Where am I going wrong?

[Turbulent]

Apologies if this has all been asked before (I am sure it has) but after trawling through the four pages of this sub-forum and perusing the rather excellent tutorials abound on here, I am still a bit stuck in my game play. I should start at the beginning:

I only started playing Silent Hunter 3 (vanilla - no mods applied) about a week ago (I bought it aaaages ago and did the tutorial, but then my studies got in the way ) and decided that although it seemed like hubris, 100% realism from the off was the way to go for me. The big attraction of the game is getting that realistic experience, and this seems like the best way to go about it. Now, after said week and a half, I have gotten to the point where I am competent when it comes to tracking ships and convoys. I can line myself up for shots rather well too if I may say so myself, frequently getting about 1-2 kms in front of a ship and at right angles to its path. So far so good! However, then the attack begins...

I just can't seem to hit anything. I range the ship by putting the bottom line of my periscope on the waterline and the top line on top of the masts, I estimate the angle on the bow and I take a couple of speed checks until I am satisfied. I wait for the gyro angle to be close to zero, open the tubes and fire, but the torpedo seems to aim for where the ship is at that moment and not where the ship will be. Can anybody please shed light on this frustrating issue? BdU must be getting sick of me; I have sunk some shipping using the deck gun but I am expending torpedo after torpedo for no success!

Thanks for any replies