CHAPTER V

STEEL VESSELS

Drive

BALLAST Bulkheads- Isherwood System Methods of Propulsion -Triple Expansion - Steam Turbines - Electric Forced Draught-Motor Ships-Oil Fuel-Speed-ConsumptionDeadweight - Displacement — Loadline-Freeboard - TonnageBoard of Trade-Cubical Measurement-Lifeboats-Food-Personnel-Classification-Special Surveys-Surveyors.

MODERN vessels are almost invariably constructed of steel. For special trades wood is used, particularly if plentiful where built, and during the Great War a number of vessels were built of ferroconcrete, but these latter do not seem to have obtained any popularity.

Steel Vessels.

Steel vessels owe their forms of construction in the first instance to a process of evolution from those of the wooden ship, in which the wood transverse framing was sheated to form the skin of the ship and wood beams connected one side to the other, on which decks were laid.

The steel vessel of to-day still has her transverse frames covered with steel plating, which forms the skin or shell, as it is more generally called. Steel beams connect between these frames, and form the deck or decks.

In the wooden ship, the keel formed the foundation from which the frames rose to form the structure, and in the earlier iron and steel vessels this system was adopted for a time.

Ballast.

When the wooden sailing vessel could not get a cargo from the port where she had discharged, it was necessary to ballast her to permit proceeding safely to the next loading port. The ballast usually took the form of broken stone, sand, or anything heavy and cheap which was available. It was soon found, however, with the iron vessel (because iron followed wood before steel) that the metal construction lent itself to forming tanks in the vessel, which were

filled with water as ballast, till to-day we have the cellular double bottom and/or other tanks in many varieties, carrying not only water but fuel oil.

The water ballast tank has also the great advantage of being easily and quickly filled, or emptied, at little cost; the filling is usually done by gravity and the emptying by steam pump.

The double bottom serves an additional purpose in the event of stranding, where the double skin has to be pierced before flooding of holds takes place.

Bulkheads.

Bulkheads also form a special feature of the construction of all vessels. These subdivide the vessel into its different necessary compartments, and in passenger steamers are necessarily more numerous than in the tramp; they also help to give strength to the structure. The number of bulkheads is prescribed by the Registry Society, but as this is more of a technical matter it need not be elaborated here. We may mention, however, that greater attention has been paid to this in recent years, with a view to obtaining designs which would ensure the ship still floating with at least one of her compartments open to the sea. This is not always actually obtainable in general cargo vessels, but is imperative in passenger carrying ships.

Isherwood System.

While transverse framing is the usual method in the Mercantile Marine, there have been introduced in recent years forms of longitudinal framing, notably the Isherwood system.

Fig. 6 (p. 49) illustrates in perspective the midship section of a single deck cargo vessel designed on the ordinary transverse system. Fig. 7 (p. 51) shows a similar vessel on the longitudinal Isherwood system. The principal advantages claimed are: ability to carry increased deadweight; increased longitudinal strength; increased bale and grain capacity; increased local strength; and less vibration.

The Isherwood system was introduced in 1907, and since then it has steadily gained favour with owners of various nationalities, as is shown by the figures on page 48, kindly supplied with the illustrations 6 and 7 by Messrs. Isherwood.

The 1922 total shows that the average vessel is of over 8,000 deadweight carrying capacity.

Methods of Propulsion.

As already mentioned, the sailing ship is declining in commercial value, so we shall confine ourselves to the methods of mechanical propulsion only.

The types of engines are

Steam reciprocating.

Steam turbines.

Motors (internal combustion engines).

According to the 1922-23 edition of Lloyd's Register the gross tonnages of vessels propelled as above were at that time respectively

51,653,324 tons gross

This means in round figures, that for every 100 tons gross

Triple Expansion.

From these figures it will be seen that at the present time the steam reciprocating engine is the most popular. The reason is not hard to find. It has been in use now, in the form principally of the triple expansion engine, for nearly 50 years. It has, therefore, the great advantage of having the third generation of engineers engaged in its manufacture and running, so that to-day it is very nearly a standard engine. Even the most ordinary marine engineer knows the troubles he may expect with it, and, what is equally important, knows how to remedy them without undue delay, and, speaking generally, the triple expansion, or quadruple expansion, engine of to-day can, in capable hands, be relied on to give regular service year in year out with moderate upkeep costs, on a coal consumption "all in" not exceeding 1.5 lb. per indicated horse-power hour.

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Steam Turbines.

The steam turbine is well known to be more economical than the reciprocator, where full advantage can be taken of its high revolutions. No modern power station has anything else but turbines where steam is used.

On board ship, however, the proposition is very different. The high revolutions of the turbine have to be reduced to a speed at which the propeller will give reasonable results. This is achieved by the introduction of steel toothed wheels and known as reduction gear. For high-speed steamers with fast running propellers, one set of gear suffices, and this is known as single reduction gear, and very excellent results have been, and continue to be, given thus.

For slow-speed steamers with slower running propellers, an additional set of gear is required, known in this case as double reduction gear, and it is here that considerable trouble has been experienced in several cases. Until a suitable metal is found for the gear able to withstand high revolutions, in addition to the unknown stresses and strains to which an engine secured to a floating body is always subject, it is probable that double reduction gears will continue to give trouble. Another factor to be considered as well as the suitability of the metal, is the mechanical accuracy