[Pisces]

Just to make sure, did you mix T1 and T2 torpedoes in those tubes? If so then it may be due to a game bug. It doesn't update the torpedo speed calculation when you switch between T1 and T2 tubes. So the torpedo goes to the wrong gyro angle. If you do have both T1 and T2 in the tubes make sure you reset the right speed and re-aim before you shoot.

Other than that it might have been made worse by the 3m vs 3m15s confussion. If you want better resolution in the speed then take multiples of 3m15s: 6m30 and divide by 2, 9m45s and divide by 3, 13m and divide by 4. The latter gets you down to a quarter knot preciscion, which is what the user interface allows you to set visually anyway. You can't see it much more precise.

Target speed isn't always constant though. If it is wrestling with the waves then it temporarily slows down, which might just make it dodge the bullet. Remember that the 3m15s rule is an average speed.

Ships can go at any fractional speed that the campaign designer desires to enter in the editor. (the ship's max speed permitting ofcourse) So it's just a question how lazy he was. Don't count on whole numbered knots if you want to shoot accurate. Nature and physics doesn't count that way either in the real world.