approach underwater, safe from the guns of both the capital ships and the destroyers (although not for long), and fire a salvo as deadly as a torpedo boat's. Limited range and speed, especially underwater, made these weapons difficult to use tactically. Submarines were generally more effective in attacking poorly defended merchant ships than in fighting surface warships, though several small-to-medium British warships were lost to torpedoes launched from German U-boats. Oil was just being introduced to replace coal, containing as much as 40% more energy per volume, extending range and further improving internal layout. Another advantage was that oil gave off considerably less smoke, making visual detection more difficult. This was generally mitigated by the small number of ships so equipped, generally operating in concert with coal-fired ships. Radio was in early use, with naval ships commonly equipped with radio telegraph, and merchant ships less so. Sonar was in its infancy by the end of the war. Aviation was primarily focused on reconnaissance, with the aircraft carrier being developed over the course of the war, and bomber aircraft capable of lifting only relatively light loads. Naval mines were also increasingly well developed. Defensive mines along coasts made it much more difficult for capital ships to get close enough to conduct coastal bombardment or support attacks. The first battleship sinking in the war — that of — was the result of her striking a naval mine on 27 October 1914. Suitably placed mines also served to restrict the freedom of movement of submarines. Theaters North Sea The North Sea was the main theater of the war for surface action. The British Grand Fleet took position against the German High Seas Fleet. Britain's larger fleet could maintain a blockade of Germany, cutting it off from overseas trade and resources. Germany's fleet remained mostly in harbor behind their screen of mines, occasionally attempting to lure the British fleet into battle (one of such attempts was the bombardment of Yarmouth and Lowestoft) in the hopes of weakening them enough to break the blockade or allow the High Seas Fleet to attack British shipping and trade. Britain strove to maintain the blockade and, if possible, to damage the German fleet enough to remove the threat to the islands and free the Grand Fleet for use elsewhere. In 1918 the U.S. Navy with British help laid the North Sea Mine Barrage designed to keep U-boats from