In practice I have never seen Class B AIS transponders filtered, but the reasons one might want to do so, and the reason why this is a designed option, is obvious. Normally in inshore waters an AIS unit on a ship is showing maybe a couple dozen other AIS contacts. These appear on the ECDIS display along with plotted radar contacts and also the navigation
chart features, route
, waypoints, etc. Sometimes, like on holiday weekends and such when the weather
is perfect for taking the boat out, if everyone were transmitting their AIS data there could be hundreds of AIS contacts to deal with. Now there could reach a point where there is simply too much stuff on the screen
. Remember, you are mostly concerned with vessels that will approach you within, say, 5 boat lengths. A 700 foot ship is also concerned with more or less the same approach distance but his boat lengths are bigger than yours. On your boat you are very much concerned with anyone ahead of you who is coming within twice or thrice your stopping distance. A ship is also concerned with anything ahead within a multiple of his stopping distance, which is far, far greater. You may be quite comfortable with your radar on half mile to three mile scale... a ship under the same circumstances might have a 10cm radar on 12 mile range and a 3cm on 6 mile range, double that at sea and away from the inshore traffic. So information overload is a real possibility. The screen
simply gets too cluttered. I have never yet seen the AIS put into "B filtering" mode but I can see how it might under extreme circumstances be deemed necessary and appropriate, and definitely not irresponsible.
Basically, yes, you should get the transciever, so you can be seen also, and not just see others. But more important than an AIS is a good radar reflector. Fiberglass
boats don't paint
as well on radar as a steel
vessel does. The aluminum mast
that has mostly replaced wooden ones, still is not a very big radar target. A corner reflector is easy to make, not expensive to buy ready-made, and is very effective. Plus it uses no power. It could save your life or that of your crew or guests.
Until you have been on the bridge of a ship in heavy traffic, it is pretty hard to envision being concerned with vessels that are still 6 or 7 miles away, but that is indeed the case. This is often at the limit of visibility by eye or radar for small boats from a ship's bridge. Pleasure boaters often maneuver erratically and unexpectedly, further confusing the watch on the ship. A ship being steered by hand is holding a course to within 2 or 3 tenths of a degree and making definite and obvious course changes at waypoints. A yacht being steered by hand will be all over the place, sometimes navigating by the "do it like ya feel it" style, and with no definite destination
other than eventually getting back to the marina. From a ship's bridge the horizon is well over 10 miles away. We often spot other ships visually at better than 20 miles distance. That is a lot of ocean to scan. From a salboat cockpit
the horizon is way way less than that. It is a whole different perspective. The sailboat can stop almost instantly. The ship can stop, with no danger
of damage to the engine
, in SEVERAL MILES. Even in an emergency
, with a likelyhood of significant damage to an engine
with pistons big enough for you and your wife to dance on, the stopping distance is hard to envision when you have never stood watch on a ship's bridge.
Yes, when not in a narrow channel, and when the ship is not constrained by draft
, and you are under sail, the rules clearly state that the ship, as a motor
vessel, must keep clear of you. However, a reality check might have you reconsider pushing the issue, not because he is bigger than you or that his vessel cost 60M while yours cost 60K, but because he has a lot more on his plate to deal with than you do and simply cannot maneuver like you can.
You need to see everybody else, including commercial vessels and including ships. You need to be visible to everyone else, too. The only reason for getting an AIS receiver instead of a transciever is you simply cannot afford the $300 or so difference. I say if you are gonna go for a reciever, do without something else, and get the transciever. And put up a radar reflector, please! And do try to avoid close encounters with a ship, or doing anything to confuse them as to your intentions.