Sivut kuvina
PDF
ePub

unsuitable as naval missiles. Properly made steel shells would perforate any thickness that shots would penetrate, and then explode within the ship, as had been long since proved by Sir Joseph Whitworth. For steel shells, there was a safer and quite as powerful a material as any which had yet been manufactured abroad, and the mechanical ability to apply every kind of explosive in the best manner was likewise to be found in the United Kingdom. He believed that the proposed large battle-ships of 14,150 tons, or a few hundred tons more for larger coal supply, were the very ships now most needed for the navy, and the least likely to become obsolete. The author had directed attention to the imperfect gun-supply, and had shown the practical value of guns of 22 tons weight; but trustworthy 10-inch guns of about 30 tons would be still more suitable for ships, as their diameter would admit of the more powerfully explosive shells which would more quickly complete the destruction of the sides of an enemy's vessels.

PROFESSIONAL NOTES.

THE CORROSION AND FOULING OF SHIPS.-ANTI-
FOULING COMPOSITIONS.

By M. Holzapfel, Nc-wcastli-on-Tyne.
[Reprinted.]

The invention of a means for protecting the bottoms or immersed parts of ships against fouling, and the worm in the case of wooden ships, and subsequently against fouling and rust in the case of iron ships, has long occupied human ingenuity. The Phoenicians over 2000 years ago already employed a composition consisting chiefly of asphaltum for this purpose, and relics of the ancient Romans prove that they employed lead and copper sheathing on the immersed parts of their ships. Before iron shipbuilding was introduced, as early as the middle of the 16th century, an anti-fouling composition was patented in this country; but after the invention of yellow metal, compositions in the form of paint were seldom used, till iron shipbuilding necessitated the use of a paint or composition to protect the iron against rust and fouling. Hardly a month now passes in which one or more patents are not applied for under the name of " anti-fouling compositions." This is a proof that the desire to invent a means for preventing the fouling of iron and steel ships occupies many minds. It is therefore surprising that there should be an almost total absence of literature on this subject, and that a perusal of mostly all the specifications which are filed from month to month should prove that even many of those who may be supposed to have given a great deal of time to the study of this subject should, to judge from their specifications, appear to be quite unaware even of the most elementary principles on which alone a successful anti-fouling composition can be based. In 1867 this subject was dealt with at considerable length by Mr. Charles F. T. Young, C. E., who gives the results of a number of important experiments by the British and French Admiralties and others, but who deduces from them the most conflicting and illogical theories.

At this year's meeting of the Institution of Naval Architects, Professor Lewes read a short paper, the outcome of a very exhaustive and painstaking research, but even he seems hardly to grasp the subject and to misinterpret some of the phenomena.

With our daily extending fleet of iron and steel ships, this question continues to grow in importance, and I will therefore endeavor to lay down the broad principles on which a successful anti-fouling composition should be manufactured.

The way to obtain this object is to imitate as nearly as possible the action of copper and yellow metal. When iron ships were first introduced, great difficulty was experienced to apply any substance which would even in a measure preserve the iron and prevent fouling for a reasonable period. Dry dock accommodation at foreign ports then was exceedingly limited, and a vessel could only be dry-docked after the return from her voyage. A man-of-war on commission in foreign or colonial waters, for an extended period would, before she came home and before she could get cleaned, become so foul as to be almost unmanageable and unseaworthy. What the government and private

[ocr errors][graphic][ocr errors][ocr errors]

metal only poison when they exfoliate that they become anti-fouling, and it is only metals and compositions which in exfoliating produce poisons that are effective anti-foulers. Mr. Young, as well as Professor Lewes, give instances in their papers of cases where the galvanic action of the salt water on the copper has been neutralized by iron and zinc protecting bars, and where consequently a great deal of fouling was found on the copper. They both say that when the copper ceased to exfoliate it ceased to be anti-fouling, and they try to deduce from this that it is only the exfoliation which is wanted, not the poisoning. My argument is this, that when the copper exfoliates it produces a poison (oxychloride of copper) which is highly destructive to the lower animal and vegetable life, and that this poison is the active anti-fouler which kills the fouling matter before it can attach itself. Now, if the exfoliation of copper alone prevents fouling, why does not the exfoliation of metallic iron also prevent fouling? I may say that it does so to a very limited extent, but not because it exfoliates, but because by coming into contact with salt water the surface of the iron plate is transformed into rust, which also is to some extent poisonous, but not sufficiently to prevent fouling in any but the coldest climates. Now I think everybody present will bear me out that an iron plate when exposed to the action of salt water for four years will lose at least five times as much of its weight,;'. e. will suffer five times as much exfoliation as a copper or yellow metal plate: if, therefore, exfoliation were the real and true factor which prevents fouling, iron itself should be a much more effective anti-fouler than copper.

Some people may argue that the success often obtained from the use of zinc white and tallow, which cannot be considered in themselves highly poisonous substances, is a proof that after all exfoliation, if not the only, is still an important factor as an anti-fouler. But zinc white and tallow when in contact with salt water become highly poisonous, and their chief merit lies in the poisoning principle. If a mixture of pure vaseline, mixed with chalk, were applied, the exfoliation would be equally good, but I need hardly say that the coating would quickly be covered with fouling matter to such a thickness as to stopthe progress of exfoliation altogether. Exfoliation can only act as an anti-fouler in regard to substances which adhere lightly to a ship's bottom, not in regard to shell which seem to eat into the protective coating till they finally find a firm hold on the bare iron. I may, therefore, say that exfoliation is not an active factor which prevents the adhesion of animal and vegetable life, but a passive agent, which may under circumstances cause the formations which have already adhered to again detach themselves from the ship. For in dealing with the first developments of animal and vegetable life which constitute fouling, we have not a body heavier than water, which, if it does not find a Dtly hard hold to keep on the ship would fall off by itself, but we have a ist insidious animal or plant, which will live on anything that is •ison.

re this, that the anti-fouling substance must

[ocr errors]
[ocr errors]

ned water which destroys all animal in animal unless it absorbs some lly absorbed and must be replaced. thing, as well as in anti-fouling e main sulstance, which, in being per, arsenic and zinc, in sitions, and they must pidity to admit of at the same time of time for which ay now take it as piderahle amount lowest developyou like,

[graphic]
[graphic]

exfoliate. Whether these poisons are poisons in the ordinary acceptation of the word, or not, does not matter, they must merely be poisons to the class of animals and vegetables which try to attach themselves to a ship's bottom, and they must carry the greatest possible efficacy in the smallest possible volume, and as already stated these animals and vegetables must be poisoned in their first stages of development, in which only they try to attach themselves to a ship or other solid substance, for when they have once attached themselves they grow rapidly, and can stand a great amount of various sorts of poisons, which is evidenced by the fact that a mussel can thrive on a quantity of verdigris which would poison a healthy man, also by the fact that large sized shells several inches long are not infrequently seen on copper sheathing where they could not fail to absorb a large amount of poison from surrounding parts of copper. It will be seen therefore that even copper is not a perfect anti-fouler when exposed to trying circumstances, when, for instance, animal and vegetable life in the surrounding water is so strong that all the small particles of poison get absorbed, and before new formations of poison can take place the animals attach themselves to the ship.

Now when they have once attached themselves the poisoning factor generally becomes useless, the animals grow and the ship comes home foul, often to the astonishment of the owner, captain and composition manufacturer, and this not only occasionally takes place with the best compositions, but also with copper and yellow metal.

I referred above to preparations of mercury, copper, arsenic and zinc. All of these are supposed to cause corrosion of the iron, particularly copper, which consequently should not be used at all, or only to a very limited extent. But in each case the anti-fouling coating should be separated from the iron by a coating of anti-corrosive paint. Many years ago red lead was taken for this purpose, but it has now been almost completely displaced by quicker-drying and more protective varnish paints, which should be so constituted that even if they are exposed to the action of the salt water, their dissolution would be so slow and gradual as to be almost imperceptible. Many vessels of the mercantile marine have such a solid and hard body of these protectives on their bottom that absolutely no rust can be seen on them except on places where the paint is chafed off. As to the anti-fouling, or second coating, I have already stated that the varnish conveying the anti-fouling ingredients should be so constituted as to allow of a gradual but very slow dissolution in salt water, so as to set the anti-fouling or poisonous matter free. Now in varying waters varnishes of various hardness may be used, in the tropics a soft and rather quickly dissolving varnish, in northern waters a hard and slowly dissolving varnish.

In an experience of over 10 years I have absolutely satisfied myself that most mercurial varnish paints on competitive trials will invariably show a better result than any other compound, while the preservation of the iron and the smoothness of the surface are unequalled. These paints, moreover, are so cheap, and dry so rapidly, that the total expenditure of a steamer for docking, cleaning and painting during four years is considerably less than a single outlay for docking and sheathing with yellow metal, which under favorable circumstances will only last the same period, i. e. four years. It seems.to me very doubtful whether a composition lasting longer than 12 months, provided it were dearer than those now in use, would meet with favor, because most shipowners are already under an obligation to their underwriters or the Board of Trade to dock their vessels at least once in 12 months (which, moreover, is desirable in order that the sea-cocks, propeller, rudder, etc., may be inspected), and the cost of anti-fouling paint, if intelligently bought, is a very small one indeed. The tendency, on the contrary, seems to be to economize further in the cost of the paint, and to dock the ships more frequently, for the prices for dock dues and labor for painting, and the time required for this purpose, are in most parts only one-third or one-fourth of what they were 15 years ago, while the additional speed of a newly cleaned and painted bottom is a great desideratum. The only

« EdellinenJatka »