Three days after its strike action, the ISU asked the Pacific Coast District of the ILA to agree that neither of them would return to work until the demands of both have been met. Within days, the Masters, Mates and Pilots and the Marine Engineers Beneficial Association announced their numbers were joining the strike. Shipowners belittled the impact of the strike call on the ranks of their licensed employees. Herman Phleger, the shipowners' attorney, doubted the seamen's economic power. "The sailors unions were totally unorganized, but when they saw that the longshore workers were making progress, the leaders of the sailors union, desiring to obtain the same results that the longshore workers might, participated in the strike."

Before two weeks had passed, the count of strikers neared 22,000: 12,000 longshoremen; 5,000 from the Sailors Union; 2,100 from the Marine Firemen; 1,500 from the Marine Cooks and Stewards; 900 from MEBA; and 200 from MM&P.