Chapter 7

_Screw Propeller_.--In Mr. Marshall's paper of 1881 it was said that "the screw propeller is still to a great extent an unsolved problem." This was at the time a fairly true remark. It was true the problem had been made the subject of general theoretical investigation by various eminent mathematicians, notably by Professor Rankine and Mr. William Froude, and of special experimental investigation by various engineers. As examples of the latter may be mentioned the extended series of investigations in the French vessel Pelican, and the series made by Mr. Isherwood on a steam launch about 1874. These experiments, however, such as they were, did little to bring out general facts and to reduce the subject to a practical analysis. Since the date of Mr. Marshall's paper, the literature on this subject has grown rapidly, and, has been almost entirely of a practical character. The screw has been made the subject of most careful experiments. One of the earliest extensive series of experiments was made under the writer's direction in 1881, with a large number of models, the primary object being to determine what value there was in a few of the various twists which inventive ingenuity can give to a screw blade. The results led the experimenters to the conclusion that in free water such twists and curves are valueless as serving to augment efficiency. The experiments were then carried further with a view to determine quantitative moduli for the resistance of screws with different ratios of pitch to diameter, or "pitch ratios," and afterward with different ratios of surface to the area of the circle described by the tips of the blades, or "surface ratios." As these results have to some extent been analyzed and published, no further reference need be made to them now.

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.

+-----------+-----+-----------+-------+--------+-------+ | Length | | Cylinders, | Boiler |Indi- | | between | | two sets in all |pressure|cated | Vessels. | perpen- |Beam.| cases. | per |horse- | | diculars. | |-----------+-------| square |power. | | | |Diameters. |Stroke.| inch. | | -----------------+-----------+-----+-----------+-------+--------+-------+ | Feet. |Feet.| Inches. |Inches.| Lb. | | City of Paris. |\ | | | | | | | } 525 | 63¼ |45, 71, 113| 60 | 150 |20,000 | City of New York.|/ | | | | | | -----------------+-----------+-----+-----------+-------+--------+-------+ Teutonic. |\ | | | | | | | } 565 | 58 |43, 68, 110| 60 | 180 |18,000 | Majestic. |/ | | | | | | -----------------+-----------+-----+-----------+-------+--------+-------+ 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.| } 440 | 51 |32, 51, 82 | 54 | 160 |10,125 | Empress of China.|/ | | | | | | -----------------+-----------+-----+-----------+-------+--------+-------+ 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.

[Footnote 3: Transactions Northeast Coast Institution of Engineers and Shipbuilders, vol. 6, 1889-90, p. 253.]

_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.

+----------------+-----------------------+-------+-----------+ | | |Length | | Year.| Name of vessel.| Diameters of | of |Indicated | | | cylinders. |Stroke.|horsepower.| -----+----------------+-----------------------+-------+-----------+ | | Inches. |Inches.| | 1881 |Alaska | 68, 100, 100 | 72 | 10,686 | -----+----------------+-----------------------+-------+-----------+ 1881 |City of Rome | 46, 86; 46, 86; 46, 86| 72 | 11,800 | -----+----------------+-----------------------+-------+-----------+ 1881 |Servia | 72, 100, 100 | 78 | 10,300 | -----+----------------+-----------------------+-------+-----------+ 1881 |Livadia yacht | 60, 78, 78; 60, 78, \| 39 | 12,500 | | | 78; 60, 78, 78 /| | | -----+----------------+-----------------------+-------+-----------+ 1883 |Oregon | 70, 104, 104 | 72 | 13,300 | -----+----------------+-----------------------+-------+-----------+ 1884 |Umbria |\ 71, 105, 105 | 72 | 14,320 | 1884 |Etruria |/ | | | -----+----------------+-----------------------+-------+-----------+ 1888 |City of New York|\ 45, 71, 113; \| 60 | 20,000 | 1889 |City of Paris |/ 45, 71, 113 /| | about | -----+----------------+-----------------------+-------+-----------+ 1889 |Majestic |\ 43, 68, 110; \| 60 | 18,000 | 1889 |Teutonic |/ 43, 68, 110 /| | | -----+----------------+-----------------------+-------+-----------+

In war vessels the increase has been equally marked. In 1881 the maximum power seems to have been in the Inflexible, namely, 8,485 indicated horse-power. The following will give an idea of the recent advance made: Howe (Admiral class), 11,600 indicated horse-power; Italia and Lepanto, 19,000 indicated horse-power; Re Umberto, 19,000 indicated horse-power; Blake and Blenheim (building), 18,000 indicated horse-power; Sardegna (building), 22,800 indicated horse-power. It is thus evident that there are vessels at work to-day having about three times the maximum power of any before 1881.

_General Conclusions_.--The progress made during the last ten years having been sketched out, however roughly, the general conclusions may be stated briefly as follows: First, the working pressure has been about doubled. Second, the increase of working pressure and other improvements have brought with them their equivalent in economy of coal, which is about 20 per cent. Third, marked progress has been made in the direction of dimension, more than twice the power having been put into individual vessels. Fourth, substantial advance has been made in the scientific principles of engineering. It only remains for the writer to thank the various friends who have so kindly furnished him with data for some of the tables which have been given; and to express the hope that the next ten years may be marked by such progress as has been witnessed in the past. But it must be remembered that, if future progress be equal in merit or ratio, it may well be less in quantity, because advance becomes more difficult of achievement as perfection is more nearly approached.

* * * * *

THE LITTLE HOUSE.

BY M.M.

One of the highest medical authorities is credited with the statement that "nine-tenths of the diseases that afflict humanity are caused by neglect to answer the calls of Nature."

This state of affairs is generally admitted, but is usually attributed to individual indolence. That, doubtless, has a great deal to do with it, but should not part of the blame be laid upon the often unpleasant environments, which make us shrink as from the performance of a painful duty?

In social life, unless from absolute necessity or charity, people of refined habits do not call on those whose surroundings shock their sense of decency; but when they go to pay the calls of Nature, they are often compelled to visit her in the meanest and most offensive of abodes; built for her by men's hands; for Nature herself makes no such mistakes in conducting her operations. She does not always surround herself with the pomp and pride of life, but she invariably hedges herself in with the thousand decencies and the pomp of privacy.

But what do we often do? We build what is sometimes aptly termed "an out-house," because it is placed so that the delicate minded among its frequenters may be made keenly alive to the fact that they can be plainly seen by every passer-by and by every idle neighbor on the lookout. This tiny building is seldom weatherproof; In consequence, keen cold winds from above, below, and all around find ready entrance, chill the uncovered person, frequently check the motions, and make the strong as well as the weak, the young as well as the old, very sorry indeed that they are so often uselessly obliged to answer the calls of Nature. It is true, the floor is sometimes carpeted with snow, but the feet feel that to be but cold comfort, though the door may enjoy rattling its broken hasp and creaking its loose hinges.

How often, too, are the nose and the eye offended by disregard of the Mosaic injunction, found in the twelfth, thirteenth, and fourteenth verses of the twenty-third chapter of Deuteronomy! Of course this injunction was addressed to a people who had been debased by slavery, but who were being trained to fit them for their high calling as the chosen of God; but is not some such sanitary regulation needed in these times, when a natural office is often made so offensive to us by its environments that it is difficult for us to believe that "God made man a little lower than the angels," or that the human body is the temple of the Holy Ghost?

Dwellers in the aristocratic regions of a well drained city, whose wealth enables them to surround themselves with all devices tending to a refined seclusion, may doubt all this, but sanitary inspectors who have made a round of domiciliary visits in the suburbs, or the older, neglected parts of a large city, of to any part of a country town or village, will readily affirm as to its general truth.

This unpardonable neglect of one of the minor decencies by the mass of the people seems to be caused partly by a feeling of false shame, and partly by an idea that it is expensive and troublesome to make any change that will improve their sanitary condition or dignify their daily lives.

The Rev. Henry Moule, of Fordington Vicarage, Dorsetshire, England, was one of the first to turn his attention to this matter. With the threefold object of improving the sanitary condition of his people, refining their habits, and enriching their gardens, he invented what he called the "dry earth closet."

"It is based on the power of clay and the decomposed organic matter found in the soil to absorb and retain all offensive odors and all fertilizing matters; and it consists, essentially, of a mechanical contrivance (attached to the ordinary seat) for measuring out and discharging into the vault or pan below a sufficient quantity of sifted dry earth to entirely cover the solid ordure and to absorb the urine.

"The discharge of earth is effected by an ordinary pull-up, similar to that used in the water closet, or (in the self-acting apparatus) by the rising of the seat when the weight of the person is removed.

"The vault or pan under the seat is so arranged that the accumulation can be removed at pleasure.

"From the moment when the earth is discharged and the evacuation covered, all offensive exhalation entirely ceases. Under certain circumstances there may be, at times, a slight odor as of guano mixed with earth, but this is so trifling and so local that a commode arranged on this plan may, without the least annoyance, be kept in use in any room."

The "dry earth closet" of the philanthropic clergyman was found to work well, and was acceptable to his parishioners. One reason why it was so was because dry earth was ready to hand, or could be easily procured in a country district where labor was cheap. But where labor was dear and dry earth scarce, those who had to pay for the carting of the earth and the removal of the deodorized increment found it both expensive and troublesome.

But a modification of this dry earth closet, the joint contrivance of an English church clergyman and his brother, "the doctor," residents of a Canadian country town, who had heard of Moule's invention, is a good substitute, and is within the reach of all. This will be briefly described.

The vault was dug as for an ordinary closet, about fifteen feet deep, and a rough wooden shell fitted in. About four feet below the surface of this wooden shell a stout wide ledge was firmly fastened all around. Upon this ledge a substantially made wooden box was placed, just as we place a well fitting tray into our trunks. About three feet of the back of the wooden shell was then taken out, leaving the back of the box exposed. From the center of the back of the box a square was cut out and a trap door fitted in and hasped down.

The tiny building, on which pains, paint, and inventive genius had not been spared to make it snug, comfortable, well lighted and well ventilated, was placed securely on this vault.

After stones had been embedded in the earth at the back of the vault, to keep it from falling upon the trap door, two or three heavy planks were laid across the hollow close to the closet. These were first covered with a barrowful of earth and then with a heap of brushwood.