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1903.] MARINE REVIEW AND MARINE RECORD: 31 ON SCREW SHAFTS.* By John List. Increased size and power in merchant steamers has led to the more general adoption of twin screws, while at the same time there has been a distinct tendency towards comparatively full models, giving good carrying capacity on a moderate draught. This ful- ness of form has led naturally to the bossing out of the hull to house the stern tubes and shafts in place of fitting outside brackets, although the latter are retained in warships and other vessels of especially fine form. Where bossing out is adopted the stern tubes and propeller shafts are usually similar in all respects to those fitted in a single- screw ship, and the shafts can be drawn inboard for examination without. difficulty. Where, in very fine-lined ships brackets are fitted with long unprotected shafts outside the hull it is necessary in many eases to draw the shafts outboard, which involves coup- lings removable from the shafts, thus introducing some mechanical difficulties, esnecially in the case of large and heavy shafts. Where the usual form of stern tube is fitted the shaft may be covered with gun-metal throughout its length, which gets over, to a great ex- tent, the risk of corrosion and consequent weakening of the shaft. In recent years the changing conditions of trade (with the United States particularly) have led to cargo steamers being sent on long voyages in ballast. In the case of single-screw ships in this condi- tion the propellers are usually only partially immersed, with the result that the consequent severe racing has led in many cases to broken shafts. So great has been the loss from this cause that the registry societies have found it necessary to modify their rules in the direction of increasing the size of the shafting. A shaft made of ingot steel by a Sheffield firm was removed on survey after hav- ing run about 743,000 miles. A point at the forward end of the after liner had been lapped with marline to prevent grooving at the end of the gun-metal and that part had not wasted at all, whereas immediately in front of the lapping the reduction in diameter was quite 9-16 in. and the surface of the steel at that part was cracked all round. The depth of the cracks appeared to be 3-8 in. In this case the stern bush was not worn down nor was the shaft at all out of line. When disconnecting the shaft it was found to be strongly magnetic; that, however, is not uncommion with shafts which have been many years at work, The question of the life of steamship shafting i is one of consid- erable importance, and it is no doubt desirable that shafts and other important working parts should be renewed, as in the case of locomotive crank-shafts, after running a given mileage. Com- petition in ocean carrying is, however, severe, the conditions of trade are constantly changing, and ships become obsolete or un- profitable for the trade they were built for, and pass into' other hands, so that renewals are not, as a rule, carried out unless de- *Paper read at Enginee ing Conference, Loudon. fects incur rejection on survey by the board of trade or the reg- istry societies. The practice, which is now becoming general, of building cargo steamers with one or more hold ballast tanks in addition to their double bottom ballast capacity, will, by giving greater immersion to the propellers in the light condition, tend to reduce the risk of broken propeller shafts. Shafts of moderate size are in some cases forged from selected scrap-iron, but the amount of really good scrap in the market is small, and shafting for high-power ships is now generally forged from ingot steel. In first-class work about 40 per cent. of the top end of the ingot and about 5 per cent. of the total weight of the in- got from the bottom should be rejected in the forging. It is-also, in some cases, specified that the sectional area of the finished forg- ing is not to exceed one-sixth of the original sectional area of the ingot. It may safely be affirmed, however, that for the ordinary. run of steel shafting for merchant steamers considerably smaller ingots, in proportion to the finished forgings, are used. While for high-power cruisers propeller shafts of, say, 24 in. diameter are fitted, having the internal diameter about 75 per cent. of the external, it is not usual in full-power passenger ships to find such extreme proportions adopted. In the case of the Atlantic passenger ship Deutschland, the particulars published give the external diameter of the propeller shafts as 25.2 in., with an internal diameter of 10 in. The conditions are, however, different. In the case of warships an excessive maximum power on the min- imum weight is essential, and the question of first cost must be entirely subordinated to the attainment of that result, whilst at the same time the vessel generally steams at low power. Also the fineness of form necessitates very long outboard shafts with bearings far apart. In the case of the passenger ship the maxi- mum sea-going power is continuously exerted and the shafts are carried in stern tubes housed in the bossing,,and with long bear-: ings spaced close together. For hard and continuous work of the Atlantic high-speed service the arrangement adopted is that best suited to the requirements; but it is not applicable to a warship, where the extreme proportion of beam to length necessitates an entirely different form of model and arrangement of shafting. Nickel steel shafting has been largely adopted, not only in full- power ships on the Atlantic, but also in the shafting of many for- eign warships. The crank-shafts, thrust and propeller shafts of the Deutschland, before referred to, are of nickel-steel, and in the later and still higher power ships of that class the same ma- terial is used. The ultimate tenacity of some heavy shafting forg- ings of nickel steel made on the continent is 38 to 40 tons per square inch, with about 20 per cent. elongation, and a ratio of elastic to ultimate stress of about 67 per cent. 'Rather softer steel made in this country, and showing 3 to 4 per cent. of nickel on analysis, has been extensively used for. shafting, so far, with sat- isfactory results. Its ultimate tenacity is 36 to 38 tons per square inch, with about 30 per cent. elongation on 2 in., and an elastic English Royal Navy - ~~ - . Russian Imperial Navy Argentine Navy i - The " Messageries Maritimes" Cas BELLEVILLE WATER-TUBE BOILERS NOW IN USE (FEBRUARY, 1903) On Board Sea- -going Vessels, NOT INCLUDING New ins -stallations Building or Erecting, French Navy - Poe - Japanese Imperial Navy - 3 Austrian Imperial Navy -- - : : Italian Royal Navy' - _ ; Chilian Navy - - - Chemins de fer de 'Ouest: (The French siete a Railway Co.) Steamships plying between Dieppe and Newhaven Total Horse Power of Boilers in Use 276,460 H. P. ahs : : 849,300 " : ee - $93,900 " j eee 122,700. " 7 32,900 " : : 3 13,500 " goa 6.500 : : : 13,000 " pee = 8E@00 © 48,500 6 - 1,634,360 Works and Dock Yards at Saint-Denis (Seine), France. DELAUNAY BELLEVILLE CO.--tncorporated 7 CAPITAL: 6,000,000 FRANCS Telegraphic Address: Belleville, Saint-Denis-Sur-Seine