Plot it's position, range and bearing. Wait 3 minutes 15 seconds and plot it's position again. Draw a line between the two spots. Measure the length of the line by 100 to get speed in knots. So if the ship moved 800m in 3m 15s, it's traveling at 8kts. If it moved 1500m it's moving 15kts.
Pictures of this method here:
http://www.subsim.com/radioroom/show...93&postcount=4