MARINE ENGINES.

487

by the Admiralty and mercantile firms with good results. Theblades can be made thinner and smoother than with steel, the lattersuffering from corrosion. The cost of the former is greater thansteel, but the endurance being in excess economy appears to beattained by its use.

Plan for Loivering the Propeller below the Keel of the Skip .—In long ships the pitching in a heavy sea, and the vertical motionof the waves, injuriously afifect the action of the propeller whenfitted in the usual way, causing the engines to “race” dangerously,as well as diminishing the speed of the vessel. In Fig. 372, thepropeller is shown in its normal position, the shaft being in astraight line with the rest of the shafting. On the ship arriving inshallow water the screw is raised so that the point of the bladesrotate above the level of the keel; and the shaft can be still furtherraised, exposing the boss, when the ship is in light trim. In thisway a broken blade can also be replaced without dry dock accom-modation.

To effect this movement of the screw the two last lengths ofthe shafting are coupled with a universal joint, the inboard shafthaving a fixed pillow block, while the screw shaft is fitted with asliding bush, lined with Strips of lignum vitae in the usual manner.The bush is guided by the two cheeks of the stern post, and it israised by means of a rack which passes upwards and is connectedby gearing to a steam engine placed on the upper deck. Theshaft works through a slot in the after bulkhead, the water beingexcluded by a radial gland working on two centres; while theuniversal joint works in a chamber in the after end of the screwalley, and is at all times accessible to the engineers. The liftingrod and gearing are inclosed in iron casings, which extend to theupper deck, and exclude any water that may rise. To preventwarps from fouling the screw when raised in shallow water, a slidingkeel is run out by a mechanism placed on the upper deck. Thewant of the usual keel piece joining the inner and outer stern postis fully compensated by the increased width and extra thickness atthe head of the screw aperture, the centre of effort of the rudderbeing raised above the ordinary height. A false foot is fitted tothe bottom of the rudder post, which can be readily removed,allowing the screw boss to be changed in a dry dock without ais-turbing the shaft.