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Old 01-02-2021, 08:46 AM   #1
BritTorrent
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Default Question about using the U-jagd

Hi all,

I've just watched a tutorial about using the U-jagd chronometer to gauge the speed of a target. It sounds pretty simple, just measure the time it takes for the ship to transit the vertical periscope line and then cross reference the time on the relevant length line.

However, does this only work for certain angles on bow? Are there any conditions where this method won't work?

Thinking about it, if you've got a really shallow or really steep wide angle on bow then surely a ship of given length will take less time to transit your scope than it would with a AoB of 90?

Is the U-jagd only accurate for for AoB of 90? Or is it just a case of being less accurate the further you get from 90?
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Old 01-02-2021, 08:59 AM   #2
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Quote:
Originally Posted by BritTorrent View Post
Hi all,

I've just watched a tutorial about using the U-jagd chronometer to gauge the speed of a target. It sounds pretty simple, just measure the time it takes for the ship to transit the vertical periscope line and then cross reference the time on the relevant length line.

However, does this only work for certain angles on bow? Are there any conditions where this method won't work?

Thinking about it, if you've got a really shallow or really steep wide angle on bow then surely a ship of given length will take less time to transit your scope than it would with a AoB of 90?

Is the U-jagd only accurate for for AoB of 90? Or is it just a case of being less accurate the further you get from 90?
Careful using that "have your scope up for a long time while you determine target speed" method around a convoy with competent escorts.
Having said that you won't find many such escorts in the game.
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Last edited by John Pancoast; 01-02-2021 at 09:19 AM.
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Old 01-02-2021, 09:30 AM   #3
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This is a common misconception. The ship will take the same amount of time to transit the wire at any angle. You can do this with your finger and a pencil at home. Put your fingertip on the table and run a pencil along it at different angles, and you will see that it takes the same amount of time end to end to run along your finger. The only time it would take less time is if the ship were moving sideways.

That said, the larger the AOB, the easier it is for you to recognize where the bow starts and the stern ends.

Also, if you want to do the math yourself, simply take the target length divided by the number of seconds, times 1.94, or 2 if you want to keep it simple.

I echo John’s sentiments about the scope exposure. Gather your data on the surface while overtaking, and if you need to estimate speed while submerged, use this method with care OR simply eyeball the speed. The rate of its bearing change is a good indication of speed, as is bow wave.
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Old 01-02-2021, 09:33 AM   #4
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Quote:
Originally Posted by BritTorrent View Post
Hi all,

I've just watched a tutorial about using the U-jagd chronometer to gauge the speed of a target. It sounds pretty simple, just measure the time it takes for the ship to transit the vertical periscope line and then cross reference the time on the relevant length line.

However, does this only work for certain angles on bow? Are there any conditions where this method won't work?

Thinking about it, if you've got a really shallow or really steep wide angle on bow then surely a ship of given length will take less time to transit your scope than it would with a AoB of 90?

Is the U-jagd only accurate for for AoB of 90? Or is it just a case of being less accurate the further you get from 90?
Best results with U-Jagd to avoid errors, is to use it when your sub is not moving at all.
And best is to have at least between 90° and 45° AOB.

Where it comes to be really difficult with U-Jagd, is to get correct speed of destroyer moving fast or very fast!
Quite impossible with our U-Jagd. You’ll have to visually estimate (as in real)

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Old 01-02-2021, 09:59 AM   #5
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I'll echo derstosstrupp's comments about visually determining speed. With a little bit of practice it's easy to determine speed via shadowing/matching speed, the bow wave and/or the smoke column.
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Old 01-02-2021, 03:51 PM   #6
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Thanks for the answers guys. Really informative!
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Old 01-03-2021, 06:22 PM   #7
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Salute !!!
Here is my method when timing for target speed:


Set my boat course to about 20 Degrees or so to the targets course.

Set my speed to dead slow... (subs cannot realistically hold depth at zero speed.)
Periscope trained at 0 (180 for stern shots).
Only raise the scope for very short period of time, just enough to start the chronometer when the target crosses the wire.

Down scope... pay some attention to the sound man reports as he calls out the target position changes.
Raise the scope quickly and note how far along the ship the wire is... down scope!
When you think the time is getting close, raise the scope just enough to see and stop the chronometer when the wire crosses the stern of the target. Now is a good time to get the AOB. Be quick about it....

Down scope.. Ahead One Third!, Rudder Full Over to get your boat on the attack course. Input the AOB into TDC.

Now do the math or use the calculation wheel to get the target speed and enter it into the TDC.
You did note the target AOB when you did the target stern crossing the wire, didn't you?


The key is to keep observations as short as possible while getting the maximum information from each peek.


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Old 01-03-2021, 09:18 PM   #8
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Default welcome back!

^Andrewsdad! Nice silent run! While you're practicing target speed on those oil tankers south of the Bay Bridge on the commute into SF, here's one I nailed at 0 degree; 400 yard; doing 0 knots!
 
Alright Alright! I'm taking 'liberties'with the truth: it's the USS Liberty Jeremiah O'Brien, D-Day survivor, at the recent devastating Pier 45 waterfront fire getting slightly scorched but escaping relatively undamaged! Her loss would have been real bummer; but for a fast tugline and the fireboat!
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Old 01-04-2021, 09:54 PM   #9
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When crossing the Benicia Bridge or going southbound on I-680 I estimate AOB on the freighters in the mothball fleet. You gotta be quick because the view is short due to trees and terrain also since I am driving it wouldn't do to make a bad course change into another vehicle.


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Old 01-22-2021, 08:25 AM   #10
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Quote:
Originally Posted by BritTorrent View Post
Are there any conditions where this method won't work?

?
Please note that you have to be stationary (SH lets you be stationary - wether you want that unrealistic behaviour or not is up to you)
or
your periscope needs a bearing of 0, so there's no lateral movement from your sub to mess things up.
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Old 01-22-2021, 12:47 PM   #11
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If you are looking along your centerline/0/180 degrees/backwards, then you can be moving forward at speed. Slow or fast shouldn't matter. At any other direction your speed will drag the periscope line sideways and mess up your target speed measurement.

Think of your periscope line as a wall projected through the water. No matter which angle the target passes through that, it still will take the same time. As long as the wall does not move sideways.

I don't know specific periscope models, but at least some used to have a mechanism in there that had a vertical line indicator that turned slaved to the gyro-compass. Within a limited bearing range. So it kept pointing to a specific true bearing. Germans called this 'Feste Linie im Raum.', in English this would translate to something like Fixed Line through space. A simple form of inertial reference, at least bearing-wise. Years ago I learned this from a website of a person working at a German naval museum. But I think I've also read references to this on the www.tvre.org/en site. This would have made it easy for them to measure this with the boat turning around on the waves.

I knew it: http://www.tvre.org/en/stabilized-azimuth-line

Last edited by Pisces; 01-22-2021 at 01:01 PM.
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Old 01-22-2021, 01:06 PM   #12
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Can confirm, the C/2 StaSr attack periscope, as well as the observation periscope had the fixed line. I know at least for the attack scope, the play was 2 1/2° to either side.
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