My suggestion - forget about him and concentrate on sinking the merchant ships first.
The Naval Academy training convoy has two escorts, a Destroyer leading and an ASW trawler following. Sinking the DD is a good idea, that takes out your most capable opponent. The ASW trawler does not have active SONAR/ASDIC, he only has passive sonar. You will be detected at the start by the tracks of the torpedoes you've fired. And, if you leave the periscope up for "sightseeing" they will all see it...make short observations and use slow speed; it's your engine/propeller noise that the Trawler will use to accurately locate you and attack.
Start your attack the same way you've been doing, sink the DD with one eel and then the C-2 freighter with a 2-torpedo salvo. Stay at periscope depth and use minimum speed to position to take out the first of the two tramp steamers with the stern tube and the tanker with a forward tube shot. Then the trick is to maneuver as necessary to line up for a bow tube shot at the second small steamer; that's when it gets interesting, playing cat & mouse with the Trawler while lining up for a good shot at the freighter.
The thing to keep in mind with the ASW Trawler is that he can only get an accurate location by hearing you or seeing your periscope. If he's got an accurate position and is making an attack run, use "Flank" with rudder amidships to gain speed and then go to silent running and change course and depth while you're "coasting down".
I've used the last small freighter as "bait" a few times to get the Trawler in a good position...about 600-700m ahead of me (running at silent speed) and torpedoed him. After that, you can reload tubes and surface to "leisurely" pursue the last freighter. I've also used myself as "bait" a few times; after taking out the lead DD, leave the scope up to get the Trawler lined up dead ahead (or astern) of me in an attack run and put a "down the throat" shot into him at 600m. Then the merchant ships are all sitting ducks.
Practice makes perfect...sink 'em all!
