In 1886, Mr. R.E. Froude published in the Transactions of the Institution of Naval Architects the deductions drawn from an extensive series of trials made with four models of similar form and equal diameter, but having different pitch ratios. Mr. S.W. Barnaby has published some of the results of experiments made under the direction of Mr. J.I. Thornycroft; and in his paper read before the Institution of Civil Engineers in 1890 he has also put Mr. R.E. Froude's results into a shape more suitable for comparison with practice. Nor ought Mr. G.A. Calvert's carefully planned experiments to pass unnoticed, of which an account was given in the Transactions of the Institution of Naval Architects in 1887. These experiments were made on rectangular bodies with sections of propeller blade form, moved through the water at various velocities in straight lines, in directions oblique to their plane faces; and from their results an estimate was formed of the resistance of a screw.
One of the most important results deduced from experiments on model screws is that they appear to have practically equal efficiencies throughout a wide range both in pitch ratio and in surface ratio; so that great latitude is left to the designer in regard to the form of the propeller. Another important feature is that, although these experiments are not a direct guide to the selection of the most efficient propeller for a particular ship, they supply the means of analyzing the performances of screws fitted to vessels, and of thus indirectly determining what are likely to be the best dimensions of screw for a vessel of a class whose results are known. Thus a great advance has been made on the old method of trial upon the ship itself, which was the origin of almost every conceivable erroneous view respecting the screw propeller. The fact was lost sight of that any modification in form, dimensions, or proportions referred only to that particular combination of ship and propeller, or to one similar thereto; so something like chaos was the result. This, however, need not be the case much longer.
In regard to the materials used for propellers, steel has been largely adopted for both solid and loose-bladed screws; but unless protected in some way, the tips of the blades are apt to corrode rapidly and become unserviceable. One of the stronger kinds of bronze is often judiciously employed for the blades, in conjunction with a steel boss. Where the first extra expense can be afforded, bronze seems the preferable material; the castings are of a reliable character, and the metal does not rapidly corrode; the bronze blades can therefore with safety be made lighter than steel blades, which favors their springing and accommodating themselves more readily to the various speeds of the different parts of the wake. This might be expected to result in some slight increase of efficiency; of which, however, the writer has never had the opportunity of satisfactorily determining the exact extent. Instances can be brought forward where bronze blades have been substituted for steel or iron with markedly improved results; but in cases of this kind which the writer has had the opportunity of analyzing, the whole improvement might be accounted for by the modified proportions of the screw when in working condition. In other words, both experiment and practical working alike go to show that, although cast iron and steel blades as usually proportioned are sufficiently stiff to retain their form while at work, bronze blades, being made much lighter, are not; and the result is that the measured or set pitch is less than that which the blades assume while at work. Some facts relative to this subject have already been given in a recent paper by the author.
Twin Screws.--The great question of twin screw propulsion has been put to the test upon a large scale in the mercantile marine, or rather in what would usually be termed the passenger service. While engineers, however, are prepared to admit its advantages so far as greater security from total breakdown is concerned, there is by no means thorough agreement as to whether single or twin screws have the greater propulsive efficiency. What is required to form a sound judgment upon the whole question is a series of examples of twin and single screw vessels, each of which is known to be fitted with the most suitable propeller for the type of vessel and speed; and until this information is available, little can be said upon the subject with any certainty. So far the following large passenger steamers, particulars of which are given in table II., have been fitted with twin screws. It appears t be a current opinion that the twin screw arrangement necessitates a greater weight of machinery. This is not necessarily so, however; on the contrary, the opportunity is offered for reducing the weight of all that part of the machinery of which the weight relatively to power is inversely proportional to the revolutions for a given power. This can be reduced in the proportion of 1 to the square root of 2, that is 71 per cent. of its weight in the single screw engine; for since approximately the same total disk area is required in both cases with similar proportioned propellers, the twins will work at a greater speed of revolution than the single screw. From a commercial point of view there ought to be little disagreement as to the advantage of twin screws, so long as the loss of space incurred by the necessity for double tunnels is not important; and for the larger passenger vessels now built for ocean service the disadvantage should not be great. Besides their superiority in the matter of immunity from total breakdown, and in greatly diminished weight of machinery, they also offer the opportunity of reducing to some extent the cost of machinery. A slightly greater engine room staff is necessary; but this seems of little importance compared with the foregoing advantages.
TABLE II.--PASSENGER STEAMERS FITTED WITH TWIN SCREWS.
| Vessels. | Length between perpen- diculars. | Beam. | Cylinders two sets in all cases. | Boiler pressure per square inch. | Indicated horsepower. | |
| Feet. | Feet. | Diameters. Inches. | Stroke. Inches. | Lb. | ||
| City of Paris. City of New York. | 525 | 63¼ | 45, 71, 113 | 60 | 150 | 20,000 |
| Teutonic. Majestic. | 565 | 58 | 43, 68, 110 | 60 | 180 | 18,000 |
| Normannia. | 500 | 57½ | 40, 67, 106 | 66 | 160 | 11,500 |
| Columbia. | 463½ | 55½ | 41, 66, 101 | 66 | 160 | 12,500 |
| Empress of India. Empress of Japan. Empress of China. | 440 | 51 | 32, 51, 82 | 54 | 160 | 10,125 |
| Orel. | 415 | 48 | 34, 54, 85 | 51 | 160 | 10,000 |
Weight of Machinery Relatively to Power.--It is interesting to compare the weight of machinery relatively to the power developed; for this comparison has sometimes been adopted as the standard of excellence in design, in respect of economy in the use of material. The principle, however, on which this has generally been done is open to some objections. It has been usual to compare the weight directly with the indicated horse-power, and to express the comparison in pounds per horse-power. So long as the machinery thus compared is for vessels of the same class and working at about the same speed of revolution, no great fault can be found; but as speed of revolution is a great factor in the development of power, and as it is often dependent on circumstances altogether external to the engine and concerning rather the speed of the ship, the engines fitted to high speed ships will thus generally appear to greater advantage than is their due. Leaving the condenser out of the question, the weight of an engine would be much better referred to cylinder capacity and working pressures, where these are materially different, than directly to the indicated power. The advantages of saving weight of machinery, so long as it can be done with efficiency, are well known and acknowledged. If weight is to be reduced, it must be done by care in design, not by reduction of strength, because safety and saving of repairs are much more important than the mere capability of carrying a few tons more of paying load. It must also be done with economy; but this is a matter which generally settles itself aright, as no shipowner will pay more for a saving in weight than will bring in a remunerative interest on his outlay. In his paper on the weight of machinery in the mercantile marine,[3] Mr. William Boyd discussed this question at some length, and proposed to attain the end of reducing the weight of machinery by the legitimate method of augmenting the speed of revolution and so developing the required power with smaller engines. This method, while promising, is limited by the efficiency of the screw, but may be adopted with advantage so long as the increase in speed of revolution involves no such change in the screw as to reduce its efficiency as a propeller. But when the point is reached beyond which a further change involves loss of propelling efficiency, it is time to stop; and the writer ventures to say that in many cargo vessels now at work the limit has been reached, while in many others it has certainly been passed.
Economy of Fuel.--Coming to the highly important question of economy of fuel, the average consumption of coal per indicated horse-power is 1.522 lb. per hour. The average working pressure is 158.5 lb. per square inch. Comparing this working pressure with 77.4 lb. in 1881, a superior economy of 19 per cent. might be expected now, on account of the higher pressure, or taking the 1.828 lb. of coal per hour per indicated horse-power in 1881, the present performance under similar conditions should be 1.48 lb. per hour per indicated horse-power. It appears that the working pressures have been increased twice in the last ten years, and nearly three times in the last nineteen. The coal consumptions have been reduced 16.7 per cent. in the last ten years and 27.9 per cent. in the last nineteen. The revolutions per minute have increased in the ratios of 100, 105, 114; and the piston speeds as 100, 124, 140. Although it is quite possible that the further investigations of the Research Committee on Marine Engine Trials may show that the present actual consumption of coal per indicated horse-power is understated, yet it is hardly probable that the relative results will be affected thereby.
Dimensions.--In the matter of the power put into individual vessels, considerable strides have been made. In 1881, probably the greatest power which has been put into one vessel was in the case of the Arizona, whose machinery indicated about 6,360 horse-power. The following table gives an idea of the dimensions and power of the larger machinery in the later passenger vessels:
TABLE III.--DIMENSIONS AND POWER OF MACHINERY IN LATER PASSENGER VESSELS.