Can't hit a sitting duck?

[bsalyers]

I have had this happen on multiple occasions and would appreciate any light shed by fellow kapitans:

I have disabled a cargo ship and she's sitting there at zero knots, dead in the water, wallowing like a beached whale in front of my bow tubes. I click the "X" on the targeting notepad to clear all info from my previous attack so that all fields revert to dashes. Now, as I read the manual, it seems to me to say that when all TDC info is cleared, the optics work as point-and-shoot aiming devices; nevertheless, I can aim my UZO directly at the stationery target and watch as the launched torepdo whizzes off well in front her at some odd angle my crosshairs weren't even near. What could possibly cause this? Despite what the manual says, do I need to enter target data for a coup-de-grace shot on a motionless target directly in front of my tubes?