The U.S. Navy is building a drone fleet to take on China. It's not going well.

During a U.S. naval test off the California coast last month, which was designed to showcase the Pentagon's top autonomous drone boats, one vessel stalled unexpectedly. As officials scrambled to fix a software glitch, another drone vessel smashed into the idling boat's starboard side, vaulted over the deck, and crashed back into the water -- an incident captured in videos. The previously unreported episode, which involved two vessels built by U.S. defense tech rivals Saronic and BlackSea Technologies, is one of a series of recent setbacks in the Pentagon's push to build a fleet of autonomous vessels, according to a dozen people familiar with the program.