I generally do the same thing, with one exception. As I approach the choke point(the part of the channel where the land on either side is closest), I'll sink one of the torpedo boats or trawllers with the deck gun, then scoot out of there at periscope depth. All the times I've done that, every warship within 50 km descends on my last known position, allowing me to run on the surface once I've gotten far enough away.
They will eventually stop looking for you and go back to whereever they came from, but you can easily evade them if you keep a sharp lookout for them.
Incidently, I wasn't keeping a sharp eye out yesterday(had to run at periscope depth for a while at dinner time) when I came back, I ran the scope up for a quick lookaround, and there was a VW right behind me(too close) on the the same course! I ordered a depth of 17meters(not knowing the actual depth at this point, and not wanting to risk pinging the bottom), and cringed while the sounds of the large ship passed directly overhead(I could swear my sub actually swayed as it entered the ships backwash). I ordered periscope depth(which is excruciatingly slow at silent running speeds) set my torp for fast and to run just below the ship, got the scope up and had the ship targetted and solution acquired before the water had even drained off the scope and nailed that bugger at 400m!
I guess it's kind of hard to dodge a fast torp at point blank range...
