The practice is sometimes adopted of burning oil and coal combined, though this is not generally done. In several evaporative trials carried out on a vessel attached to the French Navy with the boilers heated by the mixed fuel, and the burning mixture consisting of 45 per cent of petroleum, the water evaporated amounted to 11.34 lbs. a ton of fuel. This showed an evaporative increase of 25 jx.r cent over that of coal. With the proportion of oil increased to 64 per cent, 14.12 lbs. of water were evaporated, 'bowing an increase of 56 per cent over that of coal when used alone.
The records we have already given are sufficient to justify all the claims which have been made in regard to the superiority of oil as a fuel over coal. At the same time they have proved that great advances have been made in the design of appliances. There is, however, still room for improvement, and in many directions efforts are constantly being made to perfect 'heady existing appliances. The United States is the greatest consumer of fue! oil in the world, and a report, issued some little time ago by the Bureau of Mines, states that during the year 1917 about 160 million barrels of oil were used for fuel purposes. The writer of this report asserts that, of this quantity, a conservative estimate shows that no fewer than 40 million barrels might have been saved by a more intelligent operation of appliances and by proper firings. He then proceeded to state that water tube boilers meet the requisite conditions more fully than other types. The points of importance in connection with boiler design which should be kept prominently in view are : The heating surface must be arranged in such a way that the gas passages are long and of small cross section the heating surface should " see " as much of the furnace as possible, in order to increase the amount of heat imparted to it the com bustion space of the furnace must be so constructed that the burning particles of fuel shall be completely consumed before they can touch the comparatively cold boiler surface ; also, this space should enlarge in the direction of the flow of the heated and expanded gases, as the capacity of the furnace for burning oil is limited almost entirely by the furnace volume ; there should be dampers for controlling the air and the flow of the fuel gases ; uptakes and flue passages should be as free from turns as possible ; the flow of gases counter current to the circulation of the water in the boiler is desirable, as it permits the gas to leave the boiler at a temperature approaching that of the feed water as a limit ; the possibility of unequal diameter stresses should be avoioed in design ; the boiler should permit uniform and positive circulation of the water, and the steam and water spaces should be large enough to maintain uniform pressures and heating conditions.
There are, in addition, other very obvious advantages in the use of oil for power production on ships. Ex pressed briefly they are : Ease of shipping into hunkers or tanks, and with which furnaces can be supplied, the latter being mechanically controlled to a very fine point, thus enabling steam to be maintained at a constant pressure for an indefinite period. Reduction of stoke hold staff. One man can with ease, regulate the pumps and burners for from eight to ten boilers. Less stoke hold space required, thus allowing more room for cargo. Absence of coal dust and ashes, which adds considerably to comfort of passengers and crew. Capacity of forcing, when necessary ; the physical endurance of a man is the limit with coal burning. Cost of maintenance and upkeep of department is reduced by at least 100 per cent when the various items are taken into account, e.g. the saving in fire bars, cleaning materials, ash guards, buckets, fire irons, flooring or shovelling plates, and other items too numerous to mention in this article. The absence of small coal and dust lodging in inaccessible parts of the boiler room. In cases of extreme emergency how often has a rush of water dislodged these items, choking the pumps at a critical moment, with the possible loss of many lives, ship and cargo. This is impossible with oil fuel.
There is almost an absence of smoke arising from properly controlled burners, which, from a navigation point of view, is a distinct advantage—there is no chance of the bridge becoming enveloped in a dense cloud of thick black smoke, with a beam wind, thus obscuring the view of the officer responsible for the navigation of the vessel.