Sinking of large warships had rarely occurred in naval battle. Solid shot either bounced off thick wooden hulls or left a small, patchable hole. So warships normally just blasted away at each other until one of them, casualties mounting and its deck and rigging a shambles, hauled down its colors. Yet _Virginia_ had sunk or caused to eventually sink two of them in two hours. Its ironcladding allowed it to get close enough to _Cumberland_ to use an ancient but still effective technique, ramming, and close enough to _Congress_ to pound the ship at close range with its broadside shot and big rifles. While ramming would not remain a tactical option, ironcladding was universally adopted as every naval power raced to design hulls that could withstand ever more powerful explosive shells fired from rifled guns (_see pages 42-43_). As in every war, technology helped shape strategy in the Civil War and strategic considerations helped determine how new technologies were applied. The Navy’s major role in the war effort was to blockade some 3,500 miles of Southern coastline. The South’s blockade runners were typically the most advanced examples of British shipbuilding, steam-powered sidewheelers that were often iron- or steel-hulled. In the first year of the war, only about one in eleven of these runners were caught (partly because sidewheelers were still faster than screw steamers), and the Union Navy continued to build, borrow, and buy every vessel it could to strengthen the blockade. _Continues on page 36_ Steam Propulsion When steam was introduced as an auxiliary naval power source in the 1820s, paddle-wheels were the initial method of propulsion. In the late 1830s engineers began working with propellers—“screws” in naval terminology. Each technology had its partisans: the sidewheel provided greater combat maneuverability, was suited to riverine warfare, and presented no problems of leakage, as did the screw’s underwater shaft hole. However, the exposed wheels were vulnerable during combat, ate up deck space needed for guns, hindered sail handling, and created more drag than a screw when the vessel was under sail. The launching of the screw warships H.M.S. _Rattler_ in Britain and U.S.S. _Princeton_ in the United States in 1843 signaled the coming ascendancy of screw propulsion. In the historic 1845 tug-of-war between _Rattler_ and an otherwise-identical sidewheeler, the greater efficiency of the screw was publicly confirmed. [Illustration: SECTIONAL VIEW OF THE U.S. STEAM FRIGATE MERRIMAC. _1857 inboard plan of screw frigate_ Merrimack. _Screw could be hoisted into a well to reduce drag when the vessel was under sail._] [Illustration: Sidewheels: good maneuverability, but vulnerable above the water.] [Illustration: The screw: more efficient, and protected below the waterline.]