But the achievements of screw steamers are not always satisfactory at first, and time has shown some curious instances where what appeared at first sight a little thing prevented great results. To-day we know somewhat of the screw propeller, but it is very difficult, if not impossible, for the cleverest and most experienced engineer to define his knowledge or to classify his facts so as to deduce any rules from them that shall enable him to lay down fixed laws for the practical guidance of others. In past years more was professed, but still less was actually known, and that which was to be a panacea for the ills of every screw ship proved useless in many instances, and aggravated the evil in others. The patents for propellers are numerous, and some of the specifications interesting and amusing, but of them all there are less than can be counted on the fingers of one hand that have any practical value, or that have influenced the commerce of the world; and we find to-day that the propeller which gives the best results is very simple in form and its working surface a true helix. What is better understood, however, are the proportions, and in them lies the success of the instrument. It is quite true that the blades may be of such a shape and so arranged as to give bad results, but it is very difficult to alter the propeller blade now most generally used and get much improvement thereby.
The Propeller of the North German Lloyd Steamer Havel.
(From a photograph of the steamship in Handren & Robins’s Erie Basin Dry Docks, Brooklyn.)
In 1865 H. B. M. S. Amazon was found to fall short of her designed speed by nearly a knot, although the indicated horse-power was in excess of the requirements. With a four-bladed Mangin propeller, 12 feet 6 inches pitch, it took 1,940 I. H.-P. to drive the vessel 12 knots. A two-bladed Griffith’s screw of 13 feet 9 inches pitch was substituted, when 12.4 knots were obtained with only 1,664 I. H.-P. But the most remarkable case was that of H. B. M. S. Iris, which had been designed for a speed of 171⁄2 knots, but on her first trial trip, although the 7,000 I. H.-P. was exceeded, the speed was only 16.58 knots. A series of trials was then entered upon to find out the cause of this deficiency, with the result that the screws were discovered to be too large; others of 2 feet 3 inches less diameter were substituted, when a speed of 18.57 knots was attained with the same I. H.-P. Similar instances could be adduced, if necessary, to show how comparatively slight changes in the propeller can produce marked improvements in speed.
It has already been shown that the frictional resistance of the skin of the ship is very great, and generally speaking, in fast steamers, is by far the largest portion of the whole resistance. It necessarily follows, therefore, that for high speed it is essential that the submerged portion shall be as smooth as possible; and to that end ships are coated with enamel paints which, when dry, are perfectly smooth and glassy, or remain in a smooth, slimy condition. They do not, however, remain long in this state, as the action of sea-water destroys them, and even the best of these compositions admits, at times, of marine plant growth, and sometimes barnacles. The effect of a coating of weed is very serious indeed; the resistance induced thereby being greater than if the vessel were rough, from the fact that each filament of weed has to be towed through the water, and the total surface thereby exposed may be two or three times that of the ship herself. It is a sound economy in any vessel to keep the bottom perfectly clean and smooth, but in the case of high-speed steamers it is absolutely essential, inasmuch as a very moderate amount of foulness will reduce their speed by 2 or 3 knots.
The introduction of Siemens-Martin steel, about the year 1875, and its continued and extended use since, have however been really the means of rendering possible the construction of steamships of all sizes with high rates of speed now so common, and are undoubtedly the means whereby those ships can be so economically built and worked as to pay as commercial ventures. The construction of their hulls with a material fifty per cent. stronger than iron has rendered it possible to make such appreciable decrease in weight as to admit of fining their lines suitably for high speed without sacrificing carrying capacity. With this same steel, boilers can be constructed for a pressure of 150 pounds per square inch without weighing very much more than iron ones for 75 pounds. By using steel for castings, forgings, etc., the weight of the machinery has been reduced from 5 hundredweight to 2 hundredweight per I. H.-P., and when forced draught is employed it is as low as 1.6 hundredweight per I. H.-P. for large powers, and less still for such engines as are used in torpedo boats and catchers.
Recent Naval Engine.
(Made by Earle’s Shipbuilding Company, Hull, England, for twin-screw fast cruiser for the British Navy, of 9,000 I. H.-P.)
It has already been remarked that the consumption of coal, which enters as a most important factor into the question of high speed, both from the weight and cost, had been reduced, by the introduction of the compound engine, from 4 pounds to 21⁄2 pounds per I. H.-P., and latterly, as that engine was improved and higher pressures used, the consumption was further reduced to 2 pounds, and in some cases as low as 13⁄4 pound per I. H.-P. The triple expansion engine, developed within the past eight years, and later the quadruple expansion, have effected a still further saving, until with them and such other means as are now employed, the consumption is under 11⁄2 pound of coal per I. H.-P.
The success of the locomotive was very questionable until the exhaust steam was turned into the chimney so as to create a rapid draught, and the steam-blast to-day enables the locomotive to travel at its great speed by causing the comparatively small boiler to generate such a large amount of steam. When this form of boiler was tried on board ship its power would have been very much crippled had not some other means been adopted for forcing the draught, as the steam could not in this case be allowed to escape through the funnel, but must be condensed into water for the use of the boiler. By closing the stoke-hole and forcing into it by mechanical means a plentiful supply of air, this boiler was made to be as efficient for a torpedo boat as for a locomotive. This forced draught has now been adopted on large ships, and to-day the very high speed of naval vessels, and of many mercantile steamers, is due to it. Consequently, with the same weight of machinery, higher powers are developed with a corresponding increase in speed, and the cruiser Piemonte, constructed by Sir William Armstrong & Co., of which an illustration is shown on [p. 91], had her speed increased by means of forced draught from 20 knots to 22.3 knots, at which speed she was going when the picture was taken.