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Continuing our hands-on series... Words: Matt KenneY

The patrol launch in Mercer Bay. © D

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POWERBOAT & RIB MAGAZINE 89

the SAR services fit in? What is taken away when they are not there? The simple answer is: a response to your call for help when the unexpected occurs. It is the ability to find you and your crew and retrieve them to a place of safety. The aim of the SAR authorities is to secure and finalise your survival. A life jacket will stop a crewman from drowning should they fall overboard, and the raft will keep the whole crew alive should the vessel disappear below the waves – but without the intervention of a capable craft crewed by mariners or aircrew adept at searching for missing objects at sea, it is likely to be all in vain. So, in the most remote regions of the world, as is the case in South Georgia, where dedicated SAR assets are not available in the same way, living without a professional SAR network and retaining the necessity to avoid

For the majority of small-craft skippers, search and rescue (or SAR) is something carried out by professionals

in response to a call for help. In the UK, simply by making a distress or urgency call on VHF Ch16, dialling 999 on a mobile phone or setting off an EPIRB (Emergency Position-Indicating Radio Beacons), an army of professional, well-equipped and capable rescue assets are available to render assistance, and often with a friendly smile to boot. But how do you fill the void when this vital and resilient SAR framework is not there? The British Antarctic Survey (BAS) crews operating in the Antarctic live with this void every day, and this month I will show you how we prepare for the worst and keep ourselves safe, without the professional SAR safety net to catch us if we fall. Preparing for emergencies and putting in place measures to avoid accidents is second nature to most skippers. Ensuring those on board wear life jackets, investing in an EPIRB, attending a GMDSS (Global Maritime Distress Safety System) short-range radio course or making sure the vessel’s life raft is regularly serviced are all preparatory measures for the possibility of an incident occurring at sea. So where do

danger to life requires the boat coxswains and crew to put in place the framework themselves. When it comes to the crunch, crews operating in the Antarctic and subantarctic regions must look inward for solutions, rather than calling upon the coastguard and rescue services to ensure their safety. So what can be done to fill the gap? Well, again the simple answer is by robustly preventing an incident which requires an SAR mission in the first place – by equipping the vessels and crew to a higher than normal standard, and training them to a high level of competence. If then, despite these efforts, SAR action is called upon (and this is thankfully practically unheard of on BAS stations), replicating

as closely as possible the capabilities of a professional SAR framework is the only answer.

Prevention is betterthan cureEach boat is designed with independent systems in mind. For example, all four support and patrol craft are fitted with twin engines which are independently fuelled with autonomous electrical and ancillary systems and which power independent drives. Navigation systems are also diversified with the use of chart plotters, hand-held GPS units, radar, radio direction-finding equipment, fluxgate and standard magnetic compasses and, of course, well-kept paper charts and plotting instruments.

South Georgia offers some challenging conditions. A crew member lost overboard in this poor visibility could be difficult.

Conditions during the exercise where fairly benign, but still required great concentration to steer and accurate course and maintain a steady speed.

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In order to work out where an

object has drifted to, you must first know where it

started and when.


Communication systems run in the same vein with fixed VHF DSC sets, hand-held radio, and fixed and hand-held satellite communications equipment. All boats carry extensive safety and survival equipment including flares, medical kit, tools, spares, powerful ice lights and searchlights, thermal protective aids and drinking water. The area of operation has been studied closely too, and the boats assessed as to their suitability for each area. Rules are in place which mean that operating areas farther away from base require no less than two boats operating together to travel in convoy to those areas, and the remote destinations at the extreme limits of the operating area require the carriage of extensive backup and survival equipment – in addition to that normally carried aboard. During all boating operations, regular communication schedules are kept with base. These ‘ops normal’ calls pass the vessel’s position, course and speed as well as the coxswains’ intentions

and other information, for example a change in weather or sea conditions. Missed calls can – and will – trigger a rescue effort from base, so forgetting to ‘sched in’ is a cardinal sin.

How to plan a searchat seaDespite very safe working practices, no prudent mariner should ever state that an unforeseen incident at sea is impossible, and in the case of the Antarctic boat crews the lack of the UK’s extensive SAR assets still leaves a conspicuous gap. This is where the formulation of our own SAR crews and a capability to respond to emergencies to our own vessels unaided becomes crucial. We must be able to carry out a quantified and coherent search for missing boats or crew should the need arise. The most difficult and specialised skill to master when attempting a search is the determination of a search area. When the position of the search object is not known, the search team must know where

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The all weather patrol vessels provide a flexible multi-role craft equally capable tending visiting ships as acting as a rescue boat.

The chartplotter can record the

vessel’s track during the search.

FIG 4 - Each leg is steered to a fixed

heading and not by the ground track.

… every skipper should consider his or her understanding of the principles of drifting objects at sea and how maritime searches are conducted …


to go and look. Simple enough, except, of course, that the sea is a vast expanse and nothing will remain stationary at sea unless it is anchored or aground. The principal elements of formulating a search area are familiar to all experienced mariners; they pertain to drift elements, which form part of the day-to-day navigation of all craft at sea. ‘Tidal current’ and ‘leeway’ are familiar phrases to all skilled navigators, but the difficulty in search planning is how to apply these drift elements to objects lost at sea.

The rapid response methodThe simplest method of search area determination is what is known as the rapid response method. This model is used regularly by professional SAR authorities when the time the object has been missing is short, or the likely location of the object is surmised with a high degree of confidence. Let me explain how this is done:

1. Last known positions and drift start positionsIn order to work out where an object has drifted to, you must first know where it started and when. Sometimes this is straightforward if the vessel provides this information, as in the case of a Mayday call. Other times this drift start position or DSP (see ‘Learn the lingo’) is not known with any confidence, therefore it must be worked out. This is where a last known position (or LKP) comes in handy. From the last

known position, the planner must make an accurate surmise of the vessel’s movements between then and the time the incident occurred; in effect, ‘dead reckoning’ the vessel to what shall be determined as the ‘drift start position’. For instance, if an incident is believed to have occurred to one of our vessels on South Georgia due to a missed communication schedule, the position passed in the previous call becomes the last known position, and the course and speed of the vessel can be plotted from there to the supposed time of the incident, providing a DSP. As our vessels always keep 15-minute comms schedules, we know that the incident must have occurred within that 15-minute window, and to allow for the longest possible drift elapsed time and therefore the worst-case scenario, it is assumed the incident occurred immediately after the last received call. This will give us a starting point.

2. Tidal set and drift There is a simple principle which loosely applies to all objects immersed in a body of water: all objects drift with tidal current at the same rate and in

the same direction regardless of size or weight – if the objects are immersed in the same current layer (see ‘Current affairs’). That means that a person in the water floating alongside a drifting ship will drift in the same direction and at the same rate as the ship, even though it is many times larger. This makes it easy to apply tidal current as the planner merely plots the direction of the tidal flow and applies the speed of the current to mark off the distance travelled. The tidal range in South Georgia (even on spring range) is relatively small, thankfully, and the tides are generally innocuous. The effects of the wind are of more interest here.

3. LeewayLeeway is the drift created as a result of the net force applied by the wind to the exposed parts of a drifting object. The stronger the wind, the more surface area exposed and the less volume below the surface will all increase the amount of leeway made. If you consider the shapes and characteristics of objects adrift at sea, they vary greatly. Some are relatively deep-draughted, some have large

wheelhouses, and some have a great deal aloft, for example radar scanners and signal masts. Some are rounded, like life rafts, others offer more windage for’ard than aft, some vice versa. Some, in the case of a person in the water, expose very little above the surface and have the vast majority of their mass submerged and out of the wind. Therefore, each type of object will drift differently, meaning the same simple method of applying the tidal current cannot be used for leeway. The amount of leeway made by a particular object must therefore be determined in order for it to be applied. Search planners will use leeway tables which contain data collected during real-world studies of drifting objects, and the mathematical formulae given will reveal the leeway expected for a given wind speed. For example, according to the leeway tables, the formula for calculating the drift of a person in the water wearing a life jacket and survival suit is: (wind speed (kts) x 0.014) +0.10 kts Put plainly, the formula is stating that this person will be drifting at a rate equal to about 1.4% of the wind speed. In 30 knots of wind, that’s 0.52 knots of leeway or ½ a nautical mile per hour. Conversely, the formula for a shallow-draughted sports boat of around 16 feet in length, with a cuddy cabin forward, gives a leeway rate of 2 knots – or 2 nautical miles travelled over the same 1 hour period. Unfortunately, objects will rarely drift directly downwind. This is due to ‘divergence’, or the tendency for drifting objects to deviate from the downwind track due to variation in surface friction and forces acting on the object (see ‘Learn the lingo’). For the purpose of practicality, divergence is ignored during rapid-response searches.

4. The datum positionSo, by plotting both the tidal current and the leeway, we can determine what is called a ‘datum position’ – in other words the position we expect the drifting object to be by the approximate time the rescue vessel arrives on the scene. But this is a position – not an area. Why can we not just navigate to that position and locate to target? Well, because this plot is far from perfect. For a start,

RIBS

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The RIBs provide a capable fast response craft, but the electronic nav aids available on the patrol boats make them the obvious choice for navigating search patterns.

The expanding square pattern takes shape



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Fig 1

Fig 2

Fig 3

FIG 5a – The SART trace begins to show as a series of faint dashes in the direction of the distressed craft (highlighted)

The elongation of the dashed marks indicate the beacon is very close

… every skipper should consider his or her understanding of the principles of drifting objects at sea

and how maritime searches are conducted …

how confident are we in our drift start position and time? To how many decimal places was the position passed? Was it given in latitude and longitude at all? How accurate was the tidal data we used, and more significantly, how accurate was the leeway formula that we applied? And so the list goes on. In order for a search plan to address these uncertainties we need to build an error radius around the datum position.

5. ErrorsFirstly, the initial position error is applied. Again, tables of data are used to determine this. For example, an error radius of 0.1–0.5nm is typical for a GPS-derived position. Next, a more general ‘drift error factor’ is applied to the total distance the object has drifted. This will make allowance for the other uncertainties listed above. The two error radii are then amalgamated and the total error radius is plotted around the datum position. Lastly, in order to produce an easily navigated search area, a square box is placed tangentially over the error circle. Our drifting object will be located within this area – now all we have to do is search it (see Fig. 1).

6. Searching the boxIn order for the area to be searched thoroughly, a methodical approach must be adopted. In a square search area one of the most effective methods is to start at the middle (the datum position) and work out to the edges in a series of concentric squares, known as an ‘expanding square pattern’ (see Fig. 2).

The advantage of this method is that the search vessel begins at the area of highest theoretical probability of success as they begin at the datum position. It is a relatively easy pattern to navigate if the pattern begins on a northerly initial heading, as all alterations of course are 90 degrees to starboard and follow the cardinal points of the compass – north, east, south and west. Using tables derived from US coastguard studies, it is possible to accurately estimate a maximum detection range for given objects in given sea states and weather conditions. This distance, either side of the rescue vessel, represents what is known as a ‘sweep width’ (see Fig. 3).

If the gap between the successive search legs and the sweep width (known as the ‘track spacing’) is equal, then

Current affairs

Calculating the total water current of a drifting object is a complicated procedure when considering more detailed search plans. There are many different kinds of current to consider, and some are difficult to accurately account for.

tidal current – refers to the movement of water around the world due to the gravitational influences of the Sun and Moon, and to a lesser extent, other heavenly bodies. Its effect is complicated by the Coriolis Force which is caused by the rotation on the earth, and by the nature of the land masses the water flows around and over.

Ocean currents – are distinct from tidal movement, although they are affected by some of the same influences. They are predominantly created by the heating and cooling of seawater, and the mixing of warm and cold bodies of water. There are some well-known oceanic ‘gyres’, for example the Gulf Stream, the agulhas Current, and the antarctic Circumpolar Current. These gyres can travel at a significant rate, and have a profound impact on world weather systems.

Wind-driven current – Given sufficient fetch and sufficient wind strength, the surface layer of water will start to flow due to the friction of the wind acting upon it. This is known as ‘wind-driven current’ and is considered an active influence in search planning after 6–12 hours of a constant wind direction. Surface currents in these conditions may be significantly different, or even contradictory to tidal or ocean currents. Further complications are seen in such currents; for example, the ‘ekman flow’, which are cyclonic rotations of the water up to 10s of metres below the surface

the search pattern is at optimum efficiency, i.e. the visual range of the rescue vessel is filling all the gaps between the search legs and in effect leaving no stone unturned.

Putting it to the testDuring an exercise held a few weeks back, we navigated an expanding square pattern from a fictitious datum position, using a track spacing of 0.2 nautical miles. These parameters were simulating the search for a person in the water who we strongly suspected would be transmitting a 121.5 MHz homing signal using a personal locator beacon. The expanding square pattern is ideal for searching for persons in the water and other objects with low leeway rates, as the pattern is a ‘through the water’ search, meaning the rescue craft will make no attempt to counteract tidal current, in effect cancelling it out altogether – remember the rescue craft and the missing object are subject to the same tidal drift (see Fig. 4). We navigated at a set speed of 10kts and steered magnetic courses, making no allowance for the northerly wind or south-setting tide. We determined how many legs were required to cover the whole search area

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and used the simple method of timing each leg successively to know when to change course. Given the wind was blowing force 6 from the north, with 2m seas running, I was impressed by the accuracy of the pattern steered by Jo Cox, one of the South Georgia government officers, who was doing some on-type patrol boat training with me; however, this is hardly surprising as Jo has joined us from the bridge of the BAS Royal Research Ship (RRS) James Clark Ross, where she holds a master mariner’s ticket and works as the first navigating officer! Homing in on the targetFollowing the completion of the search pattern, we then simulated navigating to the signal emanating from a SART, or Search and Rescue Transponder. These transponders are popular among coastal and offshore boats, and when activated will transmit a powerful signal on the 9GHz (3 CM) X-band radar frequency. Any compatible radar scanner will display the signal, allowing the rescue vessel to navigate to the scene of a distress. Figures 5a to 5d show the signal as it first appeared at a range of about 2 miles, and then progressively the signal strengthened until circles entirely covered the

learn the lingo

Search planning at sea is full of specific terms. Here is a quick guide to the main ones:

Drift Start Position (DSP) – The position which an object was in when it began to drift with the elements (i.e. was no longer counteracting the elements).

Last Known Position (LKP) – The position a drifting object was last known to be located – this might differ significantly from the DSP.

Corrected Sweep Width (WC) – The maximum range at which a given object can expect to be sighted by any chosen means in the present weather, visibility and sea state.

Track Spacing (S) – The distance between each successive leg of a search pattern. The most efficient track spacing is one which equals the corrected sweep width of the object you are searching for.

Datum position – The calculated position a drifting object would be in at a given time if there were no associated errors.

Error Radius – The circle drawn around the datum position which includes the initial position error and drift error factors.

Drift error factor (Def) – A figure applied to the total distance an object has drifted to allow for errors in calculation and other unknown factors. This ranges from 17 to 30% typically.Initial Position Error (Ipe) – The error applied to account for the accuracy of the initial position, be it the drift start position or last known position.

Divergence – The angle in degrees an object might be expected to deviate from a direct downwind drift due to the shape of the underwater section and upper sections. Divergence is ignored for a rapid response search for practicality but its influence is accounted for in the drift error applied.

radar screen – at this point the rescue vessel was in the immediate vicinity of the distressed craft. The beacon made a significant difference to the radar visibility of the accompanying RIB which was transmitting the signal. In conclusion, the art of search planning goes vastly beyond the basic principles discussed here and offers some great challenges to those who cannot call on the professionals for help. However, with regular training and practice, the principles can be acquired and used effectively when absolutely necessary. It is important that in the UK, search planning and the execution of SAR should be left to the professionals, but every skipper should consider his or her understanding of the principles of drifting objects at sea and how maritime searches are conducted – particularly those who venture offshore and require a greater degree of self-reliance. Our crews in South Georgia and the Antarctic continent

will continue to train to be as self-reliant as possible, and to ensure that a full-blown search for a missing person or vessel never becomes a necessity. The virtually non-existent incident rate among BAS small craft reflects the quality of this training and preparation. It could be said, however, that the remoteness of Antarctica and the subantarctic regions of the Southern Ocean is still their prize feature, and their greatest challenge.Matt Kenney

Missed calls can – and will – trigger a rescue effort from base, so forgetting

to ‘sched in’ is a cardinal sin.


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