The Stand. Simple, reliable torpedo attack

[nataraj]

The Stand
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Hunters use them to wait for deer.

Yesterday, on my third patrol (ever), I had a few hours (radio message at 1040, reached position 1730) while shadowing and overtaking a convoy. I used that time to figure out where exactly to lie in wait. I did some simple calculations and found out a few facts (see .PDF with graphic):

fact 1: the solution to the gyroangle problem (what the TDC does) is independent of "Range" (but the effect of errors in the estimated target speed increases with range)

fact 2: as long as the target remains on course with constant speed (no zigzag, no circles), the optimum position, heading and moment of torpedo release can be pre-calculated very easily.

POSITION: in front of the target, slightly (600m-1000m) off target course, at periscope depth.

HEADING: target course +/- 90

WAIT: until the target comes a bit closer, take a peek: ID ship type (is it worth spending a torpedo?) and flag. Retract periscope. Enter the solution into TDC (manual mode), set up torpedoes. Faster is better, because the effect of errors increase with time.

LISTEN: when firing at a right angle, the correct moment of release depends ONLY on the relative speeds of target and torpedo. The target bearing at the moment of "fire" is

BRG = arctan( vTarget / vTorpedo )

This can be done easily on paper, not so easy with SH3 drawing tools. Here is the table:

<pre>
SPEED   (kn)   4      5      6      7      8      10     12
-----------------------------------------------------------
SLOW    30    008    009    011    013    015    018    022
MEDIUM  40    006    007    009    010    011    014    017
                              
SLOW    30    352    351    349    347    345    342    338
MEDIUM  40    354    353    351    350    349    346    343
-----------------------------------------------------------
(aft tube):
                              
SLOW    30    188    189    191    193    195    198    202
MEDIUM  40    186    187    189    190    191    194    197
                              
SLOW    30    172    171    169    167    165    162    158
MEDIUM  40    174    173    171    170    169    166    163

</pre>

"FIRE". When the target reaches the correct bearing, fire. I have not yet found out how to fire a torpedo "straight ahead", so I use F6, manual entry, set range to 800, bearing to the value from the table, AoB is 90-BRG (or BRG-270), should give a gyroangle of 0.

This also works completely submerged - I used the method first against the (only) destroyer escort of my convoy (no need to ID anything, radio had said "enemy convoy" ...)

Example: convoy course 250, I positioned myself with heading 160. Convoy speed 6 kn, I chose slow torpedo (30kn) -> target bearing 349.

Error Analysis:

I let my computer calculate the effects of errors in the estimates (speed, course, bearing etc.) and found out that at a range of 800 m, the distances between torpedo and target point (!) are mostly well within 50m, so even if the target course is a few degrees off, or the speed is <1kn off, the torpedo still hits the target ship.

at 6-8 kn target speed: error in course 25° -> 15m distance
at target speed 8,10,12kn (medium torp): +/-2kn -> 38m distance
target speed estimated 6kn, slow torp: +/- 2 kn -> distance 52m (MISS !!)

Timing: the target moves at 3-5 m/s, so 10 seconds error in firing gives an error of 30-50m ! This is the most crucial part, since near the moment of torpedo release the bearing changes quite fast.
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Sorry if this is common knowledge, I've not yet read all the guides (probably never will, better to find out some things by myself)

BTW, does anybody know exactly what bearing is reported by the sound guy: bearing to the center of the ship or to the propeller (should be propeller, but I should have had more misses or hits near the stern if that were the case). Hm

Helmut

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edit Nov.24: Last night, I had ample opportunity to test the method outlined above. Two key findings:
(1) The hydrophone operator takes about 25 seconds to complete one sweep and update the position. That is too long for good aim. But counting seconds and firing at the right moment in between works perfectly.

(2) When I had only aft torpedoes left, I found out that - at least with the KDB installed - I could not get sound bearings in the required range (170-190). went to periscope depth and used the observation periscope in a second attempt ...

[Canovaro]

nataraj,

works great, this is the way i sink 'em most of the time. it is especially useful in convoys. (with singles, i use multiple observations for estimating course and speed and a special table for estimating speed with the hydrophone as a second opinion)

I made a sheet for this some time back, it has some more data that you may be interested in;

http://opspin.net/operationspinnennetz/

look under Downloads. Torpedo bearing table.

btw there is also another sheet around which has calculations for other degree shots as well (45 degree and others). You may have to search for this a bit though.

[Canovaro]

Originally Posted by nataraj

BTW, does anybody know exactly what bearing is reported by the sound guy: bearing to the center of the ship or to the propeller (should be propeller, but I should have had more misses or hits near the stern if that were the case). Hm

The engine room would make more sense, because that's where the sound comes from!
In game, I;m not sure though, has some strange readings with these.