prize8-5, Exercise 3: Reading "Fl(2) W 15s"

Model answer (check the details):

Fl means flashing; (2) means a group of two flashes; W means white; 15s means that the whole pattern repeats every 15 seconds.

In words: the light shows two white flashes close together, then darkness until 15 seconds have passed since the first flash of the group began, and then the pattern starts again.

Why a navigator needs a watch: many lights show a similar white flash. The only way to tell this light from another is to count the flashes in a group and time the interval between groups. Two flashes every 15 seconds is different from two every 10 seconds or three every 20. With a watch, the navigator can identify the light, look it up in the list of lights, and fix the ship's position on the chart.

Care: chart abbreviations differ slightly between countries and editions, so check the key of the chart in use.