Wazoo's tutorial (
http://www.paulwasserman.net/SHIII/) has a section on using the nomograph at the end.
In summary:
- On the SH3 map, plot the position of your target: for example, by asking the watch officer for the position of the visual contact and then drawing a line from your sub to the contact's bearing/distance. Note the time.
- A few minutes later (I usually wait about 10 min but you can use whatever you want depending on the situation), do the same. Again, note the time.
- Draw a line between the two fixes, this is the route followed by your contact.
- Measure the distance between the fixes (with SH3's line tool).
- Calculate the time elapsed between the two fixes (difference of the times you noted).
- On the nomograph, with a ruler draw an (imaginary) line passing through the measured distance and the calculated time.
- Look at the point where the ruler crosses the 'speed' scale, you can read the speed of your contact!
The good thing is that using the nomograph for the calculation takes less than 5 seconds! (much quicker IMHO than using a table, a calculator or my mental capabilities!)