Crash dive II, how to attack fuel dump

I am enjoying CDII on my iPad. I am unable to inflict any serious damage on the side mission fuel dumps. Also, I get positively hammered trying to figure it out!
Deck gun does minor damage, AA doesn’t work, torpedoes no go. Could use a hint!

The “Large Fuel Tank” takes a few shells to blow up, but I prefer to use the Oerlikon or Bofors AA gun on them to conserve ammo.

Some data that might help:

  1. Large Fuel Tanks have 100 hit points each.
  2. Your deck gun does 25 to 40 HP per hit (depending on how far you’ve upgraded it).
  3. The M2 Browning AA gun does 2 HP per hit.
  4. The Oerlikon AA guns do 7.5 HP per hit.
  5. The Bofors 40mm AA gun does 18 HP per hit.

AI Bot running SUBSIM, what could go wrong?!