The Evolution of Twin Screws on Steamships (1896)
The City of New York, of the Inman and International line, in 1888 adopted twin screws, but she was by no means the first to do so. In fact, they were tried by Stevens, on the Hudson, in a small boat as far back as 1804, but she failed because, it is said, he could not make the stern pipes tight. Ericsson, too, tried them in a very ingenious way, on one shaft, in the F. B. Ogden, on the Thames in 1836, with great success. However, the City of New York was the first of the North Atlantic passenger fleet to adopt them.
They have not increased the speed much. The Etruria, a single screw, has averaged 19.65 knots per hour, Queenstown to Sandy Hook, while the best run of the celebrated City of Paris only averaged 20.70, and that of the Teutonic 20.349. However, they have added immensely to the safety of the ships, for many reasons, and it may be as well to explain them.
The dangers to which the ordinary screw steamship is exposed may be classed under five heads : —
- Fracture of main shaft, as in the case of the City of Brussels, Circassian, Umbria, Samia, and many other ships.
- Loss of screw or its fans, as in the case of the Peruvian and Sardinian.
- Loss of rudder or damage to it, as in the case of the Great Eastern and Alaska.
- Breaking down of the machinery, as in the case of the Aurania.
- Collision between two ships or with rocks, as in the case of the Oregon, Idaho, and City of Chicago.
The first four render a single screw steamship helpless, and she can only reach port by being towed or by the very tedious process of sailing under her own canvas. The fifth is usually fatal, as the cross-bulkheads are generally misplaced or too weak to withstand the pressure of a large body of water.
Now a ship with twin screws have two independent sets of engines and boilers, and in this fact is found their immunity from most of the dangers inherent in all single screw ships. Indeed, it may be said to exempt them from the consequences of the first four sources of danger, and even in the fifth case, it may prolong the ship's life, or lead to her rescue.
How? Let us see.
- It is impossible, in most cases, to repair a broken shaft at sea, but in a twin screw the only effect is to diminish her speed about one-third, say from 18 to 12 or 13 knots. As a matter of fact, the City of New York once made 382 knots with one screw in 24 hours, an average of nearly 16 knots per hour.
- This kind of accident is a very common one. The screw cannot be replaced at sea, but in a twin the effect is no worse than in the first case. Many things cause the loss of a screw. The Scythia lost hers by striking a whale; the Peruvian by striking field ice; the Sardinian through breaking the end of her main shaft. Floating timber, too, or a sunken wreck may cause it. But from all these accidents a “twin screw” is virtually free; or rather, if they do occur, the second screw is usually available.
- The 3rd is also a very common accident, and renders a single screw ship perfectly helpless. The Great Eastern became unmanageable, but having paddle wheels, slowly returned to Queenstown. The Sardinian transferred her passengers in mid-ocean, and the Alaska was assisted into New York by the Lake Winnipeg.
But in a twin screw it is possible to overcome even this disaster. By modifying the speed of the one screw or the other, as may be necessary, it is quite possible to make a fair course, sufficiently so to take the ship near to her destination, when a tug can easily be procured to assist her into port. This is exactly what the Paris did on a recent occasion. She has also the great advantage of being able to turn a circle in about her own length—an immense advantage in a narrow channel, or when fighting an enemy, as every sailor knows.
- The 4th is a kind of accident occurring from a variety of causes. Machinery can often be repaired at sea, as duplicates of many parts are carried; but in many cases, it cannot: cylinders crack, air pumps, piston rods, or condensers break, and the engine is rendered useless. The Aurania broke a connecting rod, which smashed the cylinder, and she drifted about until picked up by tugs. A paddle boat usually has two engines, and, unless the main shaft breaks, she can go ahead slowly with one engine, or even with one paddle. However, in a single screw, if one cylinder is disabled, the others will probably be rendered useless, because the steam passes from the high-pressure cylinder through the intermediate into the low, and thence into the condenser. However, in a twin screw the total breakdown of one engine only involves a diminution of speed.
- Collisions have recently become a source of terrible disaster to iron ships, and here again the utility of twin screws is seen. In a single screw further protection is obtained by means of cross-watertight bulkheads, when properly placed and constructed, though they have too often failed by an injudicious manipulation of doors through them. Double bottoms are also a great protection when a ship strikes on a sandy or level bottom, or even on rocks. In collisions between two iron ships, one usually escapes if she has a strong collision bulkhead near her bows. However, the other too often sinks in a few minutes from defective bulkhead arrangements, as referred to above, as has recently been illustrated in the case of H.M.S. Victoria and Camperdown, and of the North German Lloyd steamer Elbe and the Crathie.
The Cunard steamship Oregon, of 7000 tons, was sunk by a miserable little wooden schooner, which struck her in a vital part near the engine-room. Her bulkhead might have saved her, but sliding doors pierced it; the grooves were filled with small coal, and the doors could not be closed in time.
Twin screws having two independent sets of engines and boilers have generally in their engine-room a central longitudinal bulkhead running from the keelson to the main deck. The effect of this is threefold.
- It cuts the spaces in halves ;
- It strengthens the transverse bulkheads :
- It effectually separates the two sets of engines.
There remains to be considered one other danger, common to all ships, and perhaps the most terrible of all to landsmen—that of fire. The great advantage of an iron or steel ship, and especially of a twin screw, over a wooden ship is, that the fire may possibly be confined to one section and drowned out with water or steam from steam pumps or direct from the boilers without the passengers even being terrified by smoke. This was actually done in the case of the City of Richmond as well as in other ships.
Twin screws, too, reduce the necessity for sails, and thus the great resistance offered by cumbrous masts, yards, and rigging is lessened, simple pole masts having in most cases been substituted.
This adds much to the ship's speed in head winds, and raises the average speed of the voyage considerably. Hence, the wonderful regularity of the passages of such ships as the Paris, Teutonic, and Lucania, all twin screws, and all fitted with pole masts without yards.
The U.S. warships Columbia and Minneapolis, recently built at Philadelphia, have "triple" screws, one in the center, and one on each quarter; the latter made 21.8 knots, as the mean of 4 hours' sea trial.
Fry, Henry, “Sixth Epoch: Twin Screw, Epochs in Atlantic Steam Navigation,” in The History of North Atlantic Steam Navigation with Some Account of Early Ships and Ship Owners, London: Sampson Low, Marston and Company, Ltd. (1896): P. 50 -53