When you have a solution for a contact on your nav map, you then need to do some calculations for the angle you are going to fire your torpedo at. Having a programable calculator is very useful in this regard, as the formula for achieving this firing angle can be somewhat complex. It involves target's course, bearing, speed and range, and it's angle on bow.
Then you start finding the square root of the sum of target's range and speed times bearing squared, divided by the hypotenuse times the length of the perpendicular bisector of the bearing to the target, multiply that by half your speed and divide that all by 360 and then the remainder is your firing angle. Very simple!
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Just kidding, lol.
Go to fire control, click on a contact, assign that to a tube, then fire the weapon.