Quote:
Originally Posted by Pisces
Sorry, a 90 degree set up has nothing to do with range settings becoming meaningless. It is the gyro angle being 0 (or close to it) that makes the range setting become meaning less. If the torpedoes do not have to turn (gyro = 000), then the TDC does not have to account for the parallax angle that develops when the torpedo does need to turn and and sees the target from a different approach angle at the end of the turn.
As an aside: Parallax is the situation where you see a close object at a different direction (different things on the background horizon) than a person standing 10 meters or yards away. And is (part of) the means how your eyes perceive distance at close range. Your eye balls turn towards each other to aim at the object in focus. Your brain does the triangulation.
It might be that in this 90 degree setup you choose, by default, to wait until the target is almost in front of you. And therefore get the gyro-angle to be close to 0. But it isn't the differences in own course and target course that makes the difference in chances of hitting. Try firing at the target when your boat is aligned parallel to the target course and it comes to the bearing of 270 or 90. (which is also what the gyro angle will be close to) As you fire to your port or starboard side, you might get a hit. But it won't be the point on the hull that you aimed for at different distances. You will expect this to happen if the torpedoes move straight ahead out of the tubes. If the torpedoes need to turn, close distance makes it worse.
Ever wondered why torpedoes tend to miss when you fire too late or early? The target is on a bearing far away from dead ahead, and the distance is close. This enhances the correction that the torpedoes need to make for the parallax deviation. If the range is still from an older moment and thus further away, the torpedo is not properly aimed.
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Sorry if I confused you with my use of the word "normal." Apparently you are not familiar with the mathematical use of "normal" meaning "perpendicular to." A normal attack is an attack perpendicular to the target track, specifically to achieve a zero gyro angle.
Your lengthy discussion of "parallax" is perplexing, as parallax is only a small part of the fire control problem. A Typ VII U-boat had an LOA of 67 m, so if the periscope were set at about 2/3 of the length, the parallax correction at a firing bearing of 45 degrees would only be 1.9 degrees. At 2000 m, this reduces still further to a correction of only 0.8 degrees. At its maximum effect, at 90 degrees firing bearing, the parallax correction at 1000 m is only 2.6 degrees. At 1000m, a cargo ship of 140 m LOA subtends an angle of 8 degrees. So the impact of parallax is not going to spoil too many shots. And, since the parallax correction actually decreases with range, if parallax were a critical factor then it would follow that torpedoes should be fired at maximum range to minimize the effect of parallax error. I don't think I have ever heard that advice before.
EDIT:
Reread your post and realized there is further confusion over terminology. What you are calling "parallax" includes offsets due to parallax, but also those due to torpedo reach and torpedo turning radius.
From US Navy Submarine Torpedo Fire Control Manual, SLM-1, dated May, 1950:
121. PARALLAX CORRECTION TORPEDO TUBE:
The angular correction compensating for the longitudinal distance between the muzzle doors and the periscope. See
Plate I, figure 4.
126. REACH:
The initial straight path of the torpedo, measured in yards. Symbol: M. (See
Plate I, figure 3).
136. TORPEDO TURNING RADIUS:
The radius of the circular track, in yards, of the torpedo from the end of the initial straight path to the beginning of the final straight path. Symbol Z. (See
Plate I, figure 3)
While torpedo tube parallax correction is small and can be ignored in practice, the reach and turning radius corrections are, taken together, quite significant.
I do not have historical data for the reach or turning radius of a WW2 German torpedo, but I have measured something more pertinent: the actual reach and turning radius of a Typ G7a as used in SH3. Reach is 90m, turning radius is 98 m. So for a 45 degree gyro angle, the offset is 280 m ontrack plus 30 m crosstrack.