We recently created a hornets nest of comment here and elsewhere on the Internet when we mentioned what appeared to be a class B AIS filter on our IMO approved Furuno 2117 radar. Ben Ellison, who we consider the dean of marine tech writers, took us to task for spreading a false rumor. Ben has gotten so much conflicting comment that he went to one of the AIS tech mavens for the answer (you can read the final word on this subject here on Ben’s website). There are also a myriad of amateur and professional mariner comments, some of which are of interest.
There are many issues being addressed by commentators. We think most are missing the points which are critical to cruising yachts. To briefly summarize:
- There is both a vessel size and AIS B filter, which when activated will filter out targets. This is confirmed by Furuno and Dr. Norris, one of the AIS fathers.
- In crowded areas you had better assume that your AIS B or AIS A on a smaller size yacht is being filtered.
- Note that we turn off our AIS receiver in almost all harbors and coastal environments because of the constant annoying CPA alarms and we assume the big guys do as well.
- Offshore, properly trained ship crews will use a check list to turn theirAIS filters off. But we assume there are plenty of crews who will not take this step.
- Over the past two years we have had six occasions in the open ocean when we were forced to alter course on passage for ships that did not respond to VHF calls and were the burdened vessel. Note that COLREGs require the stand on (right of way) vessel to maintain its course so the burdened vessel can maneuver around it.
- Even though AIS B is not 100% foolproof, we suggest that it is a highly valuable safety feature, and would choose it and a good radar before a life raft, if there was a budgeting conflict.
- Carrying an AIS, A or B, even if you fudge on the programmed vessel length, does not mean a lazy, undermanned, or inebriated watch on a ship will see you.
- The highest value for an AIS transponder is going to be in dirty weather, i.e. rain or a big sea, which masks radar returns and visual observation.
- We always assume the other guys do not see us and plan accordingly.
You might think that with a careful visual watch and radar you would be OK. This is the case except for in heavy rain or fog. We have seen 20,000 ton and larger ships disappear from our radar screen in squalls. If they don’t show up, what do you think happens with a yacht?
With AIS B transponders now available for under $1000 they are worth serious consideration. Even if they are not a 100% guarantee of being seen you are a lot more likely to show up with than without.