Engineering Wonders of the World
Volume I

Propelling åvkhineryOI llRIIWHIIlilWl y///zz////5^57zy///zzr .. ,| I BY ALBERT G. HOOD, Editor of “ The Shipbuilder.” FOR long sea voyages the side paddle- wheel has been entirely replaced in the steamship by the screw propeller set at the stern. The side paddle-wheel was soon found to be unsuitable for cargo steamers, the freeboard of which varies constantly with the nature and quantity of the Paddle v. carg0 carried. Thus, when the Screw. ship was steaming light the paddles were too far out of the water, and when loaded they were too deeply immersed. In the warship, the paddle wheels and boxes presented an easy target for the enemy’s gun fire. The heavy rolling of a ship in a seaway also threw a great strain on the paddles, as each paddlo was in turn lifted clear of the water while the one on the opposite side of the vessel was buried in the waves. Again, the cumbrous paddle-boxes greatly impeded the ship when forcing her way against a head wind. On the other hand, the screw propeller, completely immersed at the after end of the ship, largely overcame these difficulties, and the side paddle-wheel is now used for river work only, or for short sea passages ; and even for the latter purpose it is now being largely replaced by the turbine-driven screw propeller. As the size and speed of steamers have in- creased, the single screw has been superseded to a large extent by twin screws driven by separate engines ; and these in turn, with the advent of the marine steam - turbine, have given place to triple or quadruple screws. One of the greatest factors in the develop- Working Pressure of Engines and Boilers. ment of the steamship has been the rapid in- crease in working steam pressure. This has been rendered possible by the great improvements effected in machinery, workmanship, and material. In the boilers of the earliest steamers the working pressure was only about 35 lbs. per square inch, or even less. The cylindrical multi- tubular boilers of the present day, commonly THE ENGINES OF THE “ DEUTSCHLAND.” alluded to as “ Scotch ” boilers, have a working pressure up to about 210 or 220 lbs. per square inch, while in the various types of water-tube boilers the pressure is fre- quently from 250 to 300 lbs. Both cylin- drical and water-tube boilers are described and illustrated later. Increased working pressure has greatly aug- mented the economy of the marine engine, owing to the greater range of steam expansion