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AIS versus radar

Jan 13, 2016
The ICOM MA-500TR.

The ICOM MA-500TR.

The current overlap of radar and automatic identification system (AIS) technology leaves sailors on small oceangoing craft in a quandary: Which is the more sensible system in terms of initial cash outlay, amperage use and overall service in ensuring safety and security?

Just as Loran-C and radio direction finders were long ago relegated to the forgotten graveyards of maritime detritus, so too has the short-lived CARD (collision avoidance radar detector) system given way to contemporary AIS technology. However, even with the convenience of AIS, radar is still regarded as the standard for dependable vessel tracking capability and collision avoidance. On the downside, radar uses significantly more amperage than the receive-only AIS systems currently available, which on a large yacht is still a negligible amount easily covered by a combination of the engine alternator and a generator set. AIS offers some — not all — of the advantages of radar at a fraction of the price, yet with much lower power consumption. Large yachts generally employ both radar and AIS in tandem for maximum protection.

On small coastal and offshore cruising craft, where every amp-hour of battery charge counts, power consumption can be a decisive factor in whether to install radar, AIS or both. A typical radar array, such as the Garmin GMR 18 xHD, consumes 3 amps and a modest chartplotter like the Garmin 7000-series GPSMAP consumes an extra amp for a total of 4 amps, or 96 amp-hours per day — more than half the total capacity of an 8D dual-purpose, flooded deep-cycle battery. Few sailing vessels under 50 feet LOA can afford such an extravagance 24 hours a day.

On the other hand, the Garmin VHF 300 AIS radio uses roughly one-third of an amp on standby, equating to a negligible 8 amp-hours per day. The integrated AIS receiver allows you to identify oceangoing cargo vessels, assuming they are also AIS-equipped. The ICOM MA-500TR AIS transceiver functions as a transponder, automatically notifying other vessels of your position. Its screen, similar to that of a radar, provides a simple dot-matrix visual display of other vessels in your area. The unit consume 1.5 amps on transmit and 0.7 amps on receive.

Of course, on a crossing from Christmas Island to Réunion in the southern Indian Ocean, there is no guarantee that large commercial vessels will be AIS compliant. By the same token, there is no guarantee they will bother to monitor the VHF radio. But we can safely assume that most commercial vessels on ocean crossings will have AIS and radar running around the clock, and also will have a radio operator on duty at all times. Even if you cannot see them, they can see you. An 800-foot containership has a lot at stake, so a few thousand dollars’ worth of electronic communications equipment is cheap insurance.

On a cruising yacht sailing well offshore and outside of shipping lanes, the chances of collision with another vessel are almost nil. However, there is always that minute chance, and AIS offers the necessary protection from large commercial vessels. Near land, particularly in dense fog with heavy shipping traffic, those chances are much greater, especially if we factor in the presence of small pleasure craft. Probably fewer than half of all U.S. recreational coastal vessels use AIS or radar.

To sum up, the best collision-avoidance strategy depends on where you sail. If you intend to limit your travels to within 100 miles of shore and your area is prone to fog, such as the California coast, radar makes plenty of sense. On the other hand, if you aim to undertake long ocean passages, spending most of your time in the tropics and closely watching your daily amperage use, then AIS is in some ways the better choice. 

Circumnavigator/author Bill Morris believes the best strategy for succeeding as an offshore voyager is to keep systems simple and, if possible, manual. Key to survival are a basic array of electronics and an aggressive alternative energy matrix to ensure maximum charge and longevity in the ship’s battery banks. Bill’s new book, Sun, Wind, and Water: The Essential Guide to the Energy-Efficient Cruising Boat, is due to be published by Seaworthy Publications in spring 2016. 

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Jan 13, 2016 04:14 pm
 Posted by  CaptBob

AIS is not real time. If there are input errors then AIS is wrong. Using AIS alone does not give you the data a radar does. Also commercial ships do not routinely have radio operators..the bridge watch might be listening but then on a voyage they also might have turned the radio dow,

Using AIS alone, especially a Class C unit which is what most recreational sailors purchase is a great way to have a AIS-Assisted collision

Jan 13, 2016 07:27 pm
 Posted by  Waughmarine

Multifunction displays such as those provided by Raymarine will overlay radar & AIS. AIS will only display vessels transmitting an AIS signal and not other vessels. (Such as other small craft). So both are needed. Having the radar set on intermittent transmit will save battery power and ensure that all targets are represented.
Please remember that both are only AIDS TO NAVIGATION and not a guarantee of safety.

Jan 14, 2016 05:48 pm
 Posted by  Tom

As a trans-Atlantic sailor with a BSEE degree, I agree with your analysis. Although we have a huge 24vdc house bank on our Oyster 49, we tend to keep the Radar on STANDBY during daylight, and ALWAYS transmitting at night.
There are lots of really bad things out there that do not transmit AIS - think fishing boats in the US. While I have back-up and independent GPS-equipped chart plotters, I am single thread in AIS, and although highly reliable, I am considering dual redundancy AIS, whereas lower-reliability redundant radar systems have more issues than benefits.
From personal experience, I would NOT say that a the chances of a collision is 'nil' in open water, but hopefully 'unlikely.' Lastly, while the US is well-plotted, I had a 1/4-mile 100' high island 1/4-mile in front of me in the Aegean in October that did not exist on the Garmin G2 charts - scary! My wife spotted it and asked me which side to pass it...
Both Radar & AIS belong on an ocean vessel, and you can power-manage the radar as it is indeed a power 'hog,' be it on a fighter aircraft or a 40' cruiser.

Jan 19, 2016 07:29 am
 Posted by  John Banim

In my opinion an AIS recover is one of the most useful and cost effective upgrades to a small sailing boat and has minimal impact on batteries.
Sailing in the Med with a 10m sailing cruiser I had a Digital Yacht AIS receiver interfaced with Standard Horizon plotter. This plotted AIS contacts relative to my plotted position which I found particularly useful for collision avoidance in coastal waters. Usually one would spot an oncoming ship visually and then check its CPA and TCPA. When a ship is standing on towards you it is most helpful when you can from the CPA and TCPA when and if she changes course. Often this is not apparent from visual observation especially when small changes such as 5 degrees is made. Knowing the ships name and MMSI is handy particularly in the neighbourhood of ports and I have contacted ships on VHF 16 a number of times to ascertain their intentions and most spoke English.
Another useful aspect of AIS is the ability to "see" targets on the far side of islands or headlands. Although AIS is VHF based and is normally line of sight I found it often worked with land in between. In the Greek Islands fast ferries often do 35/40 knots and get near very quickly especially in confined waters and AIS is invaluable in these situations.
Another point is that all Turkish Gulets although less than the mandatory 300 tons, seem to have AIS transmitters. Apart from collision avoidance, another use for AIS is the ability to check anchorages in advance for the presence of tripper Gulets with their associated noise nuisance!

Jan 21, 2016 03:59 pm
 Posted by  ssjohnson

To me the big benefit of an AIS transceiver is that on the big ships sailboat is just as large a target as a freighter...being able to be seen is pretty nice compared to just relying on a radar reflector...that may or may not do the job.

Jan 29, 2016 07:24 am
 Posted by  B1948J

When I did the Baja-Ha-Ha from San Diego to Cabo there were 200 small craft in the rally. On AIS we could see perhaps 5 or 6 at any one time. On radar we could see them all. Plus, the scariest things in the coastal waters (besides partially sunken containers) are uncharted rocks and local fishing boats. The fishing boats might have a cabin light on as the only indication they are there. Double watches are mandatory. While I love AIS to see commercial traffic I don't see how one could be safe without radar until AIS is installed everywhere (and pirates don't like to be found).

May 10, 2016 08:49 pm
 Posted by  Sailor Harry

Both have their place as long as your AIS is a transceiver because it shares the obligation to get out of the way. It has the additional advantage that the vessel can call you or at least answer your call which they always seem to do.
Nothing beats radar in fog but it looses its effectiveness in bad weather and rain which AIS does not.
Smaller fishing vessels are installing AIS for their own fleet use so the safety will increase over time. The correct settings on a good AIS a essential. Wakeup and alarm settings can be the difference between noticing the warning and doing nothing so look for an AIS with these features then set them to suit your sailing situation because it will change from coastal to ocean cruising.

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