Worked Radar Closing Assessment

A contact bears 045° relative at 6.0 NM. Three minutes later it still bears 045° relative but is at 5.0 NM. The near-constant relative bearing and decreasing range indicate collision risk. The range has closed by 1.0 NM in 3 minutes, which is 20 kn relative closing speed: 1.0 ÷ 0.05 hours = 20 kn. If that steady relative motion continued, 5.0 NM at 20 kn gives about 15 minutes to zero range.

That result is a screening calculation, not a licence to wait fifteen minutes. Bearing accuracy, own manoeuvres, target manoeuvres, sea clutter, target swap, heading error, and the short observation interval all affect the estimate. Continue a systematic plot at suitable intervals, compare relative vectors and trails, and use an appropriate range scale. A small apparent CPA created by noisy bearings should be treated conservatively until the trend is clear.

Before manoeuvring, identify whether the vessels are in sight and which COLREG rule set applies. Make an early, substantial action that produces a readily apparent change and leaves a safe passing distance, then monitor until finally past and clear. After the alteration, do not use the old CPA/TCPA as proof of success: allow the tracker to settle, observe the new relative track, and confirm that range at CPA is increasing as intended.

A Yachtmaster oral answer should state the evidence and the limitations: ‘Constant relative bearing, range reduced one mile in three minutes, about twenty knots closing, nominal zero-range time fifteen minutes if neither vessel manoeuvres. I would take early Rule-compliant action now, brief the helm and lookout, and verify the new CPA by repeated radar and visual observations.’