its own special features, the cylinders and valve chests, the connecting rods with their big ends, small ends and crosshead slides, the Stephensen valve gear and its attachments and control mechanisms, the various other pieces such as pumps, condenser, steam pipes and control gear.
An effort has been made wherever possible to reproduce each of these features in a way which gives the flavor of the original. A certain amount of artistic license has been taken, and each builder will no doubt add or remove or improve a feature as he sees fit. The main idea was to give a scale impression.
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