This boat is built to combine great speed with a moderate seaworthiness and resisting power. The dimensions range as follows: length, 57 to 85 feet; beam, 7½ to 10 feet; draft, 3 to 4 feet. They are built of steel of an average thickness of ³/₁₆ an inch, being completely decked over. Generally they are divided into six separate water-tight compartments. The forward and after ones are store-rooms; the second one is arranged for the accommodation of the crew; the third is the pilot-house; fourth, engine and fire room; and fifth, for the accommodation of officers. The single-screw shaft projects some distance beyond the stern-post to allow the double rudder to work forward of it. The screw itself is of a peculiar construction, so built as to project the water straight aft instead of radially. These launches are fitted for either spar or Whitehead torpedoes. In the former case the swivelled sleeve in which the spar works pivots abreast the pilot-house. In the latter case two methods are in use. By one, a Whitehead is carried on deck on each side mounted on a carriage running on rails, by which they may be run up to a firing-case on the forward part of the deck. This firing-case is mounted on a pivot-carriage and has depressing-gear, so that the torpedo may be pointed in any desired direction. A pointing apparatus is provided in the pilot-house by means of which allowance of angle may be made for the movements of the boat or of the attacked vessel. The pivot-circle is graduated so that the torpedo may be correctly aimed with but little difficulty. By the other arrangement, a Whitehead is carried on each side on pivoted davits in a sling, so that it may be lowered into the water; the machinery is then started by hand, and the torpedo moves off without being projected. In this case the torpedo can only be fired right ahead. [Illustration: H. M. Steam Torpedo Launch "Lightning". Thornycroft Torpedo-Launch. Length (extreme) 84 6 Beam 10 10 Draught 5 0 Speed 18 Knots] _The Yarrow Torpedo-Boat_ [Illustration: Yarrow Torpedo-Launch.]