I always get weird looks, and sometimes arguments, when I mention to people that they shouldn’t rely on AIS for collision avoidance and that ARPA is the only legal collision avoidance system.
I’m gonna be one of those people with the wierd looks. ARPA might offer a more accurate collision avoidance system than AIS most of the time. But can you cite what makes you believe it is the only legal system?
I think it’s actually the opposite but the accuracy of the information isn’t the issue. ARPA is entirely based on own ship systems whereas AIS requires the other vessel to be sending the correct information.
The IMO doesn’t recognize AIS for use for collision avoidance. If you have an accident you’ll be up shit creek if you’re using AIS only and not ARPA.
Ok, I think you are getting this from IMO in Resolution A.1106(29): “The potential of AIS as an assistance for anti-collision device is recognized and AIS may be recommended as such a device in due time.”
But the guiding law for any ship that has both ARPA and AIS is Rule 5 - Look-out “Every vessel shall at all times maintain a proper look-out by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions to make a full appraisal of the situation and risk of collision.”
I don’t see how it makes a difference whether you are only using ARPA or AIS (and not the other, as well) and have an accident. Either way you’ll be up shit creek.
I believe you stated it yourself, “…AIS may be recommended as such a device in due time.” It is not a recognized (i.e. legal) collision avoidance tool at this time. The sticking point is that AIS relies on the input from the other vessel (heading, speed, GPS position) to calculate the CPA/TCPA. When that information is incorrect (how many have seen the vessel going “sideways” due to an error in the heading input?), the collision avoidance information generated is worthless.
A classic example here : https://maritime-executive.com/article/maersk-containership-and-balearia-ropax-collide-off-spain. Word on the street was that the ferry was using AIS only for collision avoidance and the heading on the Maersk container ship’s AIS was 180 degrees incorrect.
It’s possible that some vessels somewhere could encounter a situation where having one radar sea stabilized would be a benefit.
It’s a cost / benefit thing. If one radar is used of navigation and the other for collision avoidance than the benefits might justify the inconvenience and possibility of confusion.
What about ALLISIONS.
A lot of groundings/hitting buoys/snagging flotsam.
Who besides the U.S. Navy uses yards (as opposed to nautical miles) for distance with radar?
Nobody.
I don’t recall ever using the radar water stabilized for collision avoidance so I don’t know how useful it would be. Also I agree that the idea of using APRA vectors to determine aspect when visual is available sounds like something only someone with insufficient experience would do.
That said even an experienced watch officer might, at times, check the APRA to verify what’s seen visually. In that case the water stabilized might be a better match for reality.
Just because it’s difficult to imagine a scenario where there might be confusion in this situation doesn’t mean it couldn’t happen.
EDIT: In theory having the radar vectors more closely match the actual situation is similar to the reasoning behind correcting the charts. It’s very unlikely that having a single discrepancy on a chart could lead directly to an incident. However using correct charts is part of a control measure. When we cross-check visual observations with the chart it’s better if the chart is correct.
In practice it could be a different matter. Water-stabilized might make it more difficult to understand the navigation situation.