Search Patterns for Sailors Page 2
You are immediately aware of the MOB event
Your first response should be to throw a flotation device overboard and hit the MOB function on your GPS unit. On most GPS units this function immediately records your latitude and longitude and automatically plots a course back to that position as your location changes. It is extremely helpful if the flotation device includes a self-activating light, as this will provide you with a visual reference and serve as a “place to go to” for the person in the water.
Because you saw the crew go overboard and responded rapidly, your GPS gives you an excellent approximate datum. Turn the boat around as rapidly as possible and return to datum. If you spot the person, retrieve him. If you can’t immediately locate him, you may need to start a search pattern.
The appropriate pattern for this scenario is known as a sector search (Figure 1). To execute a sector search, point the boat away from the direction of the prevailing offsetting force. For example, if there is more current than wind, and the current is from the north, you should point your boat south. Travel 1/10 mile and make a 120 degree turn to starboard, travel another 1/10 mile and make another 120 degree turn to starboard, now travel 2/10 mile and make a 120 degree turn to starboard, proceed for another 1/10 mile, and so on. If you execute the pattern properly, you will keep returning to the center of the pattern.
There is a delay before you become aware of the MOB event
Hit the MOB function on your GPS, deploy a man overboard pole or other marker, and return to the marked spot. In this case, the GPS MOB waypoint is only a crude approximation of datum. This calls for what is known as an expanding square search pattern (Figure 2).
Begin by turning your boat away from the prevailing offsetting force (be it wind or current) and travel 1/10 mile, then make a 90 degree turn to starboard and travel another 1/10 mile, make a 90 degree turn, travel 2/10 mile, make a turn, and so on. Note how this search pattern spreads out from the center, which is appropriate because you have only an approximate idea of where datum is.
Other Factors to Consider
When you’re planning and conducting a search it’s important to factor in how far and in which direction the person or object may have drifted. A good way to do this is to offset the search pattern to match the MOB’s drift. This is easily accomplished when running either of the above patterns by using your in-water speed/log rather than your GPS to measure the distance of the search-pattern legs. To avoid confusion, reset your log to zero as you begin the search. Because you are measuring distances traveled through the water, you are effectively ignoring current and thus your boat (and your search pattern) will be offset by the same amount as the person in the water. If, on the other hand, you measure distances with your GPS, the focal point of your search pattern will remain geographically fixed—which is not what is happening to the person in the water.
Some legs of your search pattern may be nearly dead upwind or nearly dead downwind, so you’ll almost certainly need to execute the search pattern under power. However, you may wish to keep a small bit of sail up and motorsail to keep your boat more stable.
Professional SAR teams often run their searches at a speed of 12 knots. This allows a lot of ground to be covered, but is not so fast that rescuers will fail to notice objects of interest. It’s a rare sailboat that can cruise at that speed; 6-8 knots is more realistic.
You’ll be extremely busy while conducting a search, but it is critical to not over-concentrate and lose track of whatever else may be going on in the area. If, for example, you’re searching near the coast, keep an eye on the chart to make sure you’re not about to run aground.
The scenarios described above assume you are searching for a person in the water. If you are looking for a small boat or a liferaft, you can increase your track spacing and leg lengths, because a small boat is much easier to see. In “good conditions,” with winds under 14 knots and seas less than 3 feet, multiply your leg lengths by 5. In poor-visibility conditions, multiply by 2.
Hopefully, you’ll never have the experience of losing a person (or anything else) overboard. Should that ever happen, you'll be glad you know how to execute a basic search pattern.