[jason210]

Or use your iPhone stopwatch :-)

As the target ship can be identified and thus its length known, then what you're doing is measuring the time it takes for a point on the ship to pass its own known distance. Then you have all the information you need to calculate its speed. What's neat about that method is you don't need to know the range, and it doesn't matter what bearing the ship is at either. The formula can be summarised as:

(ActualShipLength/TimeTaken) x 2

where the 2 is a rounded decimal that converts the metres per second to knots. Nice easy calculation.

The ship's bearing, which matters for other calculations, is more complicated to measure, because as the relative angle gets away 90 degrees, the width of the ship hides the stem or the stern, making it harder to measure the ship's apparent length. Once you know the apparent length, there's a formula you can apply but it requires tables, a whizz wheel or a calculator.

I've been working on a simple whizz wheel that does all these calculations, and which is easy and intuitive to use, and which can be printed out and assembled quite easily.

When it's done I'm going to make it available to whoever wants it. It's rather nice, I have found, to actually have items in your hand to refer to!