[magic452]

Are you sure the speed is 8 kn. kind of slow for a TF.?
Make sure you are sending speed, AoB and range/bearing to the PK.
I'm sure you are but just a though.

Open the torpedo doors, Q key, early. But at 700m you should still hit something.

At 700m your spread should be about 1.5° between shots.
Try to put one across it's bow, that will keep her from speeding up.

If the speed is 8Kn. try this:
Set MK14 to high speed. contact, 5m.

Set target speed to zero degrees. Send to PK
Set AoB to zero, A zero gyro angle shot. Send to PK
Set bearing to zero Send to PK.
If you are loading from a save all these are set to Zero.
DO NOT turn on the PK.

Aim the periscope to 10° and shoot as the target comes to the cross wire. If you have Mk10 set the scope to 12.5°

One at the bow and than work your way back.
After you shoot lock the scope on the target and you should see the impacts as the target gets to zero on the scope.
If you still miss no doubt your target speed is wrong.

If you are new to the game you might consider turning Map contacts ON for a time to get the feel of how to manual target.
If you have played SH3 the fleetboat TDC works a little differently.

Welcome to the boat mate, hope this helps.
Good luck and good hunting.

Magic