Mr. Laird was followed by other builders of iron vessels at Liverpool; the high estimation in which they were held having led to a constantly increasing demand for them. About this time Messrs. Tod and MacGregor, of Glasgow, began to take a leading position in this occupation; the Princess Royal, long engaged on the line between Glasgow and Liverpool, and launched from their yard, having been one of the finest and fastest iron packets of her time.
The Great Britain, 1839-1843.
From that period iron shipbuilding on the Clyde increased with great rapidity, but the most magnificent specimen of an iron ship of any description produced at that time was the Great Britain, to which reference will be made hereafter, constructed by Mr. Patterson at Bristol, for the Great Western Steam Packet Company.
Advantages of iron ships.
Action of salt water on iron inconsiderable.
For the information of the general reader, I may here state that the advantages of iron vessels consist principally in their durability, strength and safety, increased capacity for stowage, greater economy, and salubrity.[119] With regard to the perturbations of the compass, Professor Airy, previous to the time when Mr. Evans made his report, had published a very concise series of instructions for correcting the compass on board of iron ships; and the progress of science now bids fair to obviate any difficulty whatever ensuing from this cause. Prior to experience it was apprehended that the saline property of the sea-water would tend to corrode the iron, and, further, that this metal would be rapidly destroyed by oxidation. But experience has shown that the effect of salt water on iron alone is so small as hardly to bear a comparison with its effect upon iron in connection with wood. This remarkable difference has been observed in iron vessels in which timber had been used for the keel; the bolts driven through the keel to form its proper connections having been so rapidly acted on as almost to destroy them before the external iron plates of the hull had been perceptibly diminished in thickness: it is further of importance that the vessel should be kept in use rather than be laid up in ordinary. Vessels built in the earliest stage of this art, subsequently to that of building mere canal-boats, bore many years’ service with little need of repair, and remained in a perfectly good condition for a longer period than that to which the durability of wooden vessels ordinarily, and under similar circumstances, extends. But there is a great difference in iron plates, some are inferior and soon oxidize, while others, as will be presently shown, last for many years. As the inner surface of the plates may be almost wholly protected from oxidation, it is only from the external wear that danger may be apprehended. But, though the outer surface of the metal can be protected in a great measure from corrosion, yet iron vessels are subject to the disadvantage of having their speed diminished, after a short period of service, by the adhesion and growth of animal and vegetable matter. A coating of red lead is not a perfect preventive against this mischief, and various other scientific substitutes have been used of late; so that it cannot be doubted but this inconvenience will disappear altogether before scientific appliances. A perfectly protective varnish for the in sides of iron ships and a coating which shall effectually prevent the adhesion of animal and vegetable substances to the exterior, are desiderata of great value,[120] and will, we may hope, continue to receive the careful consideration of scientific men.
Durability, strength, and safety of iron.
All the facts yet known with regard to the superior durability[121] of iron ships are highly satisfactory. It is a consideration not to be overlooked that large ships may be rendered more durable than small vessels; for, as the weight of the hull is generally determined in a certain proportion to the whole displacement, and the plates of iron are much thicker in a large than in a small ship (the oxidizing causing an uniform waste of metal), the durability will be in proportion to the amount of wear the plates of the respective vessels can bear without danger to the ship.
But the superior strength of iron ships depends not merely upon the quality of the material employed, but also on the mode of combining it. The strength of wrought iron is well known and its power of resisting strains in almost every direction is a matter of universal experience, add to which, that its resistance to lateral pressure increases in a much higher ratio than the quantity of material. Hence, almost any amount of strength may be given to a large fabric; certainly, enough to bear the pressures and strains to which ships are exposed, with much less liability to injury than wood. With plates of iron of a substance fitly proportioned to the magnitude of the fabric, and with joints properly formed, the sides of ships have been found capable of resisting, in a remarkable manner, forces for which the strength of timber would be quite insufficient. A substance of plates sufficient to constitute this amount of strength generally, is also able to bear concussions of great force with much less hazard than timber. The uninjured state in which the Great Britain was found in Dundrum Bay, after being wrecked and lying on the beach several months during winter, exposed to various storms, proved the correctness of these views, which more extended experience has since confirmed. Experience has also demonstrated that unless the concussion takes place with extreme violence, mere indentation of the metal is generally the greatest injury sustained. Beyond this, the strain sometimes breaks off the heads of a few rivets without opening the seam, but it is uncommon for the rivets to be drawn if the metal and workmanship are good. In the case where an iron ship strikes upon a sharp pointed-like crag of rock or coral reef with considerable force, it frequently happens that a hole is made through the plate; but even when such an accident occurs the damage is generally local, the parts not immediately subject to the concussion remaining unhurt. No general leakage is, therefore, consequent on such an accident, as would be the case in all vessels built of wood.
Affords greater capacity for stowage.