Yup the speed is what killed me in the beginning.....
However what I do now is kinda 2 part. Basically I follow Neal's advice by reviewing the recognition manual and dividing it in half based on the total knots that the vessel can actually go.
Next I place the periscope or TBT crosshairs in front of the vessel by the water line until the vessel passes into the T of the crosshairs. Then I click on the stopwatch or chronometer. When the vessel clears the T then I click on the stop. Based on the seconds I have an estimate if I believe it is going fast, medium, or slow.
I follow this logic and most of the time the bad guy is at the bottom swimming with Davey Jones.