“The forced-draft trials of the Archer class,” he writes, “go far towards sustaining the objections raised by Mr. Howden against the closed stoke-hold system. The trials of the Archer, Brisk, and Cossack had to be discontinued on several occasions, owing to leakage of the boiler-tubes; and when it is remembered that these trials are for only four hours, and that no provision is made for hoisting ashes, it becomes a question of serious consideration whether the maintenance of this high power for such a short period brings with it advantages at all comparable with the continued development of a reasonably high power with an economical expenditure of coal, such as is possible with the closed ash-pit system.
“A number of steamers have been fitted with Howden’s system during the past year, among others the Celtic, of the White Star Line, and the Ohio, of the International Navigation Company. One of the latest steamers fitted with this system is the City of Venice, whose engines were converted from compound to quadruple expansion. Her boilers were designed to develop 1800 indicated horse-power with eighty square feet of grate, but on trial she could only work off 1300 indicated horse-power, owing to some derangement of the valves. She was afterwards tried with half the grate surface in use, when it was demonstrated that there would be no difficulty in developing the power so far as the boilers were concerned. Unfortunately, no data as to weight of boilers, space, or heating surface are obtainable.
“In 1886 the Alliance was supplied with new boilers, fitted with a system of forced draft designed by the Bureau of Steam Engineering. It was originally the intention of the Department to put six boilers in this vessel, as in the Enterprise and Nipsic, but with the introduction of the forced-draft system, which was purely experimental, this number was reduced to four, having a total grate surface of 128 square feet. The boilers were designed to burn anthracite coal with natural draft, and were of course unsuited to the requirements of forced draft, the ratio of heating to grate surface being only 25.6 to 1, and the water surface and steam space being small. The maximum indicated horse-power developed on trial was 1022, but any attempt to run at this or at increased power for any length of time was attended with so much priming of the boilers that the trial had to be discontinued. Alterations were made in the boilers to prevent the priming, but no continuous trial was had previously to the sailing of the Alliance. The results obtained on a measured base were, however, sufficient to demonstrate the practicability of the system, and to show that a higher power could be maintained with the four boilers at forced draft than with the original eight boilers at natural draft.
“The practical working of the system at sea presents no difficulty, as a recent run of the Alliance has demonstrated. On a continuous run of ten hours, using only two boilers with sixty square feet of grate (the grate surface of each boiler having been reduced to thirty), the mean indicated horse-power was 668 and the maximum 744, being respectively 11.1 and 12.4 indicated horse-power per square foot of grate. There was an entire absence of priming, and no difficulty was experienced in operating the forced-draft apparatus, the length of the trial having been determined by the arrival of the vessel in port. The coal burned was Welsh, of fair quality, the consumption being 29.9 pounds per square foot of grate.
“The efficiency of the system may be judged by the results obtained from an experimental boiler at the Washington Navy-yard. The boiler was of the marine locomotive type, and had a ratio of heating to grate surface of 32.73 to 1, with a water space of 245 and a steam space of 163 cubic feet. The coal burned was ordinary Cumberland Valley bituminous, and the evaporation, when burning as much as forty pounds per square foot of grate, was 6.61, while with 37.5 it was 7.24, and this with a moderate air pressure—1.5 inches in ash-pit and one inch on furnace door.”
It is unfair to attempt the explanation of this system without accompanying drawings, but it may be stated that the air, drawn by fan-blowers from the heated portion of the fire-room, is forced through a passage into the ash-pit and furnace, a portion of the current being directed by an interposed plate through the holes in the furnace frame. By the agency of a double row of holes the greater portion of the air which enters the furnace passes around the frame, thence through other apertures to the space between the furnace door and lining, and finally to the furnace through the space between the lining and furnace frame. The supply of air when firing or hauling ashes is shut off by a damper.
APPENDIX II.[61]
THE QUESTION OF TYPES.
The following letter appeared in the Times (London) of April 4, 1885: