Shipbuilding Mercantile

The spaces between the expansion trunk and the ship's side, called summer tanks, are also used for cargo when oil of light density is carried. The form of the vessel between the transverse bulkheads is maintained by a continuous deep girder right round the ship called a transverse, the transverses being usually spaced about i o feet apart. The shell and deck plating is supported by channels and bulb angles called longitudinals, spaced about 3o inches apart and extending continuously between the transverse bulkheads, to which they are attached by plate brackets. The middle line and transverse bulkheads are stiffened in a similar manner by strong vertical webs in association with bulb angle horizontal stiffeners. Several hundred vessels have been built on this system and have proved very successful, the only trouble experienced having been in the rivets attaching the longitudinals to the bulkheads. To overcome this difficulty a new design, known as the bracketless system, has recently been introduced. The outstanding feature of this system is the entire elimination of the brackets attaching the longitudinals to the bulkheads, the dis continuity at the transverse bulkheads being compensated for by the fitting of local doublings on the shell plating.

Motorships.

A revolutionary change in the mercantile marine was foreshadowed by the appearance in 1910 of the first ocean going vessel to be driven by internal combustion engines. Progress in this direction was naturally retarded by the World War, and it was only about 1921 that the construction of motorships was commenced on a large scale. Since then the development has been very rapid, and in 1938 approximately 58% of new ships were of this type.

Viewed from an economic standpoint, a comparison between motor-engined and steam driven vessels, the ship being the same in each case, indicates that— (a) there is an increased first cost; (b) the wage cost per ship tends to be less, while the cost of handling oil on the ship is much below that required for coal ; (c) there is a greatly reduced consumption of fuel—which fuel, though costly, yet costs less in the aggregate than coal.

(d) there is an increase in the deadweight available for cargo, due principally to the reduction in the weight of the fuel carried; (e) there is also an increase in the capacity of the space available for cargo.

The actual differences must necessarily vary with the size and type of ship, but for a steamer carrying about 8.000 tons dead weight the coal consumption per day would probably be about 3o tons, whereas for a motorship oil would only be consumed at the rate of about 8 tons. Allowing an average weight of coal of I,000 tons, which has to be deducted from the available dead weight of 8,000 tons, the coal-burning ship can only carry tons, whereas the motorship carries about 7,750 tons, or about o% more. The increase in the space available for cargo would amount to about the same percentage.

In the early days of motorships trouble was necessarily ex perienced from mechanical defects in the machinery, due primarily to the high temperature of combustion of the 'gases in the engine cylinders. Much patient research work has overcome these troubles, and the fact that many of the largest modern liners have been fitted with motor machinery, and perform their voyages to schedule without any question of breakdown, indicates that the motorship now occupies an established position and that these ships will in all probability form an increasing percentage of the world's tonnage.

Launching.

When the steel work of the hull has been prac tically completed the vessel is ready for launching. It will be appreciated that the operation of transferring to the water a ship whose weight in the case of the largest vessels might amount to as much as 17,00o tons requires considerable forethought and is not unattended with some risk. The launching ways are erected at about one-third of the breadth of the ship on each side and consist of two sets of ways, the fixed or ground ways and the sliding ways. The fixed ways commence near the bow and extend for the full length of the ship, being carried out as near low water mark as possible in order to ensure that there shall be ample depth of water—say from 3 to 6 feet—over the ends of the ways, to ensure a successful launch. These fixed ways consist of solid baulks of timber varying from 15 to 6o inches in breadth, according to the size of the ship to be launched, and are laid on closely-spaced supporting blocks. The sliding ways, known as the cradle, are laid on top of the ground ways, the space between the ways and the hull being filled up with timber neatly fitted to the vessel's shell plating. The ways are laid with an inclination of about 8 inch per foot in order that the ship may slide easily into the water. Two or three days before the launch, the cradle which has been fitted in place temporarily is taken adrift and the surfaces of both the fixed and sliding ways are thickly covered with melted tallow; when this has hardened it is smeared with soft soap, after which the sliding ways and the various making up pieces of the cradle are replaced. Until the moment of launch ing, the sliding ways are locked to the ground ways by means of a dagger, one either side at the forward end. To launch the vessel the two daggers are released simultaneously, and the vessel usually commences to move of its own accord. When launched in open water the vessel is brought up when clear of the ways by dropping an anchor. When launched in a narrow river the vessel must be stopped quickly before the stern strikes the opposite bank, and this is done by leading strong steel wires from the bow and attaching them to heavy piles of chain, the friction of the chain as it is dragged over the ground gradually bringing the vessel to rest. (W. S. A.)