typical flush deck destroyer had a normal crew of 105 officers and men, and was armed with four 4-inch deck guns, one 3-inch anti-aircraft gun, 12 torpedo tubes, two stern-mounted depth charge racks, along with .50-caliber machine guns and small arms. The mass-produced classes also had four boilers providing steam to a pair of steam turbines, each of which drove a screw at a combined for a top speed of about . Destroyers quickly acquired the anti-submarine warfare mission against U-boats in the Atlantic and Mediterranean, being equipped with depth charge racks, hydrophones, and eventually Y-gun depth charge throwers. However, it was very difficult to successfully attack a U-boat with World War I technology, and US anti-submarine forces only scored two kills in that war. Between the World Wars After the end of World War I, there was little need for the destroyers built, so many were laid up, and fourteen had their torpedo tubes removed and were converted to minesweepers. On 8 September 1923, seven of the ships ran aground off the coast of California in the U.S. Navy's worst ever peacetime disaster. In 1922, the Washington Naval Treaty was signed by the United States, the British Empire, the Empire of Japan, France, and Italy. The treaty called for a freeze in the size and composition of the world's major navies, including the U.S. Navy, which ceased production of large capital ships and destroyers. The London Naval Treaty, a 1930 agreement between the same parties (except France), established total destroyer tonnage limits for the navies. The treaty also defined two categories: destroyer and destroyer leader; along with the maximum tonnage of each category, and the allowable ratio of one category to another. In the London Naval Treaty, destroyers were established as "surface vessels of war the Standard Displacement (S.D.) of which does not exceed 1,850 tons and with a gun not above 5.1 inch caliber", as published in Ship’s Data for U.S. Naval Vessels. The “total completed tonnage not to be exceeded on December 31, 1936” was 150,000 S.D., but “not more than 16% of the allowed tonnage... shall be employed in vessels over 1,500 tons S.D.”. The new, higher limits rendered the existing flush-deckers obsolete, and the General Board soon moved to replace them. Since Japan was considered a probable adversary and was building destroyers through the 1920s, the General Board replaced the four-stackers with ships that could