The protection of the submerged portions of a ship's hull, and the necessity of it, whatever material the ship may be constructed of, seems to have been long acknowledged and practised. The materials employed for this purpose are almost innumerable, and we find wood, copper, lead, zinc, porcelain, Roman cement, Portland cement, leather, tin, yellow metal, india-rubber, tinned iron, woven metal and vegetable fabrics, brass, glass, plaster, &c. &c.; in fact, it would seem that almost everything had at one time or other been pressed into the service.
Atheneus, quoted by Ewbank, informs us that the ship of Archimedes was fastened everywhere by copper bolts weighing ten pounds each and upwards, whilst the bottom was sheathed with lead.
Lead was used as a material for sheathing in the second century, for Alberti tells us, in his work on architecture, which was published in the fifteenth century, that whilst he was writing that book a ship, called "Trajan's Ship," was raised from the bottom of Lake Riccia, which had been submerged for above 1300 years, and that "over all there was lead sheathing fastened on with little copper nails."
A boat, thirty feet in length, was found in the fifteenth century sunk in the Mediterranean in twelve-fathoms water, which had the deck covered with paper and linen, and sheets of lead fastened with gilt nails over all; and when stripped off, parts of the covered portion were found quite dry, the boat being estimated to have been 1400 years submerged. The hull was of cedar and larch.
A Roman ship sunk in the Lake of Nemi was found to have been coated with bitumen, over which sheets of lead had been nailed with gilt nails. The seams of this vessel were caulked with tow and pitched, the material of the hull being larch.
In the reign of Henry VIII large vessels had a coating of loose animal hair attached with pitch, over which a sheathing board of about an inch in thickness was fastened, "to keep the hair in its place." It seems, however, that sheathing the hull or building another over it was not approved of by these early navigators, and in 1668 the officers of the fleet fitting out under Sir Thomas Allen to attack the Algerians petitioned that they might not have their vessels "so encumbered," as they "were always unable to overtake the light-sailing unsheathed vessels of the enemy." This petition was granted on condition that the ships' bottoms were cleaned very frequently.
In the reign of Charles II the "Phœnix" was sheathed with lead, and soon after about twenty ships were so sheathed, milled lead fastened with copper nails being used.
In 1667, during the same reign, it is noted "that wee and divers of our subjects are att great charges in the sheathing shipps for the streights and other foreign parts." For many years, and up to the middle or latter end of the last century, the sheathing generally in use seems to have been a doubling of the skin of the ship with wood, which was kept constantly payed over with tar, grease, or a mixture of these compounds. The writer of these pages remembers a small schooner which was so constructed, and after some time had a sheathing of copper or yellow metal laid on this wooden sheathing.
In 1670 a patent was granted for the use of milled lead for sheathing, and it was ordered that "no other than milled lead sheathing should be used on His Majesty's ships." In the year 1700 lead was acknowledged to have proved a failure, and the use of wood sheathing was again resorted to.
In 1677 and 1678 Sir John Narborough and Sir John Kempthorne complained that the rudder irons of the "Plymouth" and "Dreadnought" were so much eaten as to make it unsafe for them to go to sea; and in 1682 similar complaints were made of the effects of lead in contact with iron.
In the year 1682 a commission was appointed to investigate the subject, and the result was that lead sheathing was abandoned. In the latter half of the last century, however, the "Marlborough" was sheathed with lead, and laid up in ordinary at Sheerness. In a few years after, or in 1770, she was docked at Chatham, and it was found that the lead had eaten away the iron fastenings very considerably, in consequence of which the lead was stripped off, and wood sheathing substituted.
Alloys of lead have also been tried, but all produced similar results, and were consequently abandoned.
Lead stands next to copper in its galvanic action on iron, and although it is far cheaper than copper, still its injurious effects, and the trouble, complication, and expense, necessary to insulate it, preclude its use as a material for sheathing iron ships. In 1687 a patent was obtained for using rollers to mill lead into sheets, "as well for the sheathing of ships as for any other use or purpose whatsoever," and in 1728 another patent was obtained for a "new method of sheathing and preserving the planks of ships." this consisted of "rooled copper, brass, tin, iron, or tinned plates." In 1779 was patented a "compound metal capable of being forged when red hot or when cold, more fit for making bolts, nails, and sheathing for ships, than any metals heretofore used." This was a very close approach to Muntz's metal. In 1800 was patented "red, yellow, and white sheathing," consisting of various mixtures of copper, zinc, and tin. This was again a near approach to Muntz's metal. In 1800 the manufacture of zinc into sheathing for ships was patented.
From this period numerous patents were obtained for materials or their mode of application in sheathing vessels, such as electrotyping the whole hull, and various other contrivances, most of them exhibiting an utter disregard of chemical knowledge, such, for instance, as that of Mr. J. E. M'Connell, who in 1802 proposed to apply sheets of brass or copper directly to the hull of the vessel and riveted thereto, without the interposition of any insulating material, galvanic action being thus set at defiance! Others, again, proposed to sheathe the iron hull with wood, to which the metal sheathing may be applied, as in a wooden ship; and then, again, various nostrums against fouling and corrosion are patented, such as glass, phosphorus, arsenic, &c., all of which are either so absurd or worthless as not to need further attention.
Sheathing or protecting the hulls of iron ships comprehends the following points:— Materials used, mode in which they are applied, principles involved in their action, their advantages or disadvantages, cost, durability.
The materials used have been metallic or non-metallic. The disadvantages of the metallic are that, unless carefully selected with reference to their action on the iron hull of the ship, they give rise to one or all of the following serious objections:— They do not fail to set up a serious destructive galvanic action with the hull of the ship, and thus become rapidly foul, at the same time dooming the hull to certain destruction, and their application has been a complicated and unsatisfactory process. They are not readily applicable to an iron ship, unless she be specially constructed for the purpose of being sheathed, with facilities for attaching the plates.
The non-metallic sheathing is either in the form of a paint and smeared on, or else laid on, plastered or spread as a paste or mortar, or stuck on by means of some adhesive glue or other medium. As the adhesion is merely that of plaster on a wall, and is influenced by wet, damp, or the state of the atmosphere when being laid on, and it cannot be attached with such a degree of firmness as can be done with sheets of metal, none of them are calculated in any way to fulfil the prime requirements of the case, viz. the complete protection of the hull from the action of the sea-water, and causing the action of that sea-water to induce a constant renewal and cleansing of the surface, whereby alone fouling can be prevented and the corrosion of the hull avoided. Not even the "enamelling," which is so beautifully done, is of the slightest use, as it presents a permanent surface, and does not exfoliate; besides which the difference of expansion of the iron of the hull and the enamel causes it continually to chip off. Most of the plans for sheathing or protecting the hulls of iron ships seem to have been originated with the view of their application to ships already constructed, and until 1863, when Mr. T. B. Daft, C.E., patented his improved plan of constructing ships, so as to admit of their being readily sheathed, this point seems to have been overlooked. In order to apply sheathing to ships already built, it becomes needful to attach a coating of wood, to which this sheathing is to be nailed, and the difficulty they all labour under is that they are not component parts of the structure of the ship, but are, as it were, plastered on to the hull, and consequently easily damaged and readily detached.
By the plan of Mr. Daft the hold for the sheathing nails is incorporated in the body of the vessel, and thus becomes a part and parcel of it, placing the vessel and her sheathing in the same position as a Coppered wooden hull, exposed to similar casualties in regard to stripping, &c. This plan increases the facilities for sheathing, decreases the expense, and does not increase the bulk of the hull by more than the thickness of the zinc sheathing used for the purpose, besides making a flush surface, and a better job in all respects.
Wood sheathing on the hull of an iron ship is a mistake. It is of no use to prevent fouling; it does not strengthen the ship; it becomes water soaked; it enlarges the midship section, and consequently does not increase the ship's speed; it is sure to be quickly attacked and destroyed by worms; in fact, it can only be at once and unhesitatingly condemned, more particularly when it is accompanied with the pernicious arrangement of sheet copper or yellow metal laid on it.
The cost of this complicated arrangement, viz. first giving the ship a coating or two of red lead, marine glue, cement, or some other delightful material, then a coating or sheathing of felt or india-rubber, or both combined, then a coating of wood, and then the sheathing of copper or yellow metal, is no small trifle, the "Royal Sovereign" being estimated to cost £3000, which may be looked on as thrown entirely away, first, because the plan is inefficient; secondly, because it brings in the fatal objection of galvanic action; and thirdly, because it is eminently lacking in solidity and durability — points that should never be lost sight of in carrying out such undertakings.
Zinc as a material for the sheathing and protecting of ships' bottoms is a comparatively new application in this country, for it seems that, though patented in 1805, in 1848 there were only five British ships on which it was known to be employed. So well did it answer, however, that by the year 1850 there had been 300 ships sheathed in British ports, which number had increased in 1852 to upwards of 1000, and its increase still appears to be great. All these vessels were of wood, however, and it was specially requested that in applying this material to ships fastened with copper, yellow metal, or iron bolts, layers of tarred felt should be placed over them to prevent galvanic action destroying the zinc.
Zinc rolling mills seem to have been first started in the year 1834 at Dartford, in Kent, the machinery being made by Hall, the well-known engineer of that place.
So little was the action of sea-water upon the copper or metal sheathing of a vessel understood, that it was believed and asserted to have kept clean by the oxide poisoning the zoophytes, &c., which adhered to it; and when after some time the copper sheathing of our ships of war was found to wear out, it was assumed that this action ought to be stopped, and accordingly Sir Humphrey Davy was applied to to introduce some mode of removing this evil.
For this purpose he arranged iron "protectors" in connection with the copper, which were applied to the "Magicienne," and it was found that this plan only gave local protection, the greater part of the copper oxidising as usual, only the parts contiguous to the protectors being protected, and these portions were covered with barnacles. These experiments, when carried further by means of plates of tin, zinc, or other easily oxidizable metal, applied to coppered ships, gave so perfect a protection that the ships' bottoms became covered with barnacles and weeds, thus introducing a far more serious evil than the one it was desired to remedy. It would have been thought that these facts would have afforded excellent grounds on which to form a theory of the mode in which copper sheathing operated in removing fouling, and to have led to a serious consideration of the subject; but it seems not to have excited any attention, and the poisoning theory continued, and still remains in force in some cases at the present moment.
There can be no doubt that, had proper attention been given to the chemical action of metals in contact with sea-water, we should not meet with the constant reference to the destructive action of corrosion in our iron-clads and other iron ships, which so constantly appear in the columns of the daily press, nor would there have been the enormous expense we now have to encounter from the fouling of our large iron ships, and their consequent docking, cleaning, painting, &c., and loss of time involved thereby, as due consideration must have led to the devising of some practical mode of construction and sheathing by which all this would have been obviated.
At the present moment this question is little understood by many, and some have even gone so far as to compound a mixture to be smeared over the copper, yellow metal, or zinc sheathing of ships, to prevent the very action by which it keeps free from fouling, namely, exfoliation! and advance as an argument in its favour that it will not wash off, and it completely arrests the oxidation or wear of the sheathing, stating that the full weight of the metal sheathing will be returned whenever it may be taken off, and recommend its use on account of the immense saving that it would effect.
Now, on analysing the subject and its economical results, the following particulars are obtained:— First, a ship is sheathed with copper, yellow metal, or zinc, in order that the loss of speed and fouling found to ensue if they be not used may be prevented by their exfoliation, and the hull saved from the attacks of worms. Second, in order to prevent the copper exfoliating, and thus keeping clean, and also the waste caused by that exfoliation, a mixture or compo' is smeared all over the copper. Third, this mixture or compo' is said to prevent fouling and oxidizing, and also to prevent the attacks of the worm. Therefore it follows, if this be true, that the compo' will do all that the metals can, and that consequently any metal sheathing is unnecessary.
Against any advantage that may be claimed for paints, compo', &c., must be set the certain fact that they do not remove the necessity for frequent docking, cleaning, and repaying, or plastering, with all their accompanying delays, expenses, and drawbacks; and it is certain that whatever weight of copper or other metal sheathing they may prevent the waste of, over a period of say three or four years, if their application be continued so long, it is equally certain that the use of this protective compo', its docking, renewals, and other expenses, will be found to far exceed any possible saving that might be effected, and, in fact, to be more than the cost of recoppering or sheathing the ship at the end of such a period.
It is now a decided fact that no material which presents and maintains a permanent surface is of the slightest use in preventing fouling or corrosion. The general ignorance of the action of copper sheathing in keeping clean has caused the continuance of the evil, and perpetuated a host of nostrums of the most expensive, useless, and troublesome description, without in the slightest degree abating the nuisance, added to which it has introduced and encouraged the useless and highly dangerous plan of placing sheet copper or yellow metal on iron hulls, for no other reason than because they have been used, and keep clean on wooden hulls; and this too, in defiance of all that is known in. regard to galvanic action, its causes and results. Did any individual shipowner choose to experiment on the readiest and surest way of destroying his own iron fleet, there would not be much said about it; but when such absurdities are carried out on a national fleet, on which millions have been expended, and in which all are interested, it then becomes a very serious matter.
There can be little reason to doubt that the constant advocacy of copper or yellow metal as the only proper material for the sheathing of iron ships arises from an ignorance of the action of it in preventing fouling, it being almost the universal opinion that the "poisoning" process is what causes it to keep clean. That the poisoning theory is a fallacy the success of zinc (which is admitted to be non-poisonous) when applied to iron, in keeping free from fouling, sufficiently proves; and where the mechanical assistance to the process, viz. the friction caused by the ship in passing through the water, which aids in rubbing off the exfoliation, does not take place — for example, when the ship lies at her moorings — a limited degree of fouling still takes place on the surface of the copper or yellow metal, in spite of sufficient verdigris or poisonous matter being on the surface, to give ample proof whether the theory be right or wrong.
The action of copper in keeping clean is thus stated by the 'Engineer' of December 21st, 1866:— "The beneficial action which continuously keeps a copper bottom from sea-weed is evidently due to a double process of exfoliation and of a generation of a poison (verdigris) inimical to the development of organic life."
The idea of copper keeping clean by the poisoning process killing the zoophytes was held by Sir Humphrey Davy, and in order to prevent what was then considered the unnecessary waste of copper sheathing, by corrosion or exfoliation, he protected it by zinc or iron. The experiment succeeded scientifically and thoroughly in preventing exfoliation or loss; but it failed to be a practical benefit, inasmuch as the sheathing became covered with sea-weed, shell-fish, &c., which it did not do when the copper exfoliated, consequently his plan had to be given up, to the great mortification of Sir Humphrey.
If this idea of poisoning the weeds, barnacles, polypi, and other annoyances which seem to have such great "proclivities" for ships' bottoms, were correct, there can be no doubt that such action would take place just as surely and well whether the ships were at anchor or under way; and there would not then be found the constant records of the failures which greet all the multifarious attempts which have been and are yet made to obtain success by this process, whether sought for by the application of compositions, plasters, or by the application of the metal in sheets.
The fallacy of the "poisoning" theory will be proved further on in this work from the results of actual experiments; and it is well known that all the "compositions" compounded in accordance with this theory have totally and utterly failed when tried in actual use.
It is a curious fact, that but little attention has been given to investigate the action required in whatever is used as a means for preventing the fouling of ships' bottoms; but it has at once been assumed that the action required is that of poisoning, and on this hypothesis nearly all the inventors have proceeded. The results will be found recorded in the chapter on Fouling.
It was stated at a late meeting of the Scottish Shipbuilders' Association, by the chairman, that an iron plate had been coated in squares with four of the best-known "anti-fouling" compositions, and, after having been submerged for some months, was raised for examination. The plate was found covered with barnacles, and the most ridiculous part of the affair was that the biggest barnacle of the lot, "the great grandfather of all barnacles," had stuck himself in the very centre of the plate, right on the point where all the four compo's joined, as if to show his utter contempt for and ridicule of the useless and worthless plan of "poisoning" them.
The royal mail-steamer "Atrato" had one side coated with a preparation of copper, its use being propounded in accordance with the "poisoning" theory, and when docked for examination the green copper composition was found to be covered with a coralline incrustation and patches of rust or oxide.
The "Valiant," in 1865, had one side coated with a preparation of copper, and when examined, in about five months afterwards, was found to have this "poisonous" composition thickly coated with barnacles, sea-weed, &c.
The "Orontes," iron troop-ship, was coated with a poisonous anti-fouling composition containing copper, and when examined some time afterwards was found to be, under her counter and in the run of her after portion of the hull, thickly encrusted with barnacles; and the bottom was dotted all over with patches of rust, the rivet-heads in many places showing evident signs of being affected by the copper contained in the anti-fouling composition.
In the 'Times' of January 1 7th, 1865, it was stated that the "Royal Oak" iron-clad, which is sheathed with Muntz's metal, was docked, after six months' service, in the Mediterranean, and "her bottom was found to be foul beyond conception; masses of animals, zoophytes, corallines, &c., all so encrusted with weeds in one mass that sharp scrapers will alone detach them."
It is stated that one of H. M. ships, which had been lying quiet at Bermuda for some time, had layers of oysters, &c., upwards of a foot thick, on her copper sheathing, the copper being actually taken up by the animal in the formation of its shell as a carbonate of copper, the shells of the inner layer of oysters being all of this beautiful green colour throughout. Some of these shells were a short time since in the possession of the storekeeper of the Royal William Victualling Yard at Plymouth.
In another case the interior of one of the large globes of Griffith's screw propellers, which in the navy are made of gunmetal, was found on examination to be completely filled up with full-grown mussels and barnacles, although the aperture through which the spawn had entered was not more than half an inch in area. Where, then, is the "poisoning" theory in all these instances?
The French Government seems to maintain a perfect indifference to the fatal injury resulting to the iron hulls and armour plates of the iron-clads from the use of copper or yellow metal sheathing, in spite of the remarks of M. Becquerel, their eminent chemist, whose researches have shown that, by the employment of zinc for sheathing iron vessels in place of copper or yellow metal, the iron may be kept free from fouling, and also preserved instead of injured, which is of the greatest importance.
It is curious to see the constant and persevering attempts which are made, both in England and France, to use copper or yellow metal for sheathing iron ships, and the schemes which are tried to prevent its certain and fatal action, whilst any endeavour to give zinc an equal trial is most studiously avoided. It would seem as though there were some fascination, or important advantage to be gained by trying the former, which does not exist, or has not been discovered in the case of the latter. But it remains to be seen whether the tax-paying public will much longer permit these useless and dangerous experiments to be tried on the costly iron-clads.
M. Roux, a captain in the French Imperial Navy, seems to have succeeded in convincing the naval authorities of that country that copper can be readily applied to iron hulls without causing any damage! It is stated in the 'Engineer' of December 21st, 1866, that "It was M. Dupuy de Lôme who first suggested to Captain Roux the idea of applying copper directly on the iron shell of the vessel, observing that the effects of any galvanic current were not to be feared, 'as long as the plan employed absolutely prevented the presence of sea-water at the points of contact of the two metals.'"
The same journal goes on to say that "The next steps were, therefore, to find a mode of isolating the copper from the iron, and some mechanical plan for securely fixing the copper sheets." In order to obtain the former, it is said that he coats the iron with red lead, over which he places a composition, and on this composition he places the copper. For the latter, viz. "a mechanical plan for securely fixing the copper sheets," it is stated that the inventor "professed to drill into the iron a small conical hole five millimetres deep and six millimètres (about ¼ inch) in diameter, and then to spread out a suitable rivet in this hole. By this means any defects resulting from the drilling of the hole practically disappeared, the soft rivets completely filled up any irregularities, and were firmly clenched in."
Of this plan the `Engineer' remarks, "It is certain that the process has a simple and rational look, whatever may be the ultimate results."
Now, there are generally about 120 holes in a sheet of copper as used for sheathing wooden ships, and a ship of 400 tons generally requires 1140 such sheets. If, therefore, the same proportion holds good for iron ships, it follows that the amount of holes drilled into the hull for the reception of the "soft rivets" will amount to something considerable (in the case of a 4000 ton ship, on one plan of sheathing, there are 45,663 holes at 3d. each = £570 15s. 9d.); but as it is only "drilling holes," there can be no doubt of the "simplicity" of this part of the process.
It will be interesting to examine the mode in which the experiments with Captain Roux's plan were carried out, from which sufficient results were obtained to satisfy those in authority of its feasibility and advantages, and also to prove to them that the destructive action of copper on iron does not present any real difficulty in actual use.
We are told that the "first trial was made with a large flatbottomed iron boat, the plates of which were very much corroded. After a service of nine months at sea, seven of which were passed in active service at the arsenal of Toulon, the boat was taken on land and examined by a commission and M. Dupuy de Lôme. No vegetation was found on the outside, and only one copper sheet above the water-line was found injured, probably by scraping against another vessel; on stripping more of the copper sheets the iron was found in a perfect state of preservation, without oxidation, and all the rivets in a good state."
We are further told that, "in consequence of this success, the iron-plated corvetice 'La Belliqueuse' was placed in M. Roux's hands to be sheathed by his process before beginning a long cruise in the Pacific. He expects that the copper sheathing will last twelve years, as his plan is based upon the principle that the copper protects the composition, the composition the red lead, and that, lastly, the red lead protects the iron." Since the sheathing of "La Belliqueuse," M. Roux has been ordered a few weeks ago to apply his sheathing to the iron-plated frigates "La Savoie" and "La Revanche."
A few moments' reflection will show how insufficient such a plan of trial is, and how well calculated to cause error. The boat, we are told, was "seven months in active service at the arsenal at Toulon;" when she was hauled up and examined the iron was found "in a perfect state of preservation, without oxidation."
Now, here were conclusions drawn from the results of circumstances which never exist in the case of large ships in active service, and it does not seem that it ever occurred to any one that this was the case. This flat-bottomed boat, we are told, was coppered "above the water-line; she was employed at the arsenal at Toulon," and consequently in smooth water, not being subjected to the constant washing of the water over the copper and the iron hull, as would be the case of a large ship at sea in active service, and which it is found wholly impossible to prevent.
It is this contact of the water with the copper or yellow metal and the iron hull which causes the destructive galvanic action to be set up. Keep the three from contact and no result follows, but it matters not how perfectly or completely insulation may be secured between the iron hull of the ship and the copper sheathing by the interposition of non-conductors, if when the ship is afloat the water is allowed to produce contact between them by washing indifferently over both.
It has actually been proposed to drill the skin of an iron ship full of holes, and tap copper bolts or studs into them with the view of riveting on copper or yellow-metal sheets, interposing a sheet of iron and some insulating material between; and it is gravely asserted that this will prevent the water following or obtaining access to the skin of the ship, and thus prevent the corrosion and galvanic action between the different metals and the water which would otherwise take place!
This ingenious, cheap, and thoroughly practical arrangement, which is perfectly in accordance with the knowledge required to devise an arrangement to set up galvanic action to destroy the positive or weaker metal, namely, the iron, has actually been approved of and pronounced extremely simple, in the following words:— "Altogether, this is the simplest and apparently the most efficient form of sheathing for iron ships that we have seen"!
It has been well and truly remarked that "It should be the function of engineering papers to explode all palpable bubbles, and to guide public opinion aright in matters beyond popular analysis and appreciation;" otherwise "will the public lose confidence in those blind guides by whom they have been so much misled."
The "Caledonia," iron-cased ship, has been sheathed with teak three inches thick, and this covered with a new description of metal sheathing, being copper on one side and pewter on the other, the idea being that no galvanic action prejudicial to the iron can be excited by this arrangement.
The "Zealous" has been sheathed with teak planks three inches thick above the water-line and four inches thick below it. In carrying out this plan it has been determined in future to bolt the planking directly on the plates, to avoid weakening the planks by the grooves they formerly cut in them.
Ever since the fouling and corrosion of the hulls of iron ships showed the necessity of preventing them by sheathing, it has been a desideratum to obtain a simple, practical, and efficient mode of attaching the plates of metal or sheathing to the iron hulls; and though many plans have been patented and proposed, all more or less ingenious or objectionable, none can at present be said to be in use or capable of fulfilling the requirements of the case in a satisfactory or efficient manner.
In 1862 Mr. Mallet, who is considered by some people as no mean authority on such matters, said that "any attempts at coating an iron ship directly with any metal sheathing is quite out of the question; the mechanical difficulties are insuperable. You cannot attach sheathing firmly to the skin of an iron ship in any way, but you injure the strength and diminish the safety of the ship enormously. If it were possible to get the hull of an iron ship into a condition that it would not corrode at all, there would no longer be any difficulty about preventing it from fouling at all."
It may be that, on examining Mr. Daft's plan of constructing iron ships and vessels and sheathing the same, Mr. Mallet will be disposed to alter his extremely decided opinion, and, as a mechanic, recognise the truth of the saying that "there is no finality in engineering," before he again ventures to pronounce so decided and dogmatic an opinion on a matter which in a twelvemonth afterwards is fully and completely solved in the two points, one of which was "quite out of the question," and the "mechanical difficulties" of the other were "insuperable."
Numerous attempts have been made to attach the metal used in sheathing to the bottoms of iron ships, and "sticking" them on by means of some adhesive material has been a tolerable favorite; next has been that of drilling through the hull or skin of the ship a quantity of holes, through which the bolts holding on the wood sheathing passed, on which wood the sheathing metal was nailed; next, ribs or ridges of iron were attached to the hull, vertically or horizontally, and the wood sheathing wedged or jammed in between them, and the metal sheets put on this; in fact, several modes have been proposed or patented for this purpose, but, so far as experience shows, have met with but a very limited amount of patronage. All the plans for building a wooden ship on an iron hull, and then coppering, yellow-metaling, or sheathing it, are but very unsatisfactory and objectionable plans, increasing the midship section of the ship without adding to the strength of the vessel, and bringing a metal of a superior galvanic power into close connection, by means of damp wood and water, with a weaker one.
We often hear it dogmatically asserted that zinc is of no use for sheathing iron vessels, because where it has been used on wooden ones it soon fails, and this from the formation of an oxide or carbonate which is so hard as to turn it into a permanent surface. That this is true there is not the slightest doubt, and the author has seen wooden ships sheathed with zinc in a most surprisingly foul state. A little consideration of the subject, however, would have shown that this failing arises from the lack of the very circumstances required to prevent it, viz. exfoliation, which, it may be observed, is most readily and surely obtained, and to any required amount, when the zinc is placed on the hull of an iron ship.
Prior to 1833 the copper used on the bottoms of the ships of the Royal Navy did not waste at a greater rate than one ounce per square foot per annum, whilst subsequent to this the average loss per sheet was four ounces per square foot per annum. There can be little reason to doubt that this was owing to the employment of iron fastenings, and that galvanic action took place between the iron bolts and the copper sheathing, thus preventing the exfoliation of the copper and destroying the iron, causing the ship to become "nail-sick."
When, however, copper and yellow-metal fastenings were introduced and iron given up, the action of the sea-water on the copper sheathing had uninterrupted sway, and the copper exfoliated at the increased rate mentioned, causing great excitement amongst the Government authorities at this increased waste, and no small amount of inquiries and investigations were made to find out the "reason why;" but it was finally concluded that it was the fault of the copper, which proved itself not to be so good as that formerly used!
Mr. Scott Russell remarked at the Institution of Naval Architects, in 1866, "The great merit of copper is that it decays. You all know how rapidly it wears out. And what do we pay the price for it for? We pay the price of copper in order that by its rapid decay, and its passing off and taking the barnacles with it, we may keep our ships pure and clean."
The general idea in regard to the sheathing of ships seems to have been that it ought not to waste away, and that any tendency to this was an evil that ought to be checked. This wish to "eat the cake and have it too" is very bad policy, and shows a complete ignorance of the mode in which the sheathing acts; for the fact has been patent to any one that took the least trouble to know the "reason why," that the sheathing must "perish," in order to keep clean;" and that if it does not perish the sheathing is preserved, but at the expense of a constantly dirty bottom to the ship. It is now known to all who have studied the subject that the cause of copper, yellow metal, zinc, and other metallic alloys placed on a ship's bottom, keeping clean and free from fouling, is the exfoliation of the metal and the constant renewal of the surface caused thereby, through which the adherent matter is, as it were, sent adrift, by the friction of the water against the metal sheathing washing off the exfoliated parts or films. From this it will be evident that there is just such a rate of exfoliation as will secure this great and important advantage without causing too rapid and unnecessary a destruction of the metal, which it is most important should be made to last as long as possible, provided it does not introduce any other disadvantage, or interfere with the proper carrying out of the end for which it is applied.
A vessel with an iron hull, cased with wood and sheathed with copper or yellow metal, is in a state of perpetual risk and danger, as a blow, grounding, galvanic contact of the two metals, or the detachment of the sheathing, must render her docking an immediate necessity, unless it be preferred to run the risk of allowing galvanic action and corrosion to take place. In fact, a ship so circumstanced can only be looked on as a huge galvanic battery, always ready to do mischief, and rapidly destroy the positive or weaker metal, viz. the iron of the hull of the ship, when sufficient metallic contact shall be produced, and always in operation through the contact afforded by the water or dampness in a less rapid degree.
Rear-Admiral Sir F. B. Nicholson, C.B., remarked, at the Royal United Service Institution, on the 20th March, 1865, that in the "Caledonia" "a belt of wood was originally placed between the Muntz's metal on her bottom and her iron plates. It was well known that damp wood is a partial conductor; still it was supposed that this would be a better plan than bringing the Muntz's metal right under and in contact with the armour plates. If that had been done there is no doubt that the armour plates would have been ruined."
Charles F.T. Young: The Fouling and Corrosion of Iron Ships: Their Causes and Means of Prevention, with Mode of Application to the Existing Iron-Clads.
The London Drawing Association, London, 1867. pp 36-49.
Transcribed by Lars Bruzelius.
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