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When the targets bow reaches the 000 mark, start the Stop Watch.
When the targets stern reaches 000, stop the Stop Watch.
(The same Stop Watch that times your torpedoes).
That number will remain on the watch, until you remove it, or fire a torpedo.
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So basically you are saying let the ship pass directly in front of my sub (bearing 000)???? Don't you then miss the shot? Or do I then, now having the speed, have to go race in front of them again for a shot?
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Yes, in such a situation you have figured out the speed, but too late to make use of it.
What Snestorm forgot to mention was that you can turn your uboat towards the target until the bow or tail is right infront of the target. Then with the periscope line at 0 (or 180) you do the time measurement. (now your speed won't influence the result because it's also along the periscope center-line) Afterwards when you're done you can turn back perpendicular to the target track. Depending on how close you are you may not have the time to do this. But you don't need to do this when close to the target track at all. (where his bearing changes quickly) You can do it whenever you see an AOB between 30 and 150 and are quite far away. Like during the time you are trying to get ahead of him. Just temporarily turn towards him.
As for the number crunching. It's pretty 'simple'. Just divide target length in meters, by time in seconds. Then times 2 and you have knots. The accurate m/s-to-knot conversion factor is 1.944, which is 3600 seconds divided by 1852 meters. But you'll appreciate the simplicity of a nice round number I'm sure.
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Are you saying all I have to do is set AOB to 90, lock my TDC at 000 periscope bearing, then turn the periscope until I have a 000 gyroscope reading, and then shoot when the ship crosses the reticule?
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Exactly!! You seem to know how to do the right thing, or know what's what. Speed is the most critical value. As it defines directly the amount of lead. The only situation where speed is not important is when you fire a down-the-throat or up-the-butt shot. You'll have to be ON the target track for that. However, as soon as the torpedo is spotted the target can evade by just a slight coursechange. So not very desirable.
Uncertainty of the other values (AOB and range) can be neutralised or migitated by firing with 0 gyro angle AND shooting perpendicular to the target track. (which means the sub should also be perpendicular to the track) But now uncertainty of the speed has the biggest impact on lead. That is the life of a Uboat Kaleun unfortunately. But luckily the above mentioned speed measurement method is quite accurate. And even in a historical sense. They had a moving line in the scope optics that was synchronised with the gyrocompass. Any slight turn and the line moved against it.