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Originally Posted by mr darcy
I've put together a little spreadsheet for calulating target speed, does anybody know a way to upload the file if anybody wants it?
All it needs is:-
your current speed (knots)
initial bearing and distance to target (degrees & yards)
the time between sightings (seconds)
second bearing and distance to target (degrees & yards)
It will may need some testing, it's worked okay so far for me. (touchwood)
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Nice, but you should really use one of these instead:
Actually, either the top one or the bottom one. You can find the template for the bottom one at:
http://www.sphere.bc.ca/test/build.html
Here is how it works:
You take observation 1, say, 7300 yards at bearing 220
Then, a few minutes later, observation 2, 6100 yards at bearing 213.
Subtract 213 from 220, giving 7 degrees.
Fiddle with the S scale on the slide against the A scale until the distance between 6100 and 7300 on the A scale matches 7 degrees. In this case, it comes to about 52 degrees matching with 6100 (actually, 6.1 on the scale). That is the AOB at the second observation.
Without moving the slide, move the cursor along the S scale down to the 7 degree mark on the S scale. This will give the distance traveled on the A scale, in this case about 1,480 yards (it will actually read 1.48).
Now, you remembered to time the observations, right? If you are using a variant of the 3 minute 15 second rule, you are golden, as you can convert directly to knots.
However, if you had to go take a dump or whatever between observations, and couldn't do it exactly, don't sweat it.
Lets say that you had to wait for 4 minutes, or 240 seconds, between observations. Align the distance, 1,480 yards on the A scale (again, actually 1.48) with the time in seconds on the B scale (actually, 2.4). We then read the speed in yards per second on the B scale under the 1's on the A scale, in this case 16.2 YPS (actually, it will read 1.62).
To get knots, simply align the middle '1' on the B scale with 5.7 on the A scale. Slide the cursor to the speed in YPS, or 16.2 (1.62) in our example, on the B scale. You can then read the speed in knots on the A scale right under the cursor, or in this example 9.35 knots.
If you are doing your calculations in meters instead, you would use 5.2 on the A scale to give speed in knots.
You now have everything you need, distance, AOB, and speed in knots. You could print up a chart with common YPS/Knot equivalencies if you are lazy.
I know it sounds like a horrendous procedure, but it is easier to do than it is to describe. And, it is authentic: This is how those calculations were done (although probably with a circular slide rule, which are available at the link above. For our purposes, the C and D scales on a circular rule work just as well, but you *MUST* have an S scale on it).