Well I'm not sure of the actual numbers in the video, since I can't watch it right now. But i can tell you how to determine speed.
I will use generic numbers but the formula is always the same.
Lets say you find the position of an enemy ship.
Then you wait 15 minutes and find the position again. Use the ruler to measure the distance between the 2 points. Lets just say that the distance is 2.25 miles.
So we know that the target traveled 2.25 miles (approx 4500 yards) in 15 minutes.
So now we need express that in nautical miles per hour, also known as knots.
15 minutes is 1/4 of an hour. So we multiply 15 X 4 = 60
Now multiply 2.25 X 4 = 9
Target is doing 9 knots.
Lets do another harder one.
Lets say another target travels 2350 yards in 10 minutes.
There are approximately 2000 yards in 1 nautical mile.
So we divide 2350 / 2000 = 1.175 miles.
1.175 miles travelled in 10 minutes.
10 minutes is 1/6 of an hour. So we multiply
1.175 X 6 = 7.05
Target is doing 7 knots.
Hope this helps
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