As divers, for the most part we will encounter two types of engines on wrecks, these being the Triple Expansion Engine and the Two Cylinder Compound Engine.
Over the years the engine room has seen a great deal of changes. The engines used in the 1850,s were of large construction in order to develop sufficient power from the low pressure boilers then available. Pressures of 12-30 psi were usual and 30 psi being the maximum for the box shaped boilers of the early colliers. During the 1850,s a great variety of engines were in use, the majority were simple engines, in which the steam expanded in the cylinder and then passed directly to the condenser. Many of the engines at this time had oscillating cylinders, in which the piston rod was connected directly to the crank on the propellor shaft. The oscilating engine was very compact and needed no valve gear, as the movement of the cylinders themselves covered and uncovered the inlets and exhausts. The steam reached the cylinders via the central pivots, sealing these joints was not to difficult at low pressures, but was more difficult in screw vessels where higher shaft speeds were needed. A speed of 20rpm was ideal for paddle wheels, but efficient propulsion by screw needed at least 60rpm. These engines served the early colliers well, but they had high fuel consumption.
Due to the high fuel consumption rate of engines at this time, cargo space was inturn lost to bunker space (coal), this giving rise to considerable interest in the development of a more efficient engine type.
One way of achieving greater efficiency lay in higher boiler pressures, however this led to fresh problems. Cylinders had to be made much stronger to withstand the high initial pressure of the steam, which then fell sharply as the steam expanded, giving a very uneven push to the crank. To overcome this the two cylinder compound engine was introduced.
TWO CYLINDER COMPOUND ENGINE
Workings of the 2 cylinder compound engine, 2 phase....The two cylinder compound engine has a small high pressure cylinder that absorbes the initial energy of the steam which then passes to a larger low pressure cylinder where the remaining expansion of the steam takes place, from here the steam passes into the condenser and is returned to its formative state, ie water and then returned to the boiler. This type of engine came into general use during the 1870,s, giving steam pressures of 70-130psi and remained unchanged for the best part of 50 years. The final development was the triple expansion engine.
TRIPLE EXPANSION ENGINE
Workings of the triple expansion engine, 3 phase... Steam is taken firstly in at the high pressure cylinder via the boiler, then passes to the intermediate pressure cylinder where it expands and then finally into the low pressure cylinder, once again expanding before being taken into the condeser where the steam passes over tubes of cold water to return the steam to its formative state, ie water and is then returned to the boiler.
As with engines, new designs in boilers were developed in order to deliver higher steam pressures to the newly developed engines.
The Scotch type boiler (drum type), a high pressure boiler typical of that used in conjunction with the compound and triple expansion engine.
The effectiveness of compounding engines depended upon higher steam pressures, however increasing steam pressures presented difficulties for boiler designers and builders, it was not simply a case of making plates thicker. By the 1860,s it was well known that flat surfaces were less able to resist steam pressure than curved surfaces and required many support stays to be fitted in order to run at even moderate pressures. Box type boilers were used as they fitted into the box shaped stokeholds of ships, providing a large steamstorage capacity with little waste of space, however there flat sides needed substantial support and operating pressures were still limited. A cylindrical shaped container is stronger than that of one with flat surfaces. By using a cylindrical shape, the pressure vessel may be made thinner for the same strength or stronger for the same thickness, in turn the development of the drum boiler offered itself perfectly to the requirments of the newly developed compounding engines.
References taken from
Steam at Sea ... Denis Griffiths.
Steam Coasters and Short Sea Traders .... C.V.Waine & R.S.Fenton.