Most of them after 1870 began the construc tion of iron hull vessels with compound pro peller engines, and by 1880 had laid aside the wooden hull vessels with theexpensive beam engine or the simple condensing propeller en gine, so that at the present time we find a fine fleet of large vessels running from all our prin cipal ports on the coast that are operated at a much less cost for same size of vessel than those in the same service 30 years or more ago.
Whalebacks.— These vessels have a main deck rounded over, and resemble the back of a whale. They are for use in rough water, the waves passing completely over them without resistance or injury. The first one was built for a Baltimore company about 1860. Such vessels are now used as grain carriers on the Great Lakes; and more recently as passenger vessels. As oil-carriers this type has been very successful upon the deep sea. Their average speed is nes 20 miles per hour.
Double Sc Ferry-boats.— This type of vessel, having a scr at each end of the ves sel, is, for ferry of comparatively re cent date, although dons - crew steam vessels were built in this country • :1 years ago. These vessels must not be confu with a twin-screw vessel having two screws •t the stern of a vessel. The pioneer of the late ' velopment was the Bergen, built its 1888 for t Hoboken ferry, having a triple expansion en gine. Since then this type of ferry-boat has been growing in favor until at this time it has practically driven the old-fashioned side wheeler out of use; except on the shorter ferries.
Screw Propellers.—After John Steven's ex periments with the screw propeller in 1802 to 1806, there was nothing done in this country to practically demonstrate the value of the screw until the Robert F. Stockton, having an Erics son propeller, was brought to this country in 1839. Then the propeller began to come into use, both on the Atlantic Coast as well as on Lake Erie and Lake Ontario, and it is alto gether probable that its largest development in the first decade was on the northern lakes.
Summary.— The progress in the develop ment of the modern steam vessel may be sum marized by divisions into decades marked by salient advances.
Between 1845 and 1855 iron was substituted for wood in the hulls of steam vessels and the boiler pressure was advanced from 10 pounds to 20 pounds per square inch; the coal con sumption being reduced from 4.5 pounds to 3.5 pounds per horse power per hour.
Between 1855 and 1865 the screw-propeller took precedence of the paddle-wheel as a means of propulsion; the boiler pressure was in creased to 35 pounds; and the coal consumption dropped to three pounds per horse power per hour.
Between 1865 and 1875 the compound engine was introduced; the boiler pressure went up to 60 pounds; and the coal went down to 2.5 pounds per horse power per hour.
Between 1875 and 1885 steel superseded iron in hull construction, and the triple-expansion engine came into use. The boiler pressure was advanced to 125 pounds, and the coal con sumption fell to two pounds per horse power per hour.
Between 1885 and 1895 twin screw propul sion with quadruple expansion engines and the principle of the forced draught were brought into use. The boiler pressure rose to 200 pounds, and the coal consumption dropped to 1.5 pounds per horse power per hour.
The decade 1895 to 1905 was one of transi tion. A few turbines appeared and the steam pressure in compound engines reached a maxi mum of 225 pounds.
Between 1905 and 1915 turbine engines came generally into use, and the boiler pressures came down to 160 to 175 pounds, at which figures the turbines were most effective. The coal consumptiorr also decreased to 12 pounds per Noise power per hour. See also SHIP BUILDING; WARSHIPS.