4, 1915; sec. 2.) (Effective beginning Nov. 4, 1915.) No vessel of one hundred tons gross and upward, except those navigating rivers exclusively and the smaller inland lakes and except as provided in section one of this Act, shall be permitted to depart from any port of the United States unless she has on board a crew not less than seventy-five per centum of which, in each department thereof, are able to understand any order given by the officers of such vessel, nor unless forty per centum in the first year, forty-five per centum in the second year, fifty per centum in the third year, fifty-five per centum in the fourth year after the passage of this Act, and thereafter sixty-five per centum of her deck crew, exclusive of licensed officers and apprentices, are of a rating not less than able seaman. Every person shall be rated an able seaman, and qualified for service as such on the seas, who is nineteen years of age or upward, and has had at least three years' service on deck at sea or on the Great Lakes, on a vessel or vessels to which this section applies, including decked fishing vessels, naval vessels or coast guard vessels; and every person shall be rated an able seaman, and qualified to serve as such on the Great Lakes and on the smaller lakes, bays or sounds, who is nineteen years of age or upward and has had at least eighteen months' service on deck at sea or on the Great Lakes or on the smaller lakes, bays, or sounds, on a vessel or vessels to which this section applies, including decked fishing vessels, naval vessels, or coast guard vessels; and graduates of school ships approved by and conducted under rules prescribed by the Secretary of Commerce may be rated able seamen after twelve months' service at sea: _Provided_, That upon examination, under rules prescribed by the Department of Commerce as to eyesight, hearing, and physical condition, such persons or graduates are found to be competent: _Provided further_, That upon examination, under rules prescribed by the Department of Commerce as to eyesight, hearing, physical condition, and knowledge of the duties of seamanship a person found competent may be rated as able seaman after having served on deck twelve months at sea, or on the Great Lakes; but seamen examined and rated able seamen under this proviso shall not in any case compose more than one-fourth of the number of able seamen required by this section to be shipped or employed upon any vessel.