Lessons
How long before she reaches your passing distance?
While her bearing stays steady, the time you have to act is the range minus your passing distance, divided by the relative speed — not the whole range, and never your own speed.
The rule
- Take the passing distance off the range first — that is the water you actually have.
- Divide it by the relative speed from the plot: that is how fast the gap is closing.
- This holds while her bearing stays steady — she is closing straight on you.
- Multiply the hours by 60 — minutes are what you act on.
- Your own speed is the closing rate only if the other ship is stopped.
Worked example
| Range of the target now | 10.7 nm |
| Passing distance required by the master's standing orders | 2 nm |
| Relative speed from the plot | 14.5 kn |
- Range to run down = range − passing distance = 10.7 − 2 = 8.7 nm
- Time = 8.7 ÷ relative speed 14.5 = 0.6 h
- 0.6 h × 60 = 36 minutes to act
Common mistake
Dividing the whole range by the relative speed.
That is the time until the range is zero, not until she is at the distance the master told you to keep — the extra minutes it shows are exactly the minutes you had to act in.