Had a similar experience while enroute to Martha's Vineyard years ago, during a crossing of the entrance to Buzzard's Bay. Fog was very thick, there was a light drizzle and no wind, so we were motorsailing from the pilothouse where our CRT
radar screen is located. We picked up an image about 30 degrees and, we guessed, a couple miles off the port bow.
I reduced throttle, until now hearing the vessel's engine, when the barely visible, ghostly image of a large tug suddenly appeared from the fog about 200 ft - dead ahead. I quickly threw the engine in reverse and nearly messed my shorts . . . had no idea it was that close, since we were still learning to read the new-to-us
radar.
The
radar looked clear, aside from some clutter and just as I began to procede, a thick steel cable lunged up from the ocean's surface, about 150 feet ahead of us. Quickly backed down hard just as the bow of a wide barge appeared to port. It was obviously carrying a full load, as evidenced by the low freeboard but for some reason, wasn't picked up on the screen until directly in front of us. I later learned that tow cables can be over a thousand feet in length.
Luckily our 90 hp diesel and 22" 3-blade prop, enabled a relatively quick stop and reverse. It was still way too close for comfort. I still dread the thought of what could have happened, if just a few more seconds had transpired.