Oscillating Paddle Engines

An account of the remarkable oscillating paddle engines of the last century and of the introduction of the surface condenser, an important development in marine engineering

MARINE ENGINES AND THEIR STORY - 3

BUILT IN 1864, the Iona, a famous Clyde paddle steamer, saw over seventy years’ service in Scottish waters. The Iona was built and engined by J. and G. Thompson, of Clydebank. She had a gross tonnage of 396 and a speed of 17 knots.

ONE of the most interesting of all marine engine types is, perhaps, that known as the oscillating engine, in which the cylinders swing on trunnions - resembling the mountings of old-time guns. The swinging action of the cylinders enables the thrust of the piston rods to act directly on to the shaft cranks without the use of connecting rods, crossheads or guides. Oscillating cylinders will be familiar to most people, as they are generally used in the simpler forms of model steam engines that serve as an introduction to the serious study of engineering. The use of oscillating cylinders had been proposed by Murdoch as early as 1785. In 1822, an iron steamer, the Aaron Manby, with a length of 120 feet, was fitted with oscillating engines of 80-hp. She made a voyage with cargo from London to Paris, and was the first iron ship to make a sea voyage.

In 1827 Joseph Maudslay adopted the oscillating cylinder arrangement independently and patented a marine engine of this type, with an improved valve gear. An interesting model of Maudslay’s oscillating cylinder marine engine is preserved in the Science Museum at South Kensington.

The engine has two cylinders, placed below the crankshaft. The condenser is between the cylinders and contains the air pump, driven from a crank on the shaft above it. The shaft is carried on A-shaped frames, which also support bearings for the cylinder trunnions. The central trunnions connect the cylinder exhaust passages with the condenser, and the outer trunnions are connected with the steam supply through packed glands.

Piston rods pass through glands in the top cylinder covers, and the D-shaped slide valves are contained in chests at the exhaust sides of the cylinders. Steam passes from the inlet trunnions to the valve chests through circular passages in the cylinder walls. The valves are driven by separate eccentrics on the paddle shaft. Maudslay fitted 20-hp engines of this type to the paddle steamer Endeavour that plied on the Thames between London and Richmond from 1829 to 1840.

Substantial improvements were made in the marine oscillating engine from time to time, notably by the famous engineer John Penn of Greenwich. Valve chests were placed on the cylinder sides between the trunnions, thus producing a more compact engine. To reduce weight, cast iron frames gave place to wrought iron columns and improvements were also effected in boiler design by the adoption of water tubes.

An oscillating engine with three cylinders was patented by Scott Russell in 1853; in this one cylinder was placed vertically and the others at 60 degrees to it. This arrangement gave an even turning movement to the paddle wheels, and was successfully employed in the Egyptian Government yacht Cleopatra in 1858. These engines were of 882-hp and drove 16 ft paddle wheels, giving a speed of 14¾ knots.

On their introduction, oscillating engines were not favourably received by marine engineers, as their movement in the confined space of a ship’s engine-room was regarded as an added source of danger. This prejudice was, however, finally overcome, and numbers of oscillating engines were built for paddle steamers as late as the ‘sixties of last century. Engines of this type ultimately reached huge dimensions and were installed in some of the largest and most famous vessels of their time.

MAUDSLAY’S OSCILLATING ENGINE, patented in 1827, consisted of two cylinders swinging on trunnions. Steam was supplied to the cylinders through the outer trunnions, which were connected to the valve chests by passages cast in the cylinder walls. The inner trunnions were connected direct to the central condenser, which was fitted with an air pump worked by an intermediate crank. The slide valves of either cylinder were worked by an eccentric fitted with a catch for hand operation when reversing. Maudslay fitted engines of this type to the steamer Endeavour that plied on the Thames between London and Richmond from 1829 to 1843.

The Pacific, an iron ship of 1,469 tons, was built and equipped with oscillating engines by J. S. Russell & Co. in 1853. These engines were of 1,684-hp and weighed 240 tons. The two oscillating cylinders were 6 ft 2-in diameter by 7 feet stroke, driving paddle wheels 27 feet in diameter, with fourteen feathering floats 10 feet long and 4 feet wide. The speed was 14½ knots.

Feathering floats were introduced in 1815, as explained in the earlier chapter “Marine Engines and Their Story”, by Baird in his paddle steamer Elizabeth, and the later development of the mechanism operating the floats is worthy of note. To provide a feathering action in large paddle wheels, floats were pivoted at either end, and to the back of each float was secured a projecting arm of iron.

The ends of the arms were connected by long rods to a ring or disk, free to revolve, but set with its axis eccentric to the axis of the paddle shaft. One of these rods, known as a master-rod, was duplicated, with two points of attachment in the disk to maintain its position relative to the paddle wheel. As the wheel revolved, the rods came into operation one after another, and held the floats in a vertical position while in the water.

The boilers of the Pacific are of special interest, as they were among the first “box” or rectangular boilers to be fitted with tubes instead of flues. The use of tubes for box boilers was suggested in 1845 by the Earl of Dundonald, and rectangular multitubular boilers were extensively employed after 1850.

Paddles and Screw

The steam pressure of the Pacific’s four boilers was 18 lb and they contained 1,760 tubes, each 6 feet long and 3-in diameter. Each boiler was 14 ft 9-in long, 18 feet wide, 12 ft 6-in high and had five furnaces. The total weight of the four boilers was 91 tons. The water capacity was 69 tons, the grate area 420 square feet and the total heating surface 9,507 square feet.

Among the largest oscillating paddle engines ever built were those of the Great Eastern, of 1858. This vessel, with a gross tonnage of 27,384, had, in addition to her paddles, a screw driven by separate engines.

The cylinders, four in number, were 6 ft 2-in diameter by 14 feet stroke, and gave 3,411 indicated horse power. The weight of the engines was 836 tons.

The Mersey, an iron-built steamer of 1,001 gross tons, was also equipped with oscillating paddle engines of large dimensions. The two cylinders were 5 feet diameter by 5 feet stroke. Steam was supplied to the cylinders through the outer trunnions, whence it passed to the valve chests through “belts” or passages cast in the cylinder walls. Exhaust steam escaped through the inner trunnions into the condenser. The slide valves were worked through rocking levers by loose eccentric reversing gear. Steam was supplied by four tubular boilers at a pressure of 20 lb and at 30 revolutions a minute the engines drove the vessel at 11½ knots. The Mersey was built in 1859 at Millwall by Samuda Brothers, for the West Indian mail service.

In 1860 a fleet of four iron vessels was built for the service between Holyhead and Kingstown (Dun Laoghaire). One of these ships was the Leinster, and models of her hull and machinery may be seen in the Science Museum at South Kensington. The engines were of the oscillating type, representing a high state of marine engine development. They indicated 4,751 horse power.

The two cylinders, placed immediately below the paddle shaft, were 8 ft 2-in diameter by 6 ft 6-in stroke. Both cylinders were provided with two valve chests arranged on opposite sides of the trunnions, so that their respective weights were balanced. The valves of either cylinder were driven by a single loose eccentric on the crankshaft.

An ingenious device was employed to ensure that the valve motion worked independently of the cylinder’s oscillation. The arrangement comprised a sliding rod driven by the eccentric; on the lower end of this rod was a curved slot accommodating two slide-blocks connected to the valve rods. A rack-and-pinion gear was provided to reverse the engines for going astern. The two air-pumps, worked by an intermediate crank on the paddle shaft, were contained in the condenser, placed between the two cylinders. At the corners of the foundation frames were the feed and bilge pumps, worked by brackets attached to the cylinders. The condenser weighed 22 tons and the cylinders over 20 tons each.

The steam generating plant in the Leinster comprised eight multitubular boilers, supplying steam at 20 lb pressure. The boilers were arranged in pairs with the backs towards the ship’s sides, leaving a central alleyway for stoking. Four boilers were placed forward and four abaft the engines.

THE ENGINES OF THE LEINSTER, an iron paddle steamer of 2,000 tons, built in 1860, are represented by this fine model in the Science Museum at South Kensington. The Leinster's engines had two oscillating cylinders, 8 ft 2-in diameter by 6 ft 6-in stroke. Either cylinder weighed over 20 tons, and the condenser, arranged between them, 22 tons. The paddle wheels were 32 feet in diameter and of the feathering type, with floats 12 feet long and 5 feet wide. The engines indicated 4,751 hp, and gave the vessel a speed of 17¾  knots.

The boilers were each 9 ft 3-in long, 18 feet wide and 12 ft 3-in high. The eight boilers contained 40 furnaces and 4,176 tubes. The total grate area was 677 square feet and the total heating surface 16,800 square feet.

Feathering paddle wheels were employed and these were 32 feet in diameter, each with 14 floats, 12 feet long by 5 feet wide. On her trial trips the Leinster attained a speed of 17¾ knots, with the engines making 25½ revolutions a minute on a steam pressure of 20 lb. The vessel was 343 feet long and 35 feet beam, with a depth of 19 feet, a draught of 13 feet and a displacement of 2,000 tons.

The beam engine of an American river steamer, illustrated below, is of particular interest. The photograph is of a model, built to a scale one-sixteenth full size, in the Science Museum at South Kensington. This type of engine was perfected by Robert L. Stevens, in the United States, about 1822, since when it has been used extensively in the Eastern States, especially for paddle steamers on the Hudson River and Long Island Sound. The model represents an engine of 1884 built by the Pusey and Jones Company of Wilmington, Delaware, U.S.A. Engines of the type are still in use.

The engine has two A-shaped gallows frames of heavy timbering, rigidly braced and bolted together and mounted on wrought iron keelsons. The cylinder has the condenser below it and is 3 feet in diameter, with a stroke of 9 feet. Placed between the feet of the forward legs of the gallows frames, the cylinder and condenser are provided with cast lugs that are bolted to the framework. The crankshaft has bearings on either side of both paddle wheels and also near the crank. Three sets of keys are used to secure the paddle wheels to the shaft.

The valve gear is of a special type patented by Francis B. Stevens in the U.S.A. in 1841. In front of the cylinder are two cylindrical valve chambers, to the top and bottom of which are connected castings containing the valve seats. There are a steam and an exhaust valve at either end of the cylinder and the valve chests are connected by two hollow columns or valve chambers. The left-hand column connects the top and bottom steam valves; the right-hand column joins the upper and lower exhaust valves and is also connected with the condenser. There is one eccentric for the steam valves and another for the exhaust valves. Driven by the eccentric rods are two rocking shafts fitted with four curved arms, known as “wipers”, that operate on four “toes” on lifting rods connected with the valves. The lifting rods are fitted with springs to ensure that the “toes” shall follow the movements of the “wipers”.

River Paddle Steamers

The Stevens steam cut-off is not adjustable and can be varied only by “gags” or stops placed below the valve gear rocking-shafts. When these “gags” are in use, the steam valves are operated by the exhaust gear and steam is used for practically the full stroke of the piston. Increased power is thus obtained temporarily when encountering heavy ice or for increased speed in emergency. Hand-operated valve-gear is fitted also; this comprises a trip-shaft having “toes” and “wipers” that operate in the same way as those of the power-operated rocking-shaft. The power-driven eccentrics are thrown out of gear by a foot lever when it is necessary to operate the valves by hand. Steam is supplied by a single cylindrical return-flue boiler working at a pressure of 65 lb Each paddle wheel is 24 ft 10-in diameter and has three sets of spokes. These carry twenty wooden floats 6 ft 6-in long, secured by U-shaped bolts. A point of interest about these wheels is the provision of narrower floats nearest the line of dead centres to facilitate starting. A vessel powered by an engine of the type described would be about 160 feet long, with a breadth of hull of 28 feet, a breadth over paddle guards of 48 feet and a draught of 4 ft 6-in.

These remarkable beam-engined paddle steamers for river service provide an interesting contrast with the small vessels employed on many tropical rivers, which are equipped with a single paddle wheel at the stern driven direct by a pair of locomotive-type cylinders, arranged horizontally.

Among the paddle engines of various types that have already been described, some used steam to create a vacuum by condensation, thus permitting the atmosphere to perform the working stroke of the piston. Others used steam at an exceptionally low pressure, necessitating the employment of large cylinders, with the attendant difficulties of finding room for the engines in the confined space of a ship’s hull. The boilers of those early days were often only strong enough to hold the weight of the water they contained. For this reason, special relief valves had to be fitted to let in the air. If they had not been fitted, the pressure of the atmosphere would have crushed in the plates, when the boiler cooled down, against the partial vacuum inside.

PADDLE ENGINES OF THE GREAT EASTERN. These engines were of the oscillating type, and comprised four cylinders, 6 ft 2-in bore by 14 feet stroke, arranged in pairs. Each facing pair of cylinders drove on to one crank. The engines indicated 3,411 hp and weighed 836 tons. Between the cylinders were placed two air pumps driven by an intermediate crank on the paddle shaft. The valve chests can be seen on the cylinder sides, one at either end, with a valve rod passing through both chests. The Great Eastern, built in 1858, was 680 feet long, 82 ft 6-in wide and had a gross tonnage of 27,384.

Very low pressures, from about 3 to 5 lb, were not generally exceeded in Great Britain until 1835, but in America the Mississippi steamers used pressures as high as 140 lb per sq in. River steamers had the advantage of unlimited fresh water for supplying the boilers. Seagoing vessels, however, used sea water for the boilers, and consequently pressures rarely exceeded 20 to 25 lb until after 1860, when the surface condenser began to be adopted for ships’ engines.

The use of sea water in boilers resulted, of course, in the deposit of a layer of salt on those surfaces from which heat should have been obtained to generate steam. This made the use of high pressures impossible, especially as the brine had to be “blown out” of the boiler periodically.

Blow-out cocks on the early boilers were opened every hour or so and the steam forced out a quantity of super-salted water, with consequent loss of heat. Sometimes brine pumps were installed to remove the salt solution continuously. As much as one-quarter of the total water fed to the boiler would be removed in this way. The condensers of the early engines consisted merely of a jet of sea water in a tank into which the exhaust steam was allowed to escape. The steam was condensed by the water jet and the condensate (condensed steam and sea water) was pumped back to the boiler.

Hall’s Surface Condenser

In the surface condenser the steam is condensed without mixing with the sea water used for cooling. This condenser comprises a tank containing a large number of tubes through which sea water is pumped. Exhaust steam enters the tank and condenses against the cool tubes, so that the water thus formed is reasonably pure and can be used in the boilers continuously. Both James Watt and David Napier had invented surface condensers, but the credit for the practical development of the device is due to Samuel Hall, who patented his condenser in 1831.

In Hall’s condenser the exhaust steam was passed through a number of copper tubes in a tank, through which cooling water was pumped. In modern surface condensers the cooling or circulating water is pumped through the tubes, against which the exhaust steam condenses externally.

Hall’s condensers were installed in the paddle steamer Wilberforce in 1837, and gave satisfactory service on the London to Hull service until 1841, when the outside of the tubes was found to be coated with Thames mud. Her surface condensers were therefore replaced by those of the jet type, and for many years there existed a prejudice against Hall’s invention.

In 1859, however, a surface condenser was installed in the P. & O. steamer Mooltan, and this was found to be quite successful in operation. Later the surface condenser was generally adopted, and its use had a marked influence on the development of the marine engine.

The surface condenser was devised primarily to keep marine boilers clean by using fresh water, and to save the heat wasted in “blowing out” the brine when sea water was used for the generation of steam.

Apart from attaining increased boiler efficiency, however, the surface condenser paved the way to higher steam pressures for marine work. The use of higher steam pressures became increasingly important with the search for greater thermal efficiency in the operation of the steam engine.

When using steam in one cylinder only, there are considerable heat losses due to the alternate heating and (partial) cooling of the cylinder walls. This difficulty is to a certain extent overcome by using steam at a high pressure, first in one cylinder and then at decreased pressure in a second cylinder. This method of using the steam expansively is known as compounding. If a third cylinder be added we have “triple-expansion” working, with further economy. Quadruple expansion with four cylinders is employed also for marine purposes.

Compounding can be resorted to satisfactorily only when the initial steam pressure is high, and this need for multi-stage operation was another reason for adopting the high-pressure boilers made possible by the use of the surface condensers and by improvements in materials and design. Compound engines were installed in paddle steamers at an early date. It is stated that in 1830 a number of steamers for service on the Rhine and the Meuse were fitted with compound engines by Gerard M. Roentgen in Holland. These vessels had a steam pressure of 70 to 80 lb per square inch.

On the Thames, experiments were carried out with small paddle steamers using high-pressure steam. Three Thames paddle steamers, the Ant, Bee and Cricket, equipped with oscillating engines, employed steam at what was then considered high pressure. Unfortunately several lives were lost by the explosion of the Cricket’s boiler in August 1847. This accident, accompanied by similar explosions in America , discounted the use of high-pressure steam for marine purposes.

High pressures, with compound or triple expansion working were, however, gradually adopted and most present-day paddle steamers have two or three cylinders, arranged athwartships, and driving diagonally upward to the paddle shaft.

This diagonal arrangement of the marine engine for driving paddle wheels was originally patented by Marc Isambard Brunel, the famous engineer, in 1822. Among the finest engines of this type were those of the sister ships Princesse Henriette and Princesse Josephine, built and engined at Dumbarton in 1888, for the Ostend-Dover cross-Channel service of the Belgian Government. These were steel vessels, each of 1,099 gross tons with a length of 300 feet, a beam of 38 feet, and a depth of 13 ft 6-in. The engines were of the compound type, with the cylinders arranged athwartships inclined upward to drive direct on the paddle shaft. The high-pressure cylinder was 4 ft 11-in diameter, and the low-pressure 8 ft 8-in diameter, with a common stroke of 6 feet. Steam to the high-pressure cylinder was controlled by a piston valve, and the low-pressure cylinder was provided with a flat slide valve of the double ported type. The valve rods were extended through both back and front covers. The valve gear was Walschaerts (now used extensively for locomotive work) and this was operated by eccentrics on the crankshaft. The necessary “lead” for

expansive use of the steam was derived from a lever attached to the main crosshead of each piston rod. The variation in the amount of steam supplied to the cylinders for each stroke, or “linking up” as it is termed, was controlled by Brown’s steam reversing gear.

With the increasing size of marine engines and the greater weight of the component parts, the provision of power-operated reversing gear, such as that used on the Princesse Henriette, became a necessity.

Paddle Steamer Built in 1932

A cylindrical condenser was used and the shell was constructed of steel plate. Two vertical air pumps, 34-in diameter by 24-in stroke, were worked by bell-cranks from the high- and low-pressure crossheads. The cooling water for the condenser was circulated by two centrifugal pumps, and Weir’s automatic pumps were installed for boiler feed water. To save weight, cast steel was largely used in the construction of the engines, and the air pumps and condenser ends were of brass.

The paddle wheels were of the feathering type, 24 feet diameter and 13 ft 6-in wide. Each wheel had nine steel floats with concave faces, and at 50 revolutions a minute gave the vessel a speed of just over 21 knots. Steam was supplied by six boilers at 120 lb and the engines at this pressure indicated 7,000 horse-power.

A fine working model of these engines can be seen in the Science Museum at South Kensington.

For general purposes, paddle steamers must be regarded as obsolete as far as ocean voyages are concerned, but they are still built for service in special conditions, especially where shallow draught is essential. Reference has already been made in this chapter to the river steamers of the U.S.A. and to the stern-wheel paddle-steamers that ply on tropical rivers in other parts of the world.

For tourist and excursion traffic round the coasts of Great Britain, the paddle steamer is much in demand. The General Steam Navigation Company, for example, owns a number of such steamers, including one that was completed as lately as 1932. This is the Royal Eagle, built for the passenger service between London and Margate, and equipped with three-cylinder triple expansion engines, arranged diagonally and developing 3,000 horse-power.

In a later chapter an account will be given of the gradual development of the paddle wheel’s successful rival - the screw propeller.

AMERICAN RIVER STEAMERS were equipped with beam engines, and the model illustrated shows the machinery of a type of paddle steamer built in 1884. The cylinder is 3 feet bore, with a stroke of 9 feet, and drives the paddle shaft by a beam supported on massive A frames The paddle wheels are 24 ft 10-in diameter, with floats 6 ft 6-in long. The valve gear controlling the steam supply is shown to the right of the cylinder, and is operated by eccentrics on the paddle shaft.

You can read more on “Coastal Pleasure Steamers”, “The Development of the Screw Propeller” and “The Mississippi”on this website.