Trying to attack in heavy fog is imho a waste of time and torpedoes. Besides this, you run the risk of running at high TC into some destroyers that your watch crew will spot when the first shells fall on the conning tower, which is very late to react...
My tactics in heavy fog are go submerged and surface for 2 hours every 12 hours for ventilating and recharging batteries, and wait for the fog to lift. It usually doen't last very long (am using SH3 weather)
Heavy fog is a defense system invented by the Brits like cold and snow in Russia.
