Maritime engines are the same in principle as those used in cars or aircraft, depending on the type. All these engines use some form of energy stored in a gaseous form to move an object that is in turn connected to some shaft which in turn connected to a propulsion device, be it a paddle wheel or a screw propeller.

The first engine was the piston type, and despite it's inneficient operation still today the most popular type of engine world wide. The first piston engines were steam driven and referred to as "Reciprocating Engines". Later varients injected flammable vapour in the form of petrol or Diesel into the cylinder and ignited it, this removed the need for a boiler system to generate the steam and is still the most popular method of powering cars today.


In a piston engine gas under pressure is released into a cyclinder containing a piston which is airtightly sealed to the interior of the cylinder. The force of the gas drives the piston down and in doing so it transmits rotary motion to a shaft. A counterweight to the piston keeps the shaft turning even when the gas is no longer exerting pressure on the piston and as a result the shaft now drives the piston back up, forcing out the expanded gas.

The engine is inefficient as part of the energy imparted to the shaft is put back into the system to send drive the piston back up.


These early steam engines did not lend themselves to ocean going ships since they used a lot of water to generate the steam which had to be stored, salt water could not be used as it was highly corrosive and ate away the engine. The first succesful steam powered vessels tended to be river boats operating on the great inland rivers and lakes of the world or in coastal waters. Similar designs were also utilised in railway engines and steam tractors where water and coal were readily available.

The paddle wheel was the first motivator of choice for boats as the engines tended to be very large and slow moving, which nicely fitted them to the task of cranking a cumbersome paddle wheel. A major advance which made these engines practical was the Double Acting Steam Engine which utilised a sliding valve to force the piston first in one direction and then back in the other direction, this is particularly important in early steam engines as the pistons, cranks and shafts are huge and heavy.

Although paddle wheels had a brief hey day in ocean going vessels their problems were insurmountable, but as inshore propulsion they lasted a very long time, as late as the 1980's paddle wheel tugs operated in HM Dockyards for instance.


An engine from the double engined paddle steamer Comet, now located in the London Science Museum. This was the first commercial passenger steam boat in Europe and was operated by a Scottish resort owner from 1812 to 1820. It was by no means the first steam boat, many attempt had been made previously, both in Europe and America , but in the US further developments of the Great Lakes paddle steamers stalled during the Civil War and other attempts in Europe had largely failed due to fear and reluctance to embrace the new technology.

Another essential component of the Steam Engine to make it effiicient, particularly for maritime work, is the condensor. The Condensor's original purpose was to maintain a vacuum to assist in the exhaust stroke of the piston, but it was soon found that an efficient condensor also recovered the exhaust steam and recaptured much of the water to enable it to be reused, making a maritime engine practical.

An old steam train engine will generate balls of steam from the exhaust, this is the steam being vented from the cylinder at each stroke. For a railway engine this is not too much of an issue, trains could stop for water, or even scoop up on the move from troughs between the tracks. The advantage of a vacuum maintenance was not an issue in the design of locomotive engines. However underground and roadside steam engines and modern power stations are required to use condensors to cut down atmospheric emissions.

But in the maritime engine the condensor recycles fresh feed water for the boilers. Another offshoot of the condensor was the desalination condensor which are also usualy fitted to warships to enable more fresh water to be extracted from salt water if needed due to damage, and to supply the needs of the crew. Desalination plants are not usualy found on merchantmen except liners which have large numbers of passengers to provide for.

T he PS Great Western (Often mistakenly designated SS) was the first purpose designed Ocean going paddle steamer, though in the event she was pipped to the first steam passenger Atlantic crossing by a cross channel steamer: Sirius in a race in 1838, Sirius had to burn most of its cabins and masts to achieve the task!

Until the Great Western steam ships had not been considered suitable for long hauls due to cost, coal cost, wind did not! But steam ships were proving their worth where regular services were required regardless of wind and weather, the concept of the Great Western was to extend the railway service of the Great Western Railway to the USA with similar fixed timetables.

Paddle Wheels though were inherently inefficient and worse so in heavy weather. Furthermore they prohibited the use of steam in warships since at the time warships were still broadside firing not only did the wheels impede the placing of guns but were considered just too vulnerable. The only paddle wheel warships tended to be auxilliaries such as tugs and support ships.

The Designer of the Great Western, IK Brunel, turned to the Screw for his next great ship, the Great Britain. Great Britain was already in build as a Paddle Steamer when the decision was taken to convert her to a single screw. The Great Britain was also to be all iron, whereas the Great Western had been Iron Clad wood, similar to the Iron Clad warships of the era. Launched in 1843 she served as an Atlantic passenger liner but without notable financial success. After running aground in 1846 she was re-built and in 1852 served the Australian run where she soon proved her worth. In 1855 she was used as a troop ship in the Crimean War and the Royal Navy got to assess her and largely copied her as HMS Warrior, the first iron hulled steam warship, classified a Frigate as she had a single gun deck, though greater firepower than most capital ships of the time, launched in 1860 she was the Dreadnought of her day.

Engines of the mid 1800's were still relatively primitive, HMS Warrior used only two cylinders for instance, pictured above, the cylinders were hoizontally mounted to fit better low in the ship and used the "Trunk" system where the inner cylinder was free to move also and so extended the actual working stroke of the piston. Her single screw had just two blades and was directly driven from the crankshaft.

The inneficiency of the early 1800 steam engine meant that only passenger liners and warships really employed the steam engine on long hauls, freighters were not economical as they had to carry huge amounts of coal to power themselves and this used up freight space. The only real steam freighters of the time carried high value cargo with low bulk, such as mail.

To increase the efficiency of an engine higher temperatures were needed to produce greater pressure, the problem was one of thermodynamics though, huge amounts of the heat in the steam was transferred to the piston and cylinder on the drive stroke, these then cooled again with the exhaust stroke and so would soak up the heat again on the next drive stroke. Heat = pressure so heating the pistons decreased the pressure.

In 1868 Alfred Holt invented the compound engine. In this engine the initial high pressure, high temperature steam is fed to a small cylinder, since it is small there is less area to transfer heat to and the piston, but not all the power is extracted from the steam, on the exhaust stroke the steam, still at pressure, feeds to a second, larger cylinder which extracts the remainder of the power from the steam.

The maritime version of the engine was an instant success, far less coal was needed and so more goods good be transported, steam ships began to operate to the far east, where previously only sail, or sail and steam ships had been able to cope with the distance.

Toward the latter half of the 1800's industrial techniques had developed to enable ever higher pressure boilers to be produced, and the development of high temperature, petroleum based lubricants. These boilers could generate more power than the twin compound engine could absorb so the Triple Expansion Steam Engine was developed and steam travel came of age at sea.

The first commercial triple expansion engined ship was the Aberdeen, 1881, which plied the Britain to Australia route. The efficiency and economy of the engine caused a revolution in ship building and within a few short years steam had overtaken sail as the primary mover of freight.

The Triple Expansion Engine continued long after the introduction of the Turbine, as late as WWII it was being produced in huge quantities to power the Liberty Freighter Ships and warships where speed was not so critical.

The final development of the reciprocating engine was the four cylinder or Quad Expansion Engine, these were used in high performance ships such as liners early in the 20th century but were a comparative rarity compared to the triple.

The Triple Expansion Steam Engine became the mainstay of maritime traffic, both military and civilian during the late 1800's and through the second world war, although challanged by the turbine it was cheaper and more efficient at low speeds and in the end it would be the Diesel that would replace it fully.

A typical maritime Triple Expansion Engine, this was the power plant from the mass produced Liberty Ships of WWII

The principle of superheating steam was well known prior to it's introduction, but was simply not useful at first as there was not the lubricants to cope with it, so boiler technology did not so much lag behind engine development, as waited for it to catch up.

Normal steam is created by boiling water, this is saturated steam, when it starts to cool some of the water condenses back out, the water is more compact than steam so pressure falls. It was found that taking the steam out of the original boiler with the water still in it producing more steam, and then heating it again produced far greater pressures and temperatures could be achieved and more power was available until condensation started to occur.

The standard RN boiler for much of the between wars and WWII was the Admiralty Triple Drum Boiler, which could have either a coal or an oil furnace. As it's name implies it comprised three water drums, cylinders, forming the three points of an inverted Vee with the furnace inside the vee, the drums were linked by a maze of pipes in which the primary heating occurred and then another set of pipes in which the superheating was applied before the steam was passed to the engines.


A typical three drum boiler, this one produced by Yarrows.