Search patterns
Governing: STCW Code Table A-VI/2-2; IAMSAR Manual Volume III. Builds on: L2.1. See also: Officer of the Watch (Yachts) <3000 GT for bridge-side MOB manoeuvres.
Table A-VI/2-2 requires knowledge of search patterns and environmental factors affecting their execution, and a practical demonstration of carrying out search patterns, taking account of environmental factors. The patterns are those in IAMSAR Volume III, which yachts on international voyages carry.
Datum first
Every search starts from a datum: the best estimate of where the person is now.
- Last known position: from the MOB button on the plotter, a lifebuoy with light and smoke, or a bearing and range.
- Time elapsed since then.
- Drift: a person in the water moves mostly with the current; wind affects them much less than it affects a boat or a raft.
Worked example. Person overboard 40 minutes ago; current 1.5 knots setting east.
| Step | Working | Result |
|---|---|---|
| Time | 40 min | 0.667 h |
| Drift | 1.5 x 0.667 | 1.0 nm east |
Update the datum as time passes, and brief it to the coxswain before launch.
The main patterns
| Pattern | Use | Shape |
|---|---|---|
| Expanding square | Small area, good datum | Square spiral outwards from datum |
| Sector search | Good datum, small target, single unit | Legs through datum at set angles |
| Track line | Target's intended route known | Along and back down the track |
| Parallel sweep | Large area, poor datum | Parallel legs across the area |
Expanding square worked through
Legs grow by one track spacing (S) every two legs: S, S, 2S, 2S, 3S, 3S, and so on, each turn 90 degrees the same way.
Example (illustrative). S = 0.2 nm, boat speed 10 knots.
| Leg | Length | Cumulative |
|---|---|---|
| 1 | 0.2 | 0.2 |
| 2 | 0.2 | 0.4 |
| 3 | 0.4 | 0.8 |
| 4 | 0.4 | 1.2 |
| 5 | 0.6 | 1.8 |
| 6 | 0.6 | 2.4 nm |
Time: 2.4 / 10 = 0.24 h = 14.4 minutes. In the meantime the datum drifts; long searches need restarting from an updated datum.
Environmental factors
- Current: moves the datum.
- Wind and sea: slow the boat, hide a head in the water, set the best approach heading.
- Visibility and light: reduce track spacing at dusk and at night.
- Sun glare: search with the sun behind you where possible.
- Crew fatigue: rotate lookouts.
Coordination
The yacht is your plot. Its bridge tracks the FRB by radar or AIS and tells the coxswain when each leg is complete. If the coastguard takes coordination, the on-scene coordinator may assign you a pattern and area; follow it and report progress.
CAUTION - Look, don't drive In a search the coxswain drives, the bowman looks forward, a third crew member looks to the sides and aft. Speed above what the lookouts can scan is wasted.
TIP - Mark the datum Drop a dan buoy, lifebuoy or smoke float at datum when you arrive. It gives you a fixed reference drifting with the same current as the casualty.