I had a similar thing happen a couple of weeks ago. I was motoring toward southwest past Ft. Mifflin just upriver from 3F, outside the channel but approaching a bulkhead on the Philly side. A large tanker was stationary in the anchorage area across the channel near Riverwinds
, but he was pointed toward the Philly side with a tug behind him. I paused for a few seconds to see if he was doing anything, but both visual observation and AIS told me he wasn't moving. So I proceeded past as quickly as I could motor. After I was past the AIS showed them speeding up, and they moved toward the dock and ultimately tied up there. I should have been more aggressive about making radio contact, especially since I knew the tug's name (its name was obscured by the tanker, but AIS showed me the name). If I knew that's where he was heading I would have gladly steered behind them. But for all I knew, the tug could have been intending to back away from the boat and I would have been in the way if I steered behind him.
Yesterday we were tacking downriver against the current in the same area. One of those big white Del Monte freighters was heading downriver in the channel. I ran out of room and had to tack toward the channel. Having resolved to be more assertive with the radio, I wanted to contact the guy to let him know that I saw him and would stay out of his way by coming about before I reached the channel. But instead of sending a general call to him, I selected his vessel on the AIS list and attempted to place a DSC call to him on Ch13. However, he did not acknowledge the call. In the minutes that I waited for him to acknowledge I was almost to the point where I wanted to tack anyway, so I gave up on calling him and just came about to get out of his way.
By the way, for those who think the lack of acknowledgment might have been a bad antenna connection, that's always possible, but I have placed DSC test calls to boats before (nonverbal digital tests), with about 50% success at getting an acknowledgement. My owners manual says that the failures were likely boats whose radios were not configured to automatically acknowledge test calls.
Despite the frustration of not getting a DSC call answered, I am absolutely thrilled with the new AIS radio. I get really clear data on what boats are coming my way (long before I can make visual ID). Often these are boats that are "sneaking up behind me." More importantly, I get very good predictions of how much time I have to get out of their way (the chartplotter program shows me 5 minute vectors for my boat and theirs), and can make decisions with much more confidence that I will avoid a collision.