Great though the advantages were of the screw propeller, it was admitted that it was not without many drawbacks. The single screw took up a lot of room, weakened to some extent the structure of the stern, and if anything happened to the engines or propeller the ship was helpless and had to depend entirely upon whatever sail power she might possess. To overcome this difficulty Messrs. J. and W. Dudgeon were the first to build, from the designs of Mr. John Dudgeon, a twin-screw ocean-going steamship. Twin and triple screws had been used before, but were driven by the one engine. Before this, however, Messrs. Dudgeon experimented with a small iron vessel, of 400 tons, called the _Flora_, which was given two independent engines and screws. The propellers were placed under the counter, and proved the advantage of this position over that of the practice, where two screws were used, of placing one before, and the other behind the rudder. The advantage of placing the screw either in a space cut in the deadwood, or, in the case of twin propellers, under the counter, was much greater than the method at one time adopted of placing the screw behind the rudder. Under the newer method the steering power of the rudder was not impaired; but under the older method, when the screw shaft was carried beyond the rudder, a slit known as a “shark’s mouth” had to be made in the rudder so that the upper and lower portions would be able to pass the screw shaft. The practice of equipping the vessels with wells or recesses into which the screw could be lifted was found to possess but slight advantages for warships, and was ere long abandoned. The best that could be said for it was that when a ship was travelling under sail only, the screw could be lifted from the water and the strain upon the wooden stern caused by dragging the screw, whether of the fixed or folded patterns, through the water, or running loose, was avoided altogether. The _Hebe_ was the third vessel on this principle built and engined by Messrs. Dudgeon, and the advantages of the twin-screw system over the single screw were again strikingly manifested during a series of manœuvres. The _Hebe_ was an iron vessel of 470 tons, and 165 feet long. The screws were three-bladed, 7 feet 6 inches in diameter, and had a pitch of 15 feet, and were worked by two separate and independent engines each having two cylinders 26 inches in diameter, with a 21-inch stroke of piston, and being collectively of 120 nominal h.p. The tests showed that the vessel with both screws working ahead could made a complete circle in four minutes or less, and in still shorter time with only one screw working and the helm thrown over, or with the two screws working in opposite directions.