Messrs. Doxford, in their latest attempt to solve the problem of the easily-shifting cargo in bulk, proposed that vessels intended for this trade should have inner upright walls fitted some distance from the hull, and so arranged that when the vessel is heeled over within the usual range of inclinations of a vessel at sea, the weight of the cargo and the buoyancy create a restoring couple in all conditions of loading. The spaces between the cargo-hold and the outer shell may be left empty or used for water-ballast as required. In some instances the bottom is reduced in depth as much as the loading regulations will allow.
Among the more notable features of recent years in cargo-boats specially adapted for the coal, iron ore, and other dead-weight trades is the patent cantilever framed type of steamer built by Sir Raylton Dixon and Co., Ltd., Cleveland Dockyard, Middlesbrough, on the Harroway and Dixon patents. This type of boat has the advantage of having totally unobstructed holds with very large hatchways and an additional 75 per cent. water-ballast, which is placed in the tanks inside the cantilever construction at the top of the holds under the deck. In these steamers the space on either side and under the decks is used for water-ballast, which is carried in triangular tanks at either side of the vessel, immediately beneath the main deck. The tanks extend from the coamings to the sides of the ship, the greatest side of the triangle being towards the cargo and supported by the cantilever framing; the tank framing and plating increase the strength of the hull materially. The sloping topsides thus formed prevent bulk cargo shifting. An advantage to the owner is that the tanks are exempt from tonnage measurement. When these tanks are filled with water and also the lower and peak tanks the vessel is seaworthy even if the cargo-space is empty.
This additional water-ballast has the special merit of immersing the ship deeper when in ballast only, consequently giving more power to the propeller and rendering the ship more manageable when light, as well as supplying unique security in case of damage, for when one of these boats is loaded and the topside tanks are empty, they correspond to the air tanks of a lifeboat and thus prevent the ship from sinking.
These vessels in some cases have been fitted with shelter decks right fore and aft for the carriage of cattle and horses, and indeed would be suitable for passenger service, for which the very easy rolling movement would be a great recommendation.
This type of vessel has been on the market for about four years and already some 200,000 tons have been built. One of the largest steamers built on this plan is the Echunga, 405 feet long, 56 feet beam, and 28 feet 8 inches moulded depth. She was built in 1908 for the Adelaide Steamship Company. Her net register is 2245 tons, her dead-weight capacity 8400 tons, and her measurement 11,000 tons. Her topside tanks contain 1350 tons, and her total water-ballast is 3200 tons.
In the steamers built by Messrs. William Gray and Co., Ltd., of West Hartlepool, water-ballast is carried not only in the double bottoms but in side tanks, the inner skin of the double bottoms being carried a considerable distance up the sides. A hull within a hull is thus formed, the intervening space being used as water-ballast tanks. Not the least advantage is the great additional strength the ship is given. The trunk system of shipbuilding adopted by Messrs. Ropner and Sons, Ltd., of Stockton-on-Tees, differs from the turret by having a double wall on each side, and has not the rounded turret base. The steamer Thor, built for a Norwegian owner, has only one hold, no less than 250 feet in length, the engines being placed aft.
Messrs. R. Craggs and Sons, Ltd., of Middlesbrough, have made a speciality of building tankers, and were the designers and contractors for the first ocean steamer to load oil in bulk. Their stringerless system of construction is, they claim, the last word in transverse framing, and has numerous advantages for single-deck vessels.
During the last three years three distinct innovations in steam-ship construction have been made. All three are of a revolutionary character, and two are likely to have no small influence upon the construction of both passenger and cargo steamers, while the third is of great importance for the rapid loading and discharging of coal and ore cargoes. The first of these is the Isherwood system of longitudinal ship construction, in which the transverse frame as ordinarily understood is dispensed with, but deep transverse web frames are placed at intervals of 15 to 18 feet apart and extending right round the ship, forming both frame and beam together. These frames are intersected by longitudinal frames consisting of sections of convenient form, preferably bulb angles, spaced about 20 to 30 inches apart, just as transverse frames are under the ordinary system. The fore and aft frames are fitted beneath the deck also, and are spaced from 30 to 50 inches apart. In the double bottom the fore and aft girders are formed of plates and angles.
The first general cargo vessel on this plan was the Craster Hall, launched in February 1908 by Messrs. William Hamilton and Co., Ltd., Port Glasgow. Her length is 392 feet 6 inches; breadth, 50 feet; depth, 29 feet to the upper deck; dead weight, 7300 tons.
