How are you using AIS?


Now you are getting to the heart of the matter. If you cannot :eyes: each other then you/we are all equal with no “right of way.” Just avoid. Done.


what I am saying is arpa is water speed and ais is ground speed
rules are based on water speed.
Unless they re wrote the col regs when GPS was invented and i missed that?
lots of radars out there with only log input for speed calc for the arpa
Perhaps a review of the terms underway and making way is needed??


Regarding water/ground stablized APRA:

This is from MSA MGN - 63 issused 1998

Users should be aware of possible hazards of using ground stabilised mode with ARPA/ATA when assessing risk of collision with approaching vessels, particularly in areas where significant tidal streams and/or currents exist. When course and speed inputs are derived from electronic position fixing systems (eg LORAN, GPS and DGPS) the display is ground-stabilised. The output data of tracked targets will relate to their ground track and, although accurate, may be highly misleading when assessing target aspect and determining collision-avoidance manoeuvre

MGN 379: “This notice replaces MGN 63” issued 2008

Where true target trails is selected, a sea stabilised display will indicate all targets’ motion through the water. A ground stabilised display will indicate all targets’ motion over the ground.
In coastal, estuarial and river waters where a significant set and drift may be experienced, a sea stabilised display will produce significant target trails from all fixed (stationary) objects possibly producing an unacceptably high level of clutter and masking. In such circumstances a ground stabilised display may reduce its effect and enable the observer to detect clearly the trails of moving targets, thus enhancing the observer’s situational awareness.


Nope. The rules aren’t based on speed at all. If you are underway you are required to follow the rules, that includes if you are drifting.

I actually expect it’s the other way around since I’ve worked on numerous vessels without a functioning log but never one without GPS into the ARPA. Even the shitty little non ARPA radars I had on tugs had GPS input.

Also, type approval for ARPA requires a ground stabilized mode.

Find where “making way” is defined in the COLREGS.

The only time it matters whether a vessel is making way or not (and as far as I can tell the only time on the whole book the term is used at all) is in Rule 35, Sound Signals in Restricted Visibility. Power driven vessels sound one prolonged every two minutes if making way, two prolonged every two minutes if not making way.


NUC and RAM show running lights or not depending or if making way or not.

The whole water/ground stabilized debate is based on the fact that in some cases with high current, ground based APRA true vectors differ from vessel aspect. The MCA decided that to solve that problem instead of just looking out the window everyone should use water stablized even though it messes up the display if you’re using trails.


Yes making way not defined yet mentioned as above and also when NUC/RAM applies so the concept exists.
I go back to my statement how did arpa and the rules work before GPS?

Narrow channel not defined as are safe speed and others

Why do we have an expensive doppler log specifically designed to give speed in water as a carriage requirement and connected to your radar and some warn you if bottom locked in shallow water?
Last vessel I was on neither radar ( furuno) allowed gps input as speed only stw, if that stopped so did the arpa function
There certainly was gps input for all the other gps stuff, position sog etc.

So there are 2 captains in court being questioned by a judge, one says we were stationary.
which vessel is he from the one not making way STW zero drifting in the current or the one making way at current speed so SOG zero?
Its an interesting subject


What does ARPA have to do with the rules? The rules work now the same way they worked before ARPA. They don’t depends on speed at all. If you are underway you are bound by the rules.

ARPA is required to have a ground stabilized mode. Without that option it isn’t an ARPA.

He must be on DP as that’s the only way to be underway and “stationary”.


What rules are you referring to? International COLREGS refer to speed in Rule 6, Rule 8, Rule 17, and Rule 19, but nowhere is the term “water speed” used.

When ARPA first came on the scene water speed was the most common reference (speed logs) used by ships and water reference ARPA was the standard.

SIRE inspected ships are required to use their ARPAs in water referenced mode. This is an obsolete requirement. Sure, if you like, set up one of your RADARs water referenced. It doesn’t hurt anything except it fucks up true trails, which I hate to see fucked up. The tanker masters I hang out with laugh at this SIRE bullshit, and have to remind themselves to switch reference modes when the SIRE auditor comes aboard to they don’t get dinged.

Ground referenced (via GPS) ARPA is my preferred mode. I’ve never once had a vessel’s aspect so out of whack with its course over ground vector that I got confused about my responsibilities in a risk-of-collision situation, so the aspect vs COG vector issue isn’t much of an issue.


COLREGS Rule 3:(i): The word underway means that a vessel is not at anchor, or made fast to the shore, or aground.

What’s so hard about that?


Just a question:

How does ARPA know about speed and direction of the currents where another ship on the Radar screen is proceeding or drifting?

Offshore in the Gulfstream it seems reasonable to extrapolate the own, calculated current to a ship in the vicinity.
However, around capes, in estuaries or in narrow straits (e.g. Messina), one mile of distance may make a big difference, or even reverse the currents.

And for COLREGS’ StandOn/GiveWay: How does the Radar know if a drifting ship is intentionally waiting dead in the moving water or if she is accidentally “proceeding” without power in the current,
AIS may indicate this… if someone deemed necessary to put it in…


Radar plotting (manually or using ARPA) does not care. If a vessel is not under command then it needs to have the proper visual indications. If it’s drifting in any way that’s not due to a casualty of some sort, that vessel still needs to comply fully with the rules.

The other vessel plotting one that’s drifting needs to consider the relative bearing of said vessel if we’re talking about two power driven vessels within sight of each other. If you sight a vessel a few points off of your starboard bow, you are the give way vessel. If your unsure if your overtaking another vessel, operate as though you are overtaking.



"He must be on DP as that’s the only way to be underway and “stationary”.

So in your view stationary is relative to the bottom and not the water.
Surely thats a concept that has only been around since GPS?


That’s the definition of the word. No one that knows what they’re talking about would call drifting “stationary”.


so the next question from the judge would be "how fast where you drifting"
well Sir there was no wind so we had zero speed, dead in the water if you like.
and the other captain says??


There is a great MAIB report of 2 box ships that crashed into each other in the middle of the day ( years ago) in an area of high current and the ships argued overtaking versus give way due to ais v water stabilised arpa. Probably caused the MGN notes that Kennebec Captain posted.
Both at fault of-course


Stopped and drifting does not absolve you from your responsibilities, unless you are Not Under Command.
Action taken to avoid collision must be ‘positive’ and both vessels have a duty to act.
There is no light signal for drifting, just the M flag - ‘my vessel is stopped and making no way through the water’.
I use true vectors with an offset true motion relative display; if the arrow heads meet then you are on a collision course, regardless of set and drift.
As I said before, I only use AIS for target identification although it is useful in fog for picking out smaller targets that may not paint on the radar yet.


I don’t mean to nit-pick but even being NUC wouldn’t completely absolve you. There would always be something more that could be done.


I’m presuming NUC means dead in the water with no engines; other than getting the oars out there is not a lot else you can do.


No. A vessel can be NUC and making way due to a steering casualty as well.


With regards to the use of AIS in general the term cross-check is a good one.

  1. verify (figures or information) by using an alternative source or method.
    “always try to cross-check your bearings”

An example (pre-ECIDS) was the requirement to use DRs on the chart combined with the requirement to take fixes at set intervals. It was sort of a “forced” situational awareness and a comparsion using two methods.

Using simple rules for the use of AIS, for example “don’t use it for collsion avoidance” are useful but better a full understand the limitations etc. Visual and radar information has limitations as well.

Cross-checking along with chronic anxiety.

The question is how do you achieve this technically?