Formed by maritime labor representatives from America's Pacific, Great Lakes and Gulf Coast regions In 1893, the ISU affliliated with the American Federation of Labor, in 1893 and in took the name International Seamen's Union of America in 1895.
The union existed at a turbulent time in the United States shipping industry. The unions within the ISU faced "continual changeover in the makeup and leadership," and weathered the historical periods of the Great Depression and World War I. Select periods were beneficial, including during World War I when a shipping boom and ISU's membership included more than 115,000 dues-paying members. However, when the boom ended, the ISU's membership shrunk to 50,000.
During its existence, the union did have a major effect on the shipping industry. Perhaps the most significant was the successful lobby for the Seamen's Act of 1915. The act fundamentally changed the life of the American sailor. Among other things, it:
Another of ISU's successes was the strike of 1919, which resulted in wages that were "an all-time high for deep sea sailors in peace time."
However, ISU had its shortcomings and failures, too. After a round of failed contract negotiations, ISU issued an all-ports strike on May 1 1921. The strike lasted only two months and failed, with resulting wage cuts of 25 percent. The ISU, as with all AFL unions, was criticized as being too conservative. For example, in 1923 the Industrial Workers of the World (I.W.W.) publication The Marine Worker referred to the ISU's "pie-cards" (paid officials) as "grafters and pimps. Additionally, the union was weakened by the loss of the Sailors' Union of the Pacific in 1934. Furuseth charged that the SUP was being infiltrated by "radicals" from the I.W.W., and demanded the SUP cease activities with the Maritime Federation. The SUP refused and the ISU revoked their charter.
The ISU was involved the West Coast longshoremen's strike of 1934. Lasting 83 days, the strike led to the unionization of all West Coast ports of the United States. The San Francisco general strike, along with the 1934 Toledo Auto-Lite Strike led by the American Workers Party and the Minneapolis Teamsters Strike of 1934, were important catalysts for the rise of industrial unionism in the 1930s.
West Coast sailors deserted ships in support of the International Longshoremen's Association longshoremen, leaving more than 50 ships idle in the San Francisco harbor. ISU officials reluctantly supported this strike. In clashes with the police between July 3 and July 5 1934, three picketers were killed and "scores were injured." During negotiations to end the strike, the sailors received concessions including a three-watch system, pay increases, and better living conditions.
In April 1935 at a conference of maritime unions in Seattle, an umbrella union was established to represent the membership of the ISU as well as maritime officers and longshoremen. Called the Maritime Federation, Harry Lundeberg was named its first president.
Believing it was time to abandon the conservative ISU, Curran began recruiting members for a new rival union. The level of organizing was so intense that hundreds of ships delayed sailing as seamen listened to organizers and signed union cards. The ISU's official publication, The Seamen's Journal, suggested Curran's "sudden disenchantment" with the ISU was odd, since he'd only been a "member of the union for one year during his seafaring career."
In May 1937, Curran and other leaders of his Seamen's Defense Committee reconstituted the group as the National Maritime Union. Holding its first convention in July, approximately 30,000 seamen switched their membership from the ISU to the NMU and Curran was elected president of the new organization. Within a year, the NMU had more than 50,000 members and most American shippers were under contract.