Of course the Nomograph is a good tool, but my favorite way to calculate speed is to use a calculator to get precise measurements. For example, a ship traveling 1.3 NM's in 5 minutes:
1.- 60 (minutes always start here) divided by 5 (Travel time) = 12
2.- 12 X 1.3 (distance) = 15.6 (Speed)
So you have this formula: 60 divided by (time) X (distance) = speed
A little thing to keep in mind is, if you use 6 minutes as your time measurement, you don't need the calculator, just move the distance measurement a decimal point to get the speed. Example: A boat traveling .5 miles in 6 minutes will get you 5 knots speed. 60 divided by 6 = 10.. ..10 X .5 = 5 So simple even a caveman can do it
This works well for precision. Some will say "Ah, just round it to the nearest even number". I say, "hell no!" ... I want my speed calculated to at least the nearest 1/4 nautical mile per hour. Many times that can be the difference between a miss or hit. If a boat is moving a constant speed and course and I have ample time, I always hit exactly where I'm aiming using this technique. It becomes extremely accurate when using the Dick O'Kane method of targeting. If you don't know about the "O'Kane" method, search these forums, you won't regret it. Some time back Rockin Robbins had a video tutorial on this method I'd strongly recommend viewing.