Compounding became worthwhile for pressures above about 40psi. That in turn required stronger boilers and a more efficient way of turning heat into steam. By the 1850s a solution had been conceived in the form of a tubular or fire-tube boiler.37 Hot gas passed through fire-tubes inside the mass of water, sucked up by a funnel or smokestack. The area of boiler water touched by hot gas was far greater than in a kettle boiler. Flat-sided box boilers could not take the higher pressure, so from the 1860s on boilers were being made oval or cylindrical. Oval boilers could handle pressures up to about 75psi; above that boilers had to be cylindrical. Designers resisted this change because the new cylindrical (Scotch) boilers wasted considerable space in a flat-sided stokehold.
By about 1890, most battleships and large cruisers typically had single-ended return-tube boilers with four furnaces each (some earlier cruisers had double-ended boilers). Second-class cruisers typically had three-furnace boilers, some with single and some with double ends. All had one combustion chamber per furnace. Single-ended boilers made it easier to subdivide power, but were heavier. Return-tubes meant that the nested fire-tubes passed back and forth through the water volume before exhausting. A typical fire-tube might be 2½ins in outside diameter, with a 7in water space down the middle of each nest of tubes. The grate area on which coal was burned was about 3 per cent of total heating area (i.e., the area of the fire-tubes), the latter typically amounting to 2.5 square feet per IHP at natural draught.
To generate more heat, hence more steam, boilers needed more air. Fans were used to build up air pressure and hence air volume in a closed stokehold. In mid-century advocates of such forced draught claimed that they could increase steam output by 30 or 40 per cent, even with low-quality coal. Greater temperatures in turn increased stress on the boiler itself. Typical British (and, presumably, foreign) naval practice limited machinery weight by using thinner boiler plating than in commercial practice. Boilers had to be rigid, to contain steam pressure, but they also had to expand at high temperatures (typically they were corrugated, to allow for expansion). This was not a good combination. To avoid bursting boilers, the Admiralty typically limited forced draught runs to a few hours, and it distinguished between a ship’s performance using forced versus natural draught. During the 1880s and 1890s DNC Sir William White often claimed that foreign cruisers reached high speeds by using high rates of forcing which could never be repeated in service; the rated speeds of the cruisers he had designed for the Royal Navy were far more realistic because they reflected much more realistic conditions. In its 1892 report the Boiler Committee recommended that specified forced draught be limited to 25 per cent beyond specified natural draught power for standard navy boilers, and 45 per cent for torpedo gunboats (presumably meaning for locomotive boilers).
With high enough steam pressure, enough was left at the outlet of the second cylinder to make a third or even a fourth cylinder worth while: triple or quadruple expansion. Higher pressure and more cylinders meant greater efficiency and thus longer range. Each boost in steam pressure bought greater economy.38 Because triple expansion increased the number of cylinders, it made crankshafts easier to balance and thus reduced vibration.
The most extreme fire-tube boilers were the locomotive boilers installed on board small fast ships from the 1870s on. In the Royal Navy, the first such boiler was on board the prototype torpedo boat Lightning (1879), and these boilers were later tried on board small cruisers. There was no pretension to efficiency; the object was to generate as much steam as possible in the smallest possible dimensions. Cylindrical boilers used relatively large-diameter fire-tubes, which could not easily be blocked by cinders from the coal fire. Locomotive boilers used the smallest possible tube diameters, for maximum heating area inside a cylinder filled with water. The tubes were straight, from firebox to smoke box (leading to funnel). Tubes could easily be blocked (and burst) by unwanted grease or cinders, but in the 1880s there seemed to be no other way to produce enough steam within small dimensions.
The alternative to fire-tube boilers was conceived (and used in a few cases) as early as the 1850s: the water-tube or tubulous boiler.39 The relationship between water and hot gas was reversed. Feed water was led through tubes passing through the furnace. Much greater water surface could be exposed to heat. Limited diameter tubes could withstand greater pressure than a large cylindrical boiler. The outer skin of the furnace did not have to withstand steam pressure. Water-tube boilers could generate higher-pressure steam, which was exactly what high-powered warships needed. As early as 1873 some liners were operating at 100psi. Proponents argued further that because the mass of water in them was relatively small, it took less heat to start them: they could start much more quickly, and they could more quickly answer demands for more steam. They were also expected to be more durable, capable of longer runs at high power. The British found themselves unable to get enough power from the available heating space, using conventional boilers.40 In 1892 the Boiler Committee recommended installing tubulous boilers in two ships for trials (Thornycroft on board the torpedo gunboat Speedy, Belleville on board the torpedo gunboat Sharpshooter), and that one at least of the new cruisers be so fitted if the trials proved successful. A third torpedo gunboat, Spanker, was fitted with French du Temple boilers. Bellevilles were chosen for the cruisers Powerful and Terrible before the Sharpshooter trials were complete because they needed so much power. The only ones considered should have relatively large-diameter straight tubes which could easily be cleaned and examined. It happened that the French Belleville fitted this description.
When the Royal Navy adopted water-tube boilers, the great advantage cited was that it was no longer necessary to force boilers to reach and maintain high power (the Germans, however, wrote that water-tube boilers were more heavily forced than cylindrical ones). The 1902 report of the Boiler Committee explained that there was greater fire-grate area for the same floor space, hence less forcing to reach full power. There would be less damage if the boiler were struck by a projectile, since there would not be a large pressurized vessel to burst. A water-tube boiler could also carry a higher steam pressure, and it was lighter for the power it generated. However, it took relatively little scale or corrosion to ruin a water-tube boiler. The Royal Navy adopted fresh water as boiler feed and its ships had to carry stocks of reserve feed water for the first time.41 With so little water in the boiler, there was no reserve to make up for slight irregularities in feed, so the rate of feeding had to be automatically controlled, and very quickly altered when more steam was demanded. Similarly, a water-tube boiler would react more sensitively to irregular stoking, and the type of fire used had to be changed. The boilers had to be fed more continuously, and with greater care than before. Water-tube boilers were not necessarily more efficient than their cylindrical predecessors – and cruisers needed efficiency as well as compact high power: the boilers worked best at a high fraction of their designed output. The solution to economical cruising was to have a large number of such boilers, only a few of which were lit off for cruising. Unfortunately, a ship in a combat zone would want most of her boilers lit all the time, so she would be quite uneconomical. Some British armoured cruisers designed about 1902 had a combination of cylindrical and water-tube boilers, the cylindrical boilers acting as, in effect, the ship’s cruising power plant.42 The agonizing period during which the Royal Navy decided both to adopt water-tube boilers and which boilers to adopt became the storied ‘battle of the boilers’. 43
A broadside 6in breech-loading gun on