Is 1-2 big ships per patroll good?

[iven72]

I am using the VIIB and when attacking the larger ships they take somtimes 4 or m ore trrpedos to sink is that average? I do use the targeting computer and GW mod. somtimes even the small ships take 2-3 torpedos. Also have you had problems getting credit for sunk ships? I had one on more than one occasion where its dead in the water 3/4 sunk I come back later after the weather calms down to finish it with the deack gun and its gone.