Sorry Xavier ,, This is the " Lionfish " mission / campaign isn't it ?
There are three Oilers in the convoy , sink them , you win .
Rise to periscope depth . ( Now remember you can pause the game , so choose , either to take another look on tactical , whilst paused , and close to periscope depth , or , take a quick look thru 'scope , and then see what is on your tactical . - pausing game then , to give you some time to take stock of positions etc ) You don't need to use pause , but maybe play it that way first .- Because you are so long at that mission - you will exhaust our charges ! Now , When you look you will see where the three oilers are . When you begin , and at depth , they do not show .
Now you can sink them . Use your stern tubes as well . Conserve torpedoes . Dive to avoid collisions , to about 25 minimum . Fire from 30ish ,, under merchants , at an Oiler behind ! Do that if they are in line , and the merchant is in the way . Or : just sink it anyway , if you can sink the Oiler too , but you might need that torpedo , if another Oiler gives a chance to sink it . Any warships .. if they look like they are getting very close .. Firing just ONE torpedo towards will deflect them . Usually . Conserve torpedoes for the three Oilers .
As you rise :: Pivot . You can at the one third speed - which will be normal , unless , and until you say " Ahead flank " etc ... Usually pivotting at the third speed , as you rise in this mission , gives you enough time to point in direction of the convoy. It is best , to point about 270 degrees. That is , just to the North side of the convoy , probably One Oiler will slide around to your North , from your starboard , quarter . Use stern tube . Use two even ! Make sure THAT one is gone ! Use three ! Now . Just go get those other TWO . !