Steamships and their story

by E. Keble Chatterton

CHAPTER XIII
SOME STEAMSHIP PROBLEMS

I have left till the end of the story the consideration of some of those points which, though of the highest interest to many who are anxious to know something of the intimate character of the steamship, may seem to some readers to possess a special rather than a general concern. However, now that I have shown the manifold manner in which the steamship has advanced from a thing of scorn to a vessel of admiration, and have indicated as far as possible within the limitations at my disposal the ways and means that have brought this about, we may pertinently stop to consider for a few moments some of the problems which still have to be encountered even to-day, when naval architecture and marine engineering have attained to such heights of perfection. I shall endeavour, as, indeed, has been my aim throughout the course of this volume, to make myself perfectly clear without the employment of more technicalities than may be necessary. To the reader who may happen to form one of that large class who regard the ship, whether propelled by sails or by steam, with an admiration that verges on affection, I need offer no apology; for no one can possibly reverence the ship and, at the same time, be content to remain in ignorance about her complex nature.

Perhaps there is no feature of the steamship which is less suspected of being misunderstood than the propeller. To the average mind, its character is apparently so self-evident as barely to require any unusual consideration. But its introduction as a means of ship-propulsion has been the cause of a good deal of miscomprehension, and has set to work the keen brains of some of the most able mathematicians in order to determine the exact relation which it bears towards the ship and the manner in which it is capable of being used for the greatest good, and with the utmost economy. Here and there in the course of the narrative I have hinted at some of these problems, but in order not to break up the continuity of the story, I deemed it best to defer until now the fuller presentation of the subject. It is not necessary to remark that the propeller’s function is, by means of its revolutions, to drive the ship ahead, and to overcome the resistance which encounters the hull. Besides the skin friction, the eddy-making, and the wave-making, there is also the resistance of the air. Now let us suppose for a moment that instead of propelling itself ahead by its own engines and screws, a liner were to be taken in tow by a powerful tug-boat. It would follow then that the pull required to cause the liner to go through the water would be equal to those total entities of resistance which we have just enumerated. But let the tug be cast off, and allow the liner to start her engines and proceed by means of her propellers. The above resistance now becomes augmented by the resistance of the propellers. The reason is that the propeller causes a suction which tends to pull the ship back.

It is a striking fact that about one quarter of the propeller’s work is wasted in friction, and slip. (By “slip” is meant the loss caused through the yielding of the water at the propeller, and the screw not progressing to the full extent of its pitch.) In designing the screw for a steamship, due regard must be paid to the amount of horse-power which the engines are to generate and the speed at which the vessel is to travel, but whether the inward- or outward-turning propeller is the more efficient has not yet been satisfactorily determined by experts, though the probability would seem to be with the outward-turning screws. An instance of this was recently afforded by one of the leading firms of ship-builders in this kingdom who had been commissioned to construct a vessel 300 feet long, with a speed of between 18 and 19 knots. The owner, who was a scientist, particularly stipulated that the ship’s propellers should be inward-turning, and was very positive of the advantages which would thus accrue. The builders, however, arranged the engines in such a manner that they could be driven either way with equal ease. After they had tried turning inwards, they tried outward-turning, and reversed the propellers with a decidedly satisfactory result. The same conclusion has also been arrived at by Professor W. S. Abell, who asserts that all his experience goes to prove that greater hull efficiency is obtained by outward-turning propellers. In this connection I might quote the case of the steam yacht Niagara II., which was built some years ago in the United States. She was about 250 feet long, with a displacement of 2,000 tons, and her deadwood aft was not cut off. Information was obtained through two six-hour trials under similar conditions, except that her screws were interchanged from side to side, so that they were inward-turning on the first trial, and outward-turning on her second. Notwithstanding that greater horse-power was used when the inward-turning propellers were employed, yet the latter did not give the ship the same amount of speed as when they were made to turn outwards. Indeed, the speed of the inward was found to average 12·8 knots, whereas the outward-turning screws gave an average of 14·12 knots. It is in the department of the propeller that fuller information is awaited with an enthusiasm that belongs to no other branch of naval architecture.

When we speak of a steamship as being of such a tonnage, we do not always thereby convey a correct idea as to her size, for there is a decided difference between one kind of tonnage and another. When we say a vessel displaces so much water, we know that her weight is exactly that amount of tons; but the tonnages which are given in a vessel’s certificate after being surveyed are of a totally different character. The Board of Trade recognises three measurements of tonnage. First of all, comes the under-deck tonnage. The “tonnage-deck” is the second deck from below when the ship has more decks than one, and the length for the purposes of tonnage-measurement is taken along this deck. This length is divided into a number of equal parts, and the transverse sectional areas are found, deductions being allowed for the thickness of the ceilings. The gross tonnage of a ship consists of the under-deck tonnage plus the tonnage of all the closed-in spaces above the tonnage-deck, excepting the spaces fitted with machinery, wheel-house, shelter for deck passengers, galleys and w.c’s. If the poops, bridges and forecastles are fitted with doors or some other means of closing them permanently, they have to be measured into the gross tonnage; but if they are not of a permanent character, they are exempt. Thus, the gross tonnage of a steamship might include the under-deck tonnage, the space between decks, the poop, the bridge, the forecastle, the captain’s and the officers’ quarters, the chart-room, the light and air space, and so on.

But the net register tonnage will be ascertained by making certain allowed deductions, which include the space taken up for propelling power, the quarters of the crew, and of the captain, as well as the chart-room, the boatswain’s store-room, and the water-ballast spaces. As instancing the curious results which are obtainable from the different measurements for reckoning tonnage, Mr. A. L. Ayre, in his “British Shipbuilding,” gives the interesting comparison of a particular steamship according to her varying tonnage. Thus the ship in question has an under-deck tonnage of 550, whilst her gross tonnage worked out at 980, and her net register tonnage at 360. It is not generally known perhaps that the complicated system of arriving at the net register tonnage gives opportunity for strange and amusing effects. Owing to the difference between the actual engine-room in a steamer and the theoretical engine-room, it is not only possible to build a ship with a negative tonnage, but this has actually occurred in the case of a certain tug, and was referred to in the report of the Royal Commission on Tonnage, 1881. The present writer was recently aboard a new 20-ton yacht, in which the owner had been fortunate enough to persuade the authorities to get the measurements down so low that the net register tonnage came out at a ludicrously low figure. Internally, nothing was more conspicuous than her roominess, which was of a quite exceptional character. The vessel was a two-masted sailing craft, but supplied also with an auxiliary motor, which did not detract from the roominess of the ship, since it was placed out of the way underneath the companion ladder. However, by the time the deductions had been made for “engine-room” space, “chart-room” (which was really the comfortable and spacious main cabin), and sundry other items, the size of the yacht had theoretically shrunk from 20 tons to something almost insignificant, and the consequence was that this bold vessel was able to escape with harbour dues as low as yachts of one quarter of her own tonnage. Not long since a humorist saw fit to write an amusing yarn, in which he depicted a certain individual who, smarting under what he believed were excessive harbour dues, determined at length to get even with the authorities, and finally had built a steam vessel rather on the lines of the screw tug than the usual steam yacht. Roominess was not the owner’s objective; all he wanted was just as much space for himself as was comfortable. But he sub-divided the rest of the ship into a large space for her engines and boilers, as well as auxiliary engines to drive capstans, together with a roomy forecastle for the crew. His own cabin was clearly marked on the plan as “Captain’s Cabin.” Finally, after the vessel was launched, and the internal capacity of the hull, as well as the spaces occupied by the machinery and the crew, had been deducted so as to obtain the net register tonnage, it was found that instead of coming out at so much net register, the figures showed that she was minus 7 tons! Consequently, the owner used to protest every time he was charged with harbour dues, that instead of being called upon to pay, it was really the harbour authorities who owed him. After this, it is not surprising to learn that the name of the vessel was the Euome. I do not suggest for a moment that this story is anything but mythical, but it is sufficiently illustrative of what may occur when the tonnage measurement rules are in a state of such confusion.

It will be readily understood that it is of the utmost importance that regard be paid to the stability of the steamship, and herein is presented another of those problems which have to be taken into account and solved as easily as may be. Now, a vessel loses a great deal of her stability when she carries loose in her hold oil in bulk, grain, rice, and such movable cargoes. A similar effect is produced, of course, by the amount of free water in her tanks. For unless these features of danger are guarded against, it follows that when the ship is inclined to one side or the other by wind or wave, the cargo will cause the ship to have a worse list, and there may be some chance of her not regaining her proper trim, and turning turtle altogether. It is not so very long since a well-known cross-channel steamer which had set out for this country disappeared during the course of her voyage, and never a man lived to say how the foundering occurred. But it was known that when she set forth a portion of her deck cargo consisted of a heavy furniture van, and this, indeed, was seen floating about at the time the disaster was thought to have occurred. The conclusion generally arrived at in the minds of the best critics was that this heavy deck cargo had caused the stability of the ship to decrease to such an extent that when the ship rolled excessively she was unable to avoid rolling right over.

We have already shown during the progress of our story how the use of tanks has gradually been employed in the ballasting of the steamship. Not merely is the double bottom used for this purpose, but, as we mentioned, tanks are placed between decks in the wings in certain ships. Although a steamship, when her double bottom tanks have been filled, becomes much stiffer and possesses a greater displacement, yet she will certainly roll more heavily, and so tend to cause heavy strains in bad weather. Many vessels possess also tanks both in the fore-peak and the after-peak, which are extremely useful for the purposes of modifying the trim of the ship. This is especially valuable when the ship is proceeding “light,” and has not the advantage of a weighty cargo on board to keep the propeller well immersed. At the same time, supposing that the after-peak tank were utilised for the purpose of immersing the stern to a greater extent, it would also follow that the bows would be raised fairly high above the water, and in the case of a beam wind, the ship would not be easy to handle, for her head would have a strong tendency to fall off in just the same way as the man in the Canadian canoe seated at the stern finds that considerable difficulty is met with in steering his little craft with her bows out of the water, and at the mercy of every puff of wind which may blow from either side. As in other respects the ship is a compromise, so in regard to stability. She has to be stiff, or else she will roll right over in a sea-way; yet she must not be too stiff, or she will roll badly, and perhaps do herself serious harm, quite apart from being extremely unpleasant to those who happen to be aboard. Therefore, the aim nowadays is to give the ship a reasonable amount of stability, and to cause her rolling in a sea-way to be of an easy character. This is brought about by additional ballast tanks, which not only give the ship greater immersion and displacement (so causing greater stability), but by raising the centre of gravity through placing additional ballast in those ’tween-deck wing tanks that we discussed when we were considering the cantilever ships, the tendency of the vessel to roll is minimised. In fact, the combination of the double-bottom tanks and the wing tanks takes away excessive stiffness and heavy rolling, and makes the ship to behave in an easy manner in bad weather, even without cargo on board.

Then, again, since salt water is more buoyant than fresh, it will follow that when a ship passes from the sea into fresh water, her draught will be increased, and, therefore, there will also be a decrease in the amount of freeboard above the water-line, and, consequently, the range of stability becomes less also.

Perhaps, like the propeller, the rudder also has been granted too scanty a consideration by most general readers, although its action is of the greatest interest. First of all, we must remember that the rudder is useless in the case of still water; that is to say, the ship must be going ahead or astern and not be stationary, and the speed of the vessel must be greater or less than that of the water. Thus, when a ship is riding to her anchor in a tide-way, the rudder is operative, and the vessel can be steered across the stream; but supposing she were to be steaming at the rate of 4 knots, and had with her a 4-knot tide, she would not answer her helm. We mentioned at an earlier stage that the ship when going ahead caused a column of water to follow after her. The screw itself drives a column of water astern, and it must be obvious that these masses of water must act on the rudder of the ship, and so on her steering. Thus, the column of following water causes a decrease in the pressure on the rudder, and so makes the rudder less operative. The column of water, however, which is driven astern by the propeller will cause a greater pressure on the rudder, and thus it is possible for steamships propelled by a screw to use a small rudder, and by cutting away the deadwood of the ship just forward of the rudder, the latter is less interfered with by the hull, and the steering qualities are improved. We quoted just now the expert opinions that better speed is obtained when the screws are outward turning rather than inward. The outward-turning screws also give superior steering results in the case when the screws are placed near the hull, though when the propellers are well out, this is not so noticeable. If one desires to have a ship which shall turn quickly this characteristic is obtained by cutting away the deadwood aft, and also the ship’s forefoot. An extreme instance of this is found in the case of a centre-board sailing craft, which, as anyone who has handled her knows full well, will turn round with a remarkable and surprising celerity.