TDC updating newbie question

[Pisces]

Originally Posted by BigWalleye

For a "normal" ( near 90 degrees) attack, the range does not matter. ...

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.

[BigWalleye]

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.

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.

[Zosete]

Specially to Pisces for that lengthy post. With everyone's suggestions and my in-game observations and rereading the FAQs I think I made my mind up on some things.

1. The only thing that gets auto updated is the AoB. For that, you need to have LOS. Periscope up suffices and lock is not necessary (but recommended).

2. You DO need to switch it back to automatic because the data you enter updates the torpedo bearing. Fundamental.

3. I should have realized before that the trajectorie is just for info. Once you get accurate data you could fire anytime for a perfect hit (of course, that's when the other variables factor in and you'd better choose the perfect split second).

Now I only have one question left. AoB is relative to course rather than ship angle, right? I mean: if i'm trying to catch a ship that it's zigzaging slowly in front of me, his bow swings wildly but the "average" course is relatively unchanged. I still don't get if the TDC is able to update AoB when the target changes its course. I reread your posts but I'm a bit puzzled about this.

[Sailor Steve]

Originally Posted by Zosete

Now I only have one question left. AoB is relative to course rather than ship angle, right? I mean: if i'm trying to catch a ship that it's zigzaging slowly in front of me, his bow swings wildly but the "average" course is relatively unchanged. I still don't get if the TDC is able to update AoB when the target changes its course. I reread your posts but I'm a bit puzzled about this.

AOB is the angle he sees you at. If he's heading directly toward you the AOB is 0. If his lookout would say "Periscope at 20 degrees!" then the AOB is 20.

[BigWalleye]

Originally Posted by Zosete

Specially to Pisces for that lengthy post. With everyone's suggestions and my in-game observations and rereading the FAQs I think I made my mind up on some things.

1. The only thing that gets auto updated is the AoB. For that, you need to have LOS. Periscope up suffices and lock is not necessary (but recommended).

2. You DO need to switch it back to automatic because the data you enter updates the torpedo bearing. Fundamental.

3. I should have realized before that the trajectorie is just for info. Once you get accurate data you could fire anytime for a perfect hit (of course, that's when the other variables factor in and you'd better choose the perfect split second).

Now I only have one question left. AoB is relative to course rather than ship angle, right? I mean: if i'm trying to catch a ship that it's zigzaging slowly in front of me, his bow swings wildly but the "average" course is relatively unchanged. I still don't get if the TDC is able to update AoB when the target changes its course. I reread your posts but I'm a bit puzzled about this.

The TVR updates AoB assuming the target sails straight ahead. If you estimate AoB based on target's base course, then the TVR will not know about zigzags. If you enter a new AoB when the target zigs, then the TVR's calculated deflection will be correct as long as the target remains on that course. Base course is useful to you for planning your approach, but it doesn't figure into the firing solution.

[Pisces]

Originally Posted by Zosete

Specially to Pisces for that lengthy post. With everyone's suggestions and my in-game observations and rereading the FAQs I think I made my mind up on some things.

1. The only thing that gets auto updated is the AoB. For that, you need to have LOS. Periscope up suffices and lock is not necessary (but recommended). ...

Not sure where you got that last part from. The longer you keep the periscope up, the longer anyone around you has the time to detect you. Only keep it up as long as is needed to determine range and AOB. The moment you know what AOB it is, you should unlock the scope to keep the scope at the bearing that it must be synchronised to. And lower it. The target bearing keeps drifting when it passes in front of you. And this happens more quickly as the AOB approaches 90. You don't want the old AOB to be synchronised to a later bearing when you flip the TDC back to auto, because the lock kept dragging it along while you played with the dials.

P.S. One more thing to know about the TDC. If you make a turn, it doesn't correct for this angle change either. The AOB after your turn must be synchronised to the new target bearing. The american TDC in SH4 does this for you when the position keeper is active.

[Pisces]

Originally Posted by BigWalleye

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.

Not confused at all. I am very well familiar with what normal and perpendicular means. Or else I wouldn't have gotten into aeronautical engineering college. I clearly understood you. And therefore I do insist to say that this perpendicular stand-off has no influence in the range dial to become insignificant, or even useless. Any other angle would do the same. It is the fact that you are shooting straight ahead that does this. That's why I said:

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....

The 0 gyro angle makes the range input meaningless.

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.

I agree that I did not use official nomenclature. I meant the parallax to be from the point of view of the torpedo, when it finnished the turn and started heading straight to the impact point. See the image below (not to scale). This parallax is then created due to the periscope to tube offset, straigh run (reach), and the segment of the turning radius which puts it even further aside. I didn't mean to imply that it was a very big correction that the range input makes. The target is still quite big at close range to make up for it (mostly). I just meant to imply that it becomes a factor in those situations when torpedoes need to turn significantly. And as an example, it often appears when shooting late and close by when the AOB is narrowing (going from 90 to 135 quickly, so no broadside shown anymore). There are plenty of reports of this, by both newbies and experienced players. I do agree it will probably hit in most cases. But not where you aimed for if the range was incorrect. And if the target is shorter than 140 meters, as some are, it might just be enough to fail.

[BigWalleye]

Originally Posted by Pisces

Not confused at all. I am very well familiar with what normal and perpendicular means. Or else I wouldn't have gotten into aeronautical engineering college.

Gosh! An aeronautical engineer! How Impressive! No wonder you're so smart!

All I ever did was shoot rockets for a living. Didn't have to be smart. Just had to know how to light the wick!

[Pisces]

Originally Posted by BigWalleye

Gosh! An aeronautical engineer! How Impressive! No wonder you're so smart!

All I ever did was shoot rockets for a living. Didn't have to be smart. Just had to know how to light the wick!

I'm sorry to hear that you feel so small. You probably earned more with it than I did over the years. And I'm not sad about that at all. So can we please be mature again?

[BigWalleye]

Originally Posted by Pisces

I'm sorry to hear that you feel so small. You probably earned more with it than I did over the years. And I'm not sad about that at all. So can we please be mature again?

At my age, I don't have to be mature any more! I have gotten old, but I have no intention of growing up!

Small? How could I not feel small when corresponding with such a clever, brilliant person as yourself? I'll bet you even have advanced degrees, too. Thank you for sharing your knowledge with the rest of us.