I've seen three large ships, so far, that were not broadcasting AIS - due to equipment failure, I assume. One was off the coast of Maine in thick fog. (We didn't have the radar on at the time as our batteries were getting low.) Three days ago, I was crossing the channel in the Delaware River, just below the C&D Canal. One ship, heading north, was about to pass me. I saw another pointing south but assumed it was anchored as it didn't show on AIS. It was heading south and passed just when I would have been crossing the channel. I think I'll be monitoring the radar more often from now on.