Calculating Speed Method / Question

[FT2/SS]

This question is mainly for those of you that have been in the submarine community (like the real one). Kinda embarassed to be asking this, seeing as I was once an FT, but if someone could doublecheck or correct my formula it would be awesome. Would provide a really nifty way to rapidly get speed off of all other observation data (its really much more simple than it seems).

Rg = Range
xRS = relative speed across the line of sight
xOS = ownship speed across the line of sight
xT = target speed across the line of sight
OS = Ownship Speed
T = Target Speed
TAoB = Target angle on the bow
DBy = Bearing Rate
K = Magic Constant (1943?) ***not sure if this value is right, pulled it off a google for Ekelund Ranges, think all the conversions are there, but not sure... I always just used 2 anyways...

Best I can remember for calculating for Dby from Rg and xRS is:

DBy = K (xRS / Rg)
(not sure if this is right, but if it is...)

therefor

DBy / K * Rg - xOS = xT

Giving us

(DBy / K * Rg - xOS) / sin(TAoB) = T

I'm not sure if this is right, but I know there's a way to do it that doesn't involve pulling out the ekelund range formula (which irronically, I have memorized). I tried to re-derive it but I seem to have forgotten too much calculus somewhere along the way. And the numbers I'm getting plugging in stuff seems to make sense.

If so there's a REALLY easy way to get speed, as long as you have your sin tables memorized anyways (or a calculator / BRC you can print out online).

Any help is appreciated on checking this.

edited: for clarity... sort of :O

[Torpex752]

Hey FT2! Well when I was contact coordinator, I loved the simple rule of:

1-Speed X 100 = Distance travelled in 3 min.
2-Distance Travelled in 3 min / 100 = speed.

They were the simplest rules I used, even used them in SH1 & 3. Hope this helps. I know that the attack plot/chart needs a few tools in order to make plotting an attack more realistic.

Frank
:cool:

[FT2/SS]

Yeah 3 minute rule is cool, but requires too much fiddling with the plot to be worthwhile.. but I just thought of a cool way to do it with sheer numbers without touching it. Thanks for the tip! :D

Use 3 minute rule to find closing speed, subtract ownship speed inside the line of sight and divide resultant by cos(AoB) to calculate speed.

Its not exact, but its close (since we cannot compute instantaniously).

Sooo...

Speed of Contact = (Difference in range in 3 minutes / 100 +/- cos(OSAoB)OSspeed) / cos(AoB)

or for quick solutions

Speed of Contact = (Difference in range for 1 minute / 33 +/- cos(OSAoB)OSspeed) / cos (AoB)

Sound right?

[Charos]

I think this may be what your after:

http://hnsa.org/doc/attack/#platexvi

Its a derivative of the Sine rule by the looks.


IE: A/Sin(a) = B/Sin(b)

B = A*Sin(b)/Sin(a)


Of course the example is for a collision course IE: No change of AOB.


The whole article from which this plate is from will most likely provide you the remaining data.

[FT2/SS]

That article is interesting, lots of stuff well before my time hehe. But it seems to be in reference to firing multiple torpedo's down a common gyro angle. Lots of old school weapons employment but little of value for TMA purposes there. The modified 3 minute rule works though, and its pretty simple to do, even mentally:

(Difference in range in 3 minutes / 100 +/- cos(OSAoB)OSspeed) / cos(AoB) = Speed

Will explain with an example incase anyone has trouble understanding what I typed, or wants to try it.




His Target
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Your Sub

not to scale :P (excuse the crappy ASCII drawing, just installed vista x64 and have yet to install Photoshop.)

Ok for above the bottom line represents your sub and the direction he's pointing, lets say the contact bears 030 relative, that makes your angle on the bow to him 30 degrees (simple enough, if he was 270 it would also be 30 obviously). Lets say he has an 70 degree angle on the bow (he's up top). Also lets say we've worked out a range of 10000 yards via periscope ranging. Also lets assume ownship's speed is 10 kts.



First take an observation with the stopwatch, then you clock 3 minutes with the stop watch immediately afterwards and take another observation.

Take the difference in range (lets say 1000 yards) and divide by 100 giving you 10 kts.

Now you take that 10 kts and subtract it by the cosine of our AoB and multiply by ownships speed, which is (10 * .8ish) or 8kts, so you have 2 knots remaining. So take those 2 kts and divide them by the cosine of 70 (about 1/3) to get 6kts, which is his speed.

Couple of things to note here though, you are better off taking a rough average of the angle on the bow calls between the first and second observation, as range rate is determined over a broad period of time. Also if your pointing away from him or he is pointing away from you you need to adjust so that it factors your speed out of the equation (your both pointing in the same direction, ie. both pointing towards the top or the bottom of the LOS diagram / TDC display, add instead of subtract). Also this will not work with a contact that has all his speed across the line of sight (Port or Stbd 090), loses accuracy as angle on the bow aproaches P/S 90, and will not work right for most zero bearing rate solutions (just set him to ownship course and speed and shoot on a zero bearing rate contact anyways, you'll hit).

Also if you want to be able to do sines real fast mentally there's an easy trick:

for 0 - 40 degrees
x / 60 = sin(x)

for 40 - 74
(x + 25) / 100 = sin(x)

For anything 75 or greater sin(x) is essentially 1

To find cosines just do sin(90-x)

Anything beyond the tenths value for this purpose is pretty much discardable, firing solutions are just a rough guestimation anyways.

[Torpex752]

I had a simplified set of sin's:

if the AOB was: 25-35 /.5 36-46/.6 47-57/.7

For anyone reading this basically if you have a range on a target thats 2000 yds, and an AOB of 25-35 Port or starboard, you take the range and divide it by .5 which puts the targets track about 1000+/- yards in front/behind you. Then all you need is speed and you are nearly at a good solution for an aft or bow shot.


Frank
:cool:

[Gaspode]

I use the following method to determine target speed when a target is close enough and the aob is nearly 90°:

1. Determine target range
2. Note target bearing
3. Wait 3 * range/100 seconds
4. Calculate bearing change, which is the target's speed
5. If aob in the previous step differed from 90 by about 20 degrees, add 10%
6. If my range was in meters, multiply by 1.1

If bearing change was far under 10 degrees and the aob is still 90 or less, I wait another 3 * range/100 seconds, then continue with step 4 and as step 7, I divide the calculated speed by 2.

That's the simplest method I can think of when I want a good speed estimation.

I have described this method accurately in a different thread previously, even for general aobs. Unfortunately, that post has become a bit lengthy... :lol:
http://www.subsim.com/radioroom/showthread.php?t=108454&page=3

[btaft]

Gaspode,

I like your approach. I will definitely have to give it a go. I have always been a 3 minute rule type person. I will probably still use the 3 minute rule when I have the time, but this should do for the quick "snap" shots. Some of these other methods, while technically correct are either too dependent upon determining an accuratre AOB or are meant for a TMA computer.

Thanks,

BT

[FT2/SS]

My main issue when using bearing rate for anything in this game is that I'm almost constantly manuevering ownship when at a close enough range to generate a bearing rate worth analyzing for (although this is my own damn fault, those damn zigging escorts always seem to want to drive right onto my track hehe), and its too much of a pain to convert everything into true bearing.

Also I've found that a proficient scope operator generally is within 5 - 10 degrees of actual regarding AoB.

Also does close range in this game display as yards or meters, I was under the assumption that it was yards, but you refered to meters?

Also on a side note, anyone found an accurate way to eyeball speed by looking at bow wakes?

[Jab_Bauer]

:huh:
Wow.. too much work for me. I just get real close and blast em. As long as Im shooting straight I can't miss. ;)

[Mav87th]

I think this may be what your after:

http://hnsa.org/doc/attack/#platexvi

Its a derivative of the Sine rule by the looks.


IE: A/Sin(a) = B/Sin(b)

B = A*Sin(b)/Sin(a)


Of course the example is for a collision course IE: No change of AOB.


The whole article from which this plate is from will most likely provide you the remaining data.

This is what Kim Ronhoff's Course Calculator Mark 4B is for. A brilliant wheel to make those calculations in a tight spot.