I was stuck in a similar situation recently with a subchaser. I was completely out of torpedoes and trying to make my way back out to open sea from the inlet between Kyushu and Honshu (I know, I know, it was simply inexcusable for me not to have saved a couple of torpedoes for defensive purposes). I was trying to creep by the subchaser, who was just creeping along in front of a deep spot in the shallow bay. I was hoping to get by the ship and get down low, and wait for the next night to finish my escape.

When I was about 2000 meters to his left and thinking I was home clear, somebody must have dropped a wrench or something. Despite silently running at ahead 1/3, I was definitely detected.

Every time the ship closed to within 200 meters or so, I would gun up the engines to flank and zag against the ship's current course. The bottom was just deep enough for periscope and no more, so it was intense. After literally an hour and a half of zagging and flanking, then silent running, I finally made it to where I could tell I could get to the edge of the deep spot within the next couple of depth charge runs. I had minor damage to exterior guns, so I was in pretty good shape. As I heard the ship approach again proving it had obviously already detected me, I gunned the engines again and peeked up the periscope to decide to break left or right.

As the ship began to depth charge a good ways ahead of my position, it turned hard right in what I guess was an attempt to have it's new course arc around and cross my new heading. Well, he caught a piece of his own depth charge, and caught fire. His speed dropped to nothing, and soon he was just a little flaming blob in the dusky light. I actually saw him sink after I had gotten a safe distance away to take a good look. Didn't get credit for it though.

So I guess my advice is patience and perseverence. With proper timing, you can deny a subchaser a good run on you.