Having worked aboard merchant vessels as well as being a yacht sailor myself, I’ve gained invaluable insights into the visibility challenges posed by small vessels and the critical importance of timely detection. Often, especially at night, small vessels without AIS are detected visually well before they appear discriminable on radar.

Factors such as fog, heavy traffic, radar clutter from land and sea can all significantly diminish both visual and radar detection ranges. Equipping vessels with an AIS (Automatic Identification System) transceiver can alleviate these limitations, enhancing vessel visibility exponentially, increasing safety and situational awareness.

AIS operates by broadcasting a vessel’s identity (static data), position, course, speed (dynamic data), and additional information such as destination and status (voyage-related data), such as sail or power, restricted in ability to manoeuvre and so on. The static, dynamic and voyage data is broadcast via VHF, AIS signals are detectable to anyone equipped with an AIS receiver within VHF Range (typically around 12 nautical miles). AIS targets then become visible and display corresponding data to receivers.

A sailing yacht can be tricky to spot from the bridge of a large ship. Photo: Dick Durham/YM

The image above right shows a radar screen overlaid with a chart. Notice the vessel located at the top left of the screen. It does not have a radar echo but it is visible due to its AIS signal overlaid on the display. The lack of a radar echo in this case is likely due to obstructing buildings and land while our vessel was alongside in port. Additionally, observe the repeated arcs of a SART (Search and Rescue Transponder) being tested along a south-westerly bearing.

It is crucial to acknowledge a vital flaw in AIS and why one should not solely rely on it for collision avoidance. Since AIS transmits information from sensors onboard vessels, inaccurate or faulty sensor data can lead to misinformation transmitted to other vessels, affecting risk of collision calculations.

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At least once a trip I come across an AIS target of a ferry seemingly defying logic, sailing astern at full sea speed. This leaves me puzzled for a few seconds but upon closer inspection, the truth always unveils itself; a simple case of incorrect AIS settings. Someone had forgotten to switch their AIS to the correct setting, causing the AIS target on our screens to show them 180º the wrong way, portraying a misleading scenario.

Understand your kit

This got me pondering my own boat and its equipment. She came equipped with a belt-driven autopilot system. Having never used it before, it took much longer than expected to grasp its operation. I vividly recall holding the control unit in my hands, toggling it from standby to on, only to find her veering off course repeatedly. Until that moment, I had naively assumed this autopilot system utilised the onboard GPS signal.

Radar screen showing AIS ‘sleeping targets’ not visible as radar targets (circled in red)

It wasn’t until I gave up in frustration and set the control unit down that it finally clicked; the heading sensor was integrated into the unit and my frantic arm movements holding the control unit were the cause of her erratic zig-zagging. This highlights the importance of understanding how the equipment you use works, its inputs and how regular checks of components is vital.

Although AIS has a lot to be said for it in terms of improving safety of the mariner, it also has its limitations and therefore all available means should be used to appraise a situation and the risk of collision.

These are the distances that your vessel ‘should’ be seen from, according to Resolution MSC.192(79) – Adoption of the revised performance standards for radar equipment

Visibility on Radar

For most small vessels fitted with a radar reflector, radar returns can be spotted without too much of a headache, assuming of course a correctly tuned radar is being used. However, this is not always the case; influencing factors such as hull material, size, swell, weather conditions, traffic density, range scale and proximity to land can all contribute to a small vessel sailing ‘under the radar’.

The photo left was taken of a radar screen approaching a headland to port, (Bardsey Island if memory serves me). At the time, a sailing vessel was spotted visually ahead, but on the radar screen, it’s a game of hide and seek – nothing conspicuous or easily discriminable. It is easy to see in this situation how AIS would benefit safety and improve situational awareness.

Rest assured, there are minimum standards that all radar equipment must meet concerning target detection. The above table is the IMO performance standards for the minimum detection ranges in a clutter free condition.

Rain and waves can make spotting small fibreglass yachts by radar very tricky. Photo: Tom Sanders

Warning time

The IMO performance standards specify a small vessel with a radar reflector should be detectable by radar at five nautical miles (in a clutter-free condition). Consider a cargo vessel travelling at 20 knots and a small vessel travelling at 5 knots on reciprocal courses.

The combined closing speed is 25 knots, allowing the cargo ship 12 minutes to detect, acquire, and take avoiding action if the small vessel has a radar reflector and is detected at 5 nautical miles. For a small vessel without a radar reflector, this detection time reduces to around eight minutes. Furthermore, allowing an additional three minutes for the radar tracking system (ARPA) to provide reliable collision avoidance data once acquired, the effective time available for the cargo ship to take action is approximately 5 minutes.

This is of course theoretical, and the likelihood is the vessel would have been detected well before. It does, however, demonstrate how AIS would emerge as a reliable ally. By continuously broadcasting vital information about a vessel’s identity, position, course and speed, AIS enhances the visibility of the small vessel allowing earlier detection so a close quarters situation can be avoided.

Transmitting AIS data is a huge help to bridge watchkeepers trying to spot your boat. Photo: Dick Durham/YM

The performance standards mentioned above are for a correctly set-up radar operating in clutter-free conditions. However, this scenario represents an ideal world with flat calm conditions and no rain obstructing visibility. In practical use, rain clutter reduction and sea clutter reduction are often employed to mitigate the effects of radar returns from adverse conditions.

Rain clutter reduction helps minimise the interference caused by radar echoes from rain, while sea clutter reduction targets the echoes bouncing off the tops of waves. However, even with minimal clutter reduction applied, small vessels may have their radar echoes diminished or disappear from the radar display entirely.

If you are unsure about your own visibility give a passing ship a call or local VTS. I routinely experience small vessels calling up when in open sea asking about their own radar return strength, AIS, or to just ask about the weather forecast. Most mariners will be happy to provide reassurance and some friendly conversation.

Conclusion

AIS allows small vessels to be considerably more conspicuous at greater ranges, which improves situational awareness for all and reduces the risk of close quarters situations. The benefits of having and using AIS far outweigh the initial costs, but like all things, AIS has limitations and it is important to be aware of these as well as being aware of how your equipment works.

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