Intercepting

[rifleman13]

I just saw a convoy course SSW just west of the Bay of Biscay.

When I come to intercept it, I can't see it. When I ordered the crash dive to 70m to check out on the hydrophones, nothing.

What's going on! Are they steaming straight or, just like in real life, zigzagging!

Thnx for any replies!

[Brag]

In GWX convoys change course frequently.

[difool2]

My one experience with being surfaced and the DD's radar detecting me indicated a range of about 16km (at which point I got the message "Enemy is engaging us sir!" Of course by the time he steamed to where I was I was already on a submerged intercept course with his convoy). Sea state may affect that.

[Pisces]

To get a target's course based on (hydro) 'bearings only' look at the following from a guy named Redwine. Because of his language (I think he's from argentina) it may be difficult to follow, but the formula works.

http://rotteufel.iespana.es/htdc_tut...ualtdc_009.htm

I found a sliderule ages ago that works by the same math but handles it in an easy way. Take 3 bearings, with equal time intervals (think long, not 3m15s, bearings move sloooowwww), line up the difference between the bearings of time1 and time3 with time1 and time2 and read off the AOB of the target at time1. BUT this is ONLY the true AOB if you are stationary. If you move (with significant speeds) the angle is NOT the AOB but skewed so-to-say by relative motion. So then it should be called 'Angle On Relative Motion'. Which is very difficult to correct for. (needs two sets of these 3 bearings while the sub is on 2 different courses or speeds and some vector-math for desert) Also, accuracy is very much hampered by the fact that Silenthunter (series... the slideruler was made back in the sh1 days I think) do not supply fine bearings (like with tenths of a degree). Bearings are all given in round degrees which makes the outcome very fuzzy/unprecise. Try it adding halve a degree either side on both bearing differences. You'll see what I mean. Also, you can't tell how fast it is, or the range it is at. But target course is a whole lot allready. You just need the time to wait for the bearing to move enough.

Here's a webcam image showing my poor attempt to make this thing (bad quality):

Let's say I took 3 bearings with 2 equal time intervals inbetween with my hydrophone (but periscope or whatever works just the same) B1=087, B2=081, B3=072, the bearingdifferences (6 and 15 degrees) result in a Target AOB at time of bearing 1 being 26 degrees. Since the target was moving from your rightside to your leftside (passing in front of you) he was 'showing' his left/port bow. Draw the angles out on paper and you can see the target's course.

I don't know who made it so hopefully they won't mind I uploaded it to my homepage, for availability sake. The .zip file contains a .htm file with information on the assembly and use of the tool by the original maker. The drawing for the tool can be print from a (autocad vector format) .dwg file, or an ordinary .gif image.

http://ricojansen.nl/downloads/gizmos.zip

[Pisces]

I updated the first link above. It was sending you to the wrong place on his site. This guy's site is sooo confusing!!! Too bad, because there's great technical knowledge in it.

[Robert Fulton]

Originally Posted by Pisces

To get a target's course based on (hydro) 'bearings only' look at the following from a guy named Redwine. Because of his language (I think he's from argentina) it may be difficult to follow, but the formula works.

http://rotteufel.iespana.es/htdc_tut...ualtdc_009.htm

...

Thanks. This is very useful. Despite knowing hardly any Spanish, I was able to work my way through this with a little help from a dictionary and also found this site with Spanish nautical terms: http://bahiaredonda.com/dictionary/dictionary.php.

Given how important accurate bearing readings are to this calculation, who has tips for accomplishing this? I find that if a target is at between 20km and 30km, I must go to the hydorphone myself. And so far my skill level is not all that precise (maybe I need better headphones ).

As an aside, this forum has immensely increased my enjoyment of SH3. I am a novice still in my 1st career with the stock game.

[Pisces]

Longer time intervals inbetween taking the bearings makes the AOB error reduce alot. But this has the disadvantage of possibly putting you well behind his 90 AOB when you are done. And you also don't know how long he is going to stay in your hydrophone range.

So it's best to start with the smallest amount of bearing drift for the initial interval (for the tool I provide a download link to: take a time until b2-b1 becomes 5 degrees) to have a rough estimate on AOB. Compute how many degrees this rough AOB can grow until it reaches 90 degrees (assuming it is not allready past it). Take that as the bearing change for 2 intervals (B3-B1), and look how many degrees that would be for the 1st one interval (B2-B1), leaving the marker set on the rough AOB. Deduct one or two degrees for good measure from this (B2-B1). And repeat the 3 bearing method by waiting until the bearing has drifted this new (B2-B1) since the very first bearing taken.

Apart from that you can get a slightly better resolution out of your hydrophone. If you listen to the sides where the sounds are barely audible, and average those, you get the bearing to the target. This works alot better than finding the maximum volume. If one of those sides is an even number and the other is an odd number the true bearing is somewhere close to xxx.5. Allways take that halve decimal when you can get it!!!

Originally Posted by Robert Fulton

...(maybe I need better headphones )

The hydro station has a volume knob built-in aswell.


Thirdly, make sure your rudder is amidships, even if you think you have no speed! You don't want to have very slow turning screw up the bearings.

[Robert Fulton]

Originally Posted by Pisces

...
Apart from that you can get a slightly better resolution out of your hydrophone. If you listen to the sides where the sounds are barely audible, and average those, you get the bearing to the target. This works alot better than finding the maximum volume. If one of those sides is an even number and the other is an odd number the true bearing is somewhere close to xxx.5. Allways take that halve decimal when you can get it!!!

The hydro station has a volume knob built-in aswell.

...

Pisces, you provide a treasure trove of valuable information. I had tried the averaging from the outer limits method, but only with modest success. My difficulty is picking up on the precise locations where I lose all contact. I had looked for a volume knob at the hydro station, but did not find it. I am now eager to do so. Is it there in the stock version? (I am away from home for a while, and cannot recheck for myself.)