[Halifax40]

Hmmm, I find this very interesting as I don't see too many people actually using the hydrophone to calculate speed. The hydrophone and counting the revolutions is probably the most accurate approach in my opinion. I use the 3.15m rule as well as double checking with a nomograph, and then triple checking with the hydrophone. Sometimes, you can get accurate speed calculations using the hydrophone in 15 seconds or less, which is the method I use first, and then check that number with the other 2.

You definitely should check out the Sound Trainer Mod hosted here, on Subsim. It includes a single mission for its training purpose but I found that using the chart and reading the instructions was enough. I jumped right into the Naval Academy and did the Torpedo mission on training - and instantly got accurate speed readings. It also helps if you have a sense of rhythm. I was in the marching band and jazz bands way back in high chool and am still very heavily into music - so it's really easy for me. A 4 bladed prop will sound very much like a 4/4 measure. 1, 2, 3, 4, 1, 2, 3, 4. All you need to do is count how many times you hear the "1" beat (how many times the prop has made a complete revolution) in 15 seconds, and multiply that by 4 and you get how many RPMs. Then just use the chart included the Sound Trainer mod for the specific ship and ta-da.. you've got an accurate speed estimation.