The first application of twin-screws on the modern principle was made by Messrs. J. and W. Dudgeon in the Flora in November 1862.
Twin-screws were tried by the Admiralty some years earlier in the construction of the iron-cased floating batteries, but were driven in those vessels by one motion from the engines. The adoption of the twin-screw in their case enabled the Admiralty to build vessels that required only a moderately light draught of water, and carried, for their tonnage, an enormous weight of armament and armour, besides the weight of their engines; but the vessels had no increased powers of turning nor could they manœuvre rapidly under steam in any circumscribed space. The double independent screws overcame these drawbacks.
A small vessel in the Clyde worked two screws also, with two rudders, the idea, as acknowledged by the adaptor, having been derived from the model exhibited in the Exhibition of 1851 by Mr. John Sturdee, master shipwright’s assistant at Portsmouth Dockyard.
An unusual degree of interest attached to the trial of the steam-ship Flora by reason of the fact that each of her twin-screws was to be operated by its own engine. In the light of future events it is worthy of note that up to this time it was thought that the twin-screw would be useful for smaller vessels and gunboats carrying six guns or less; whereas the Flora, as representative of ships capable of carrying large armaments of guns, with considerable engine-power, and a light draught of water, and with a power of manœuvring such as could not be possessed by a single-screw vessel, marked a step forward in the march of improvement which was destined to have far-reaching results, both in the Navy and the Mercantile Marine. So important was the trial deemed that the Admiralty sent special representatives to report thereon. The Flora was an iron vessel, 150 feet long, 22¹⁄₂ feet beam, and 13 feet depth, and of 365 tons. She had two independent engines and screws, the latter being placed one under each quarter, and therefore in front of the rudder, in contrast to the prevailing system of placing a single screw right astern and behind the rudder. The cylinders of the two engines were 26 inches in diameter, with a stroke of 21 inches; and the propellers were each of 7 feet diameter with a pitch of 14¹⁄₂ feet. She had two tubular boilers working at 30 lb. pressure, and one high-pressure boiler working at 50 lb. pressure, the latter boiler being intended to be used for producing a steam blast in the chimney and to dry the steam from the two common boilers. The engines were of 120 horse-power collectively. She was rigged as a fore-and-aft schooner. The principal test to which the vessel was subjected tried her capabilities of being manœuvred. With the helm hard over and the engines going full speed ahead, the first circle was made in 3 minutes 14 seconds, the next in one second less time, and the third circle in 3 minutes 16 seconds, the diameter of the circle being about three lengths of the ship, but slightly diminished each time. The ship was then tested with one screw working ahead and the other astern. One circle was made in 3 minutes 39 seconds, and another in 3 minutes 49 seconds; “in making these circles the action of the ship’s hull was extraordinary, the central part being stationary, and both ends moving round equally. The circle was made on a pivot from the ship’s midship section. The vessel was then put in a straight course, stopped, and from a state of rest the engines were started, one ahead and the other astern, the circle being completed in 3 minutes 55 seconds and the diameter being as before within the ship’s length.”[98] The Flora proved herself faster than any other steamer of her size and horse-power, and became, thanks to her speed, one of the most successful blockade-runners during the American Civil War.
[98] Illustrated London News, November 29, 1862.
H.M.S. “Minotaur.”
The experiments in the Flora, and afterwards in the Hebe and Kate, which were of about the same dimensions and power, were considered so satisfactory that a trial on a larger and more important scale was made in the summer of 1863 with the Aurora. This was an iron vessel, 165 feet in length, with a beam of 23 feet, and a depth of 13 feet 6 inches. Her engines, of 120 collective nominal horse-power, drove two three-bladed screws, each independently of the other; the screws were 7 feet in diameter and had a pitch of 14 feet 6 inches. The cylinders were of 26 inches in diameter with a stroke of 21 inches. On her trials she steered equally well with either propellers or rudder, and in the matter of speed passed everything she came across, including the Sea Swallow, one of the fastest paddle-boats on the Thames. The distance from Tilbury to the Nore, twenty nautical miles, was done in 1 hour 17 minutes, “an almost unparalleled rate of speed, considering the vessel’s horse-power of engine and hull displacement.”[99]
[99] Times, August 1863.
The Experiment was the first twin-screw boat built for the Navy. The engines were direct-acting, horizontal, high-pressure, and drove two three-bladed propellers, having a diameter of 3 feet 6 inches. She was built by Dudgeon in February 1863.