SUBSIM Radio Room Forums - Contact Intercept

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- Silent Hunter III (https://www.subsim.com/radioroom/forumdisplay.php?f=182)

- - Contact Intercept (https://www.subsim.com/radioroom/showthread.php?t=198084)

Pisces

08-31-12 08:58 AM

Quote:

Originally Posted by Captain Nemo (Post 1928431)

I find the easiest and fastest way to intercept a contacts course is by doing the following:

1. Draw a line from the contact in the direction that it is heading.
2. Ask the navigator to plot a waypoint that intersects somewhere along the line (track) that you have drawn showing the contacts course.
3. See how long it will take your u-boat to reach that waypoint (intersection) and from this work out how far the contact would have travelled in this time. From this ajust your course (waypoint) accordingly.

Example: Contact is travelling east at 6 knots. At full speed my u-boat will take 2 hours to reach the contacts track. In this time the contact will have travelled just over 22 kilometres. From this decide if you need to move your waypoint along the contacts track closer to the original contacts position or further away to make the intercept.

Hope all that makes sense.

Nemo

Ok, lets say the initial AOB is 90 degrees port. (target goes from your right to left. And your fastest speed is 16 kts. ( so the initial distance to the track is 59 km in 2 hours)

After 2 hours the contact moved 22 kilometers along it's track.
This makes your distance to the new projected meeting point 63 km. (it's 22 degrees to the left)
But this will take longer: 2 hours 8 minutes.
And so, in that time the target moves 23.7 km.
This again moves the projected meeting point even further to the left (23.6 degrees)
And so your distance to this new meeting point grows slightly to 63.6 km
It would take you now a minute more to reach that.
And so on,
and so on. But let's consider this minute close enough for now

In other situations you might have to do more repeated calculations, as your distance to the projected meeting point creeps further away.

So, how do you consider repeatedly calculating the distance that the target moves in the same time it takes you to do it "the easiest and fastest"?

To each his own method, of course. But I think those words better apply to the graphical method as described. Where no calculation is needed.

sixcoins

08-31-12 09:27 AM

Thanks so very much, to all of you. This definitely clears it all up for me... And now I can practice instead of pulling my hair out of my head.

Six.

Pisces

08-31-12 09:59 AM

Quote:

Originally Posted by sixcoins (Post 1928375)

...
Small Edit..... I DID scale my circle down until it cut through the line between myself and the contact... and when that happened, the circle ended up cutting through that line in 2 locations. ...

2 intersections is indeed possible if your chosen intercept speed is close to the minimum required. The minimum speed is when the red circle touches the (infinite) black bearing line between you and the target. Another way to draw it is to drag the corner of the yellow protractor along the black line until it is 90 degrees. You have to go faster than that, or you will never meet!

The closer intersection of the red circle and the black line is the slower way to close. The course that results is actually having you trying to get away from him along the bearing line. But his closing outways your moving away. Always use the furthest intersection.

Quote:

...
And the distance of the circle was not any factor of 12. So.... that confused me even more.

That's why I suggest not to multiply with odd-lot numbers, 2,3,4,5,6,7... . Stick to 1, or 10. And extend that bearing line if needed. No brain-burning by multiplying is required.

Maceaciadh

08-31-12 10:57 AM

Quote:

Originally Posted by Pisces (Post 1928501)

this is by far the easiest and fastest method out there(i use it). perhaps you have misunderstood what he meant by it. you draw a line of the contacts course and if you want mark crosses every 16KM(ship at 8Kts) to help visualise it(just double the speed in knots gives you a slight overestimation in KMpH) then as stated set course to an appropriate point on that line. you move your waypoint along the line and the navigator gives you a time to that waypoint. adjust accordingly and you are there. no fuss, no hassle, no time wasted drawing stuff (unless you like that way). after all the targets heading is an estimation so why solve it properly something that is wrong?

a bit hard to explain without pictures but really there are 2 bits of math
1)doubling ships speed from Kt to KMpH
2)roughly guessing where your intercept is.

FYI after the target travels 102KM it has a possibility of being beyond hydrophone range from the bearing inaccuracy (20KM radius)

after a while you develop a seamans eye and can drag it almost perfectly to an intercept

PS if in heavy fog may be better to do that whole circle business to save diving and listening alot but you can still be wrong over long distances so be careful

Pisces

08-31-12 12:06 PM

Quote:

Originally Posted by Maceaciadh (Post 1928546)

Ok, so this method is more guestimating than calculating. But still, it requires multiple checks (at hour marks) to see if you got there already.

Quote:

after all the targets heading is an estimation so why solve it properly something that is wrong?

a bit hard to explain without pictures but really there are 2 bits of math
1)doubling ships speed from Kt to KMpH
2)roughly guessing where your intercept is.

...

Because you want to get as close as possibl. Especially with fog and long range. Adding causes for error deliberately (like overestimating that 16 km equals 8 nautical mile) doesn't help with that. You want to be as exact as possible. Sure, the target course is uncertain. But that applies to both methods. Yours won't handle it any better than mine.

Quote:

FYI after the target travels 102KM it has a possibility of being beyond hydrophone range from the bearing inaccuracy (20KM radius)

Then it's a good thing that the hydrophone actually works upto 33km. It's the crew that is either too deaf, or to lazy to report what you can hear yourself. So you have until 170 km.

Quote:

after a while you develop a seamans eye and can drag it almost perfectly to an intercept

Ok, that is true. Trial and error makes those braincells learn to do it intuitively. But you'll also get the hang of making that drawing after a while. You'll do it blindfolded. Ok, not litterally. :)

Quote:

PS if in heavy fog may be better to do that whole circle business to save diving and listening alot but you can still be wrong over long distances so be careful

Yes, it all boils down to how sure you can be of the target speed and course. But also how well you can keep your own speed (and course). You always have to consider the possibilities that it's going to fail. That's why I try to reduce as much causes.

Maceaciadh

08-31-12 12:34 PM

what hour checks are you on about? it takes me 20 seconds to set course to the intercept. i miss maybe 4 out of 5 times which is probably just changing of course.

also yea i read that on another thread (guessing you read it) i am going to start listening myself soon

Pisces

08-31-12 12:46 PM

To add:

There is a fairly simple addition to the method to end up at a certain minimal distance away from the contact at the meeting point. You won't be able to figure out that easily with that alternate method.

But it would be considered more advanced. So if Sixcoins doesn't want to get overwhelmed he should cover his eyes now:

[Edit] This does require fairly reliable speed and course of the target. Most likely use is after first contact, and dodging around units in the dark.

1: You would replace the point of the contact location with a circle of minimum safe distance.

2: Then you would choose, "do I want to pass behind or in front of him?". In the image below you want to pass ahead of him.

3: draw the bearing line from the contact towards you along the edge of the minimum distance circle. Then continue from that.

http://img228.imageshack.us/img228/4...ceintercep.png

Maceaciadh

08-31-12 01:11 PM

still confused why you are putting some things forward there but if you like to draw stuff with your time who am i to argue lmao surely once you get within visual range you delete your first line and start taking more accurate visual measures or indeed hydrophone ones to find out speed and bearing.

also forgot to say on the last post but the 7% over error by just doubling makes for a good early arrival time. if you want to trail them like a convoy then just wait while they pass and take notes of all the good high tonnage ships within it. arriving early is always good even if you want to shadow.

Pisces

08-31-12 01:32 PM

Quote:

Originally Posted by Maceaciadh (Post 1928609)

Safety precautions. I want to set a course that gets me closer, but not too close. The dashed line is just shown to see how it compares with the 'straight' intercept way. It is not used.

Quote:

also forgot to say on the last post but the 7% over error by just doubling makes for a good early arrival time. if you want to trail them like a convoy then just wait while they pass and take notes of all the good high tonnage ships within it. arriving early is always good even if you want to shadow.

That works too. But with your way your 'early time' is directly proportional to the distance at the start, or how long the intercept took. So for long distance targets you will arrive more early than closer ones. I don't see the need for that. With this added circle to the method you should pass it a fixed stand-off distance every time.

CaliEs

08-31-12 05:43 PM

Quote:

Originally Posted by Pisces (Post 1928363)

Link to Kylania's image: http://www.kylania.com/sh3/Intercept_Plotting.png

This drawing make only sense when the blue line is measured as 16.0, not 6 as indicated.

Pisces

08-31-12 07:33 PM

Quote:

Originally Posted by CaliEs (Post 1928725)

This drawing make only sense when the blue line is measured as 16.0, not 6 as indicated.

I admit that it doesn't look properly to scale. But the steps are geometrically correct.

TorpX

09-04-12 12:00 AM

I haven't used this method, so I decided to break out a pencil and paper and take a closer look at it. For some reason my brain learns better from graphite on paper, than illuminated pixels on a computer screen. :)

I must admit I like this method, now that I understand it. However, there is a simpler alternative that will suffice in many cases. This is the Normal Approach Course. I'm guessing some here already know what this is, but I thought I'd outline it for those who don't.

The Normal Approach Course gets it's name from the fact that you are taking a course normal (perpendicular) to the target. That is, if the target is on a closing track, the true bearing of the target is 25 deg., and it is drawing off to the South, you would steer 115 deg. true. You don't need to know the distance, exact course, or speed of the target. (This makes it ideal in situations where you have only a sound contact.) By the way the situation develops, you will know if a interception is possible:

Case 1- You are gaining bearing on the target.
That is the true bearing of the target goes from 25 to 24 to 23 deg., etc., etc. You can maintain your speed and reach the target's track ahead of it. You can reduce speed to intercept. Or, you can maintain speed and cut into the target's track to intercept sooner.

Case 2- True bearing of the target is constant.
If you and the target hold course and speed, an interception is assured. (You are on a collision course!)

Case 3- You are losing bearing on the target. That is, the target's bearing is going from 25 to 26 to 27 deg., etc. In this case an interception at this speed is impossible. You must either increase speed, or consider weather it is advisable to continue the approach (it is sometimes possible to get close enough for a torpedo attack even though an "interception" does not take place).

The key element of the Normal Approach Course is that it allows an intercept at a minimum of speed. Another critical element is that it is only relevant for targets on a closing track (moving closer to you). Following a Normal Approach Course on a target that is already moving away from you will avail you nothing. In this case you would be better off to follow the method already explained in this thread.

Pisces

09-04-12 11:33 AM

Quote:

Originally Posted by TorpX (Post 1929971)

I haven't used this method, so I decided to break out a pencil and paper and take a closer look at it. For some reason my brain learns better from graphite on paper, than illuminated pixels on a computer screen. :)

I must admit I like this method, now that I understand it. However, there is a simpler alternative that will suffice in many cases. This is the Normal Approach Course. I'm guessing some here already know what this is, but I thought I'd outline it for those who don't.

...

This is definitely the best solution if you are submerged, and slow. It does make the intercept take quite a while though. You'll make the target do all the closing of the gap. So turning into the target whenever his bearing is changing towards your rear is very advisable to reduce the time. If all you have is a bearing, then this is the simplest way to go. With a periscope view the bearing change is even more easier to see.

Captain Nemo

09-05-12 02:04 AM

Quote:

Originally Posted by Pisces (Post 1928501)

I can see where your coming from in theory but in practice it doesn't work that way. I might make one adjustment to the waypoint that intersects the contacts course once I know how long its going to take me to get there and how far the contact would have travelled in that time (I use a pre-printed table for this), but other than that that's it. I always set the waypoint so I'll get there just before the contact does, about 5-10 Kms. I would say that unless the contact makes a course change between the contact report and the point of intercept it will work 99.9% of the time.

Nemo

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