President Donald Trump's plan to revive U.S. shipbuilding using massive fees on China-linked ship visits to American ports is causing U.S. coal inventories to swell and stoking uncertainty in the embattled agriculture market, as exporters struggle to find ships to send goods abroad. Trump is drafting an executive order that would rely on funding from a U.S. Trade Representative proposal to levy fines of up to $1.5 million on China-made ships or vessels from fleets that include ships made in China. Those potential port fees have limited the availability of ships needed to move agriculture, energy, mining, construction and manufactured goods to international buyers, according to major U.S. exporters and transportation providers in interviews with Reuters, letters to U.S. officials, and comments ahead of USTR hearings next week.