Collision Avoidance -- Dealing with Multiple Targets

[bmz]

Originally Posted by roland stockham

One thing I have not seen much mention of so far is AIS. Personally I have found it one of the most useful developments of the last few decades and would put it on a par with GPS as a safety aid. Yes like GPS there are issues with it and over reliance is one of them! In the same way that you get problems if people rely only on GPS and don't use radar, mark 1 eyeball and common sense you have problems. With AIS if you spend all your time looking at it and get into a 'virtual navigator' mode or assume that it make you visible to every one then you have a problem.
That said I think it contributes a few very important things to the mix in these complex situations. We can assume now that all large ships have AIS fitted and can confirm that by observing their signal (which is not the same as them looking at it!). If you have a transponder you also give them the same advantage. The big difference is that it also give an estimate of CPA which is faster and often I find more accurate than plotting with a compass. This allows better planning in complex situations. However I find the most useful information is that it shows rate of turn. Quite often it is hard to judge whether large ships are changing course by visual observation, you need to wait for their aspect to change. AIS tells you immediately that a target is responding to the situation by, say turning to starboard. I confirms they are taking action. I have been amazed to watch ships alter course by adegree or two at 10 miles out to adjust passing distances while they are still a dot on the horizon to me. Like GPS it takes a while to learn to get the best from AIS and maybe this is an area where we could develop some guidance for small boat sailors, whether new to sailing or new to AIS. I can thing of a few poor vis nights in the English Channel when I would have offered up my first born for such tech but then that was before we even had GPS but then again many people used to sail more cautiously, but that's another thread!!!

I agree with you; AIS is a godsend for collision avoidance. However, I have a problem with clutter on AIS; sometimes I have a half a dozen or more collision intercepts showing on my chartplotter (I don't want binnacle clutter either).

I have long since turned off my audible alarms. The technique I have found most successful is to focus on the class A intercepts, and ignore the class B until they are within a couple hundred feet of my vessel. But I have gotten a lot of negative feedback over that.

What you do about AIS clutter?

[StuM]

Originally Posted by bmz

I agree with you; AIS is a godsend for collision avoidance. However, I have a problem with clutter on AIS; sometimes I have a half a dozen or more collision intercepts showing on my chartplotter (I don't want binnacle clutter either).

I have long since turned off my audible alarms. The technique I have found most successful is to focus on the class A intercepts, and ignore the class B until they are within a couple hundred feet of my vessel. But I have gotten a lot of negative feedback over that.

What you do about AIS clutter?

200ft = 60 metres.
Two boats closing at a combined speed of just 10 knots (5 m/s) will cover that distance in 12 seconds. Doesn't leave you a lot time to take evasive action.

If the guy heading straight at you is a small powerboat doing 20 knots, what then?

[bmz]

Originally Posted by StuM

200ft = 60 metres.
Two boats closing at a combined speed of just 10 knots (5 m/s) will cover that distance in 12 seconds. Doesn't leave you a lot time to take evasive action.

If the guy heading straight at you is a small powerboat doing 20 knots, what then?

I don't know what you are doing. I asked Roland what he does about AIS clutter. That doesn't mean that no one else can respond with what they do about AIS clutter; but your post is not responsive to my question in any form or fashion.

Nonetheless this forum does try to educate, so I will respond in that vein.

What ever you do about a small powerboat closing in on you at 20 kn, it will be far more effective if you are not trying to focus on what 40 other class B AIS targets on your chart plotter are doing.

A small powerboat out joyriding may have been all over your chart plotter before he started to head for you. Those other 40 class B targets will also usually change their courses creating intercepts with you many times before seeing you and steering clear. They are basically noise and distract you from real dangers like that powerboat closing in on you at 20 kn. Of course this does not apply to class A targets which are a distinct threat, and to whom you have to yield or give way. But usually they are in channels or traffic lanes and maintain a steady course.

Whatever I do about that small powerboat has nothing to do with whether or not he or I have an AIS. I have had literally thousands of small fast powerboats crossing within a few hundred feet of me. Out of those thousands, maybe a handful times I've had to blow my horn. The only time that they are really a threat to you is if your attention is somewhere else--like the 40 other class B AIS targets on your chart plotter.

[bmz]

That should have read "usually yield or give way"

[El Pinguino]

One thing that nobody seems to have mentioned in this thread involves heading and speed. Well I mentioned it the other day but my post seems to have mysteriously disappeared.

Few yachts have class A AIS and fewer still would have 'heading' as opposed to CoG input. Most would also be relying on SoG rather than 'water speed'.

So, in using CoG and SoG one is not following 'best practice'

Simply put if relying on CoG and SoG from another ship you do not know her 'heading'.

For much of the time this will have little effect on aspect, cpa, etc, etc, but in areas with considerable tidal streams and when this is combined with ships doing maybe 10 or 12 knots it could have quite ugly consequences..... more so when people fail to look out the windows.


Lookee here...

https://coastalsafety.com/radio-ais-...rd-agency-mca/

but note -

Radio and AIS guidance from UK Maritime Coastguard Agency (MCA)

This article is an abstract from the original and is not a definitive document and should be read as information only


' a) Collision avoidance must be carried out in strict compliance with the COLREG. There is no provision in the COLREG for use of AIS information, therefore, decisions should be taken based primarily on systematic visual and/or radar observations. The availability and display of AIS data similar to one produced by systematic radar target tracking (e.g. automatic radar plotting or tracking aid (ARPA, ATA)) should not be given priority over the latter. AIS target data will only be based on the target vessels’ course and speed over ground whilst for COLREG compliance such data must be based on the vessels’ course and speed through the water.'

[El Pinguino]

A little bit more... from here...

https://www.safety4sea.com/wp-conten...IS-2015_12.pdf



'USE OF AIS IN COLLISION AVOIDANCE SITUATIONS
40 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.
41 Nevertheless, AIS information may merely be used to assist in collision avoidance decision-making. When using the AIS in the ship-to-ship mode for anti-collision purposes, the following cautionary points should be borne in mind:
.1 AIS is an additional source of navigational information. It does not replace, but supports, navigational systems such as radar target-tracking and VTS; and
.2 the use of AIS does not negate the responsibility of the OOW to comply at all times with the Collision Regulations, particularly rule 7 when determining whether risk of collisions exists.
42 The user should not rely on AIS as the sole information system, but should make use of all safety-relevant information available.'

[jtsailjt]

Originally Posted by El Pinguino

One thing that nobody seems to have mentioned in this thread involves heading and speed. Well I mentioned it the other day but my post seems to have mysteriously disappeared.

Few yachts have class A AIS and fewer still would have 'heading' as opposed to CoG input. Most would also be relying on SoG rather than 'water speed'.

So, in using CoG and SoG one is not following 'best practice'

Simply put if relying on CoG and SoG from another ship you do not know her 'heading'.

For much of the time this will have little effect on aspect, cpa, etc, etc, but in areas with considerable tidal streams and when this is combined with ships doing maybe 10 or 12 knots it could have quite ugly consequences..... more so when people fail to look out the windows.


Lookee here...

https://coastalsafety.com/radio-ais-...rd-agency-mca/

but note -

Radio and AIS guidance from UK Maritime Coastguard Agency (MCA)

This article is an abstract from the original and is not a definitive document and should be read as information only


' a) Collision avoidance must be carried out in strict compliance with the COLREG. There is no provision in the COLREG for use of AIS information, therefore, decisions should be taken based primarily on systematic visual and/or radar observations. The availability and display of AIS data similar to one produced by systematic radar target tracking (e.g. automatic radar plotting or tracking aid (ARPA, ATA)) should not be given priority over the latter. AIS target data will only be based on the target vessels’ course and speed over ground whilst for COLREG compliance such data must be based on the vessels’ course and speed through the water.'

Interesting article that you linked to with lots of good info, but for effective collision avoidance, except for determining stand on vs burdened status, I don't understand why you care what the targets heading is as long as you know what it's course is. In other words, why be concerned about which way the ship is pointing (unless it affects burdened vs stand on status) as long as you know where it's going?

I also don't understand the concern over course and speed over the ground versus course and speed through the water. Since the only difference between the two will be caused by currents, and both vessels will be in that same current when they are near enough to each other to potentially collide, it seems to me that one would be about as useful as the other. What am I missing?

[El Pinguino]

Originally Posted by jtsailjt

Interesting article that you linked to with lots of good info, but for effective collision avoidance, except for determining stand on vs burdened status, I don't understand why you care what the targets heading is as long as you know what it's course is. In other words, why be concerned about which way the ship is pointing (unless it affects burdened vs stand on status) as long as you know where it's going?

I also don't understand the concern over course and speed over the ground versus course and speed through the water. Since the only difference between the two will be caused by currents, and both vessels will be in that same current when they are near enough to each other to potentially collide, it seems to me that one would be about as useful as the other. What am I missing?

'Aspect'.... her course over the ground may suggest that she is showing you her green when in fact her actual heading is such that you will be seeing her red.... so your interpretation of who is give way and who has right of way * may be at variance with the actual situation.... difficult concept to explain but someone at IMO considers it important .

Lets say almost end on with her a point to port ... poor vis.... you are steering 090.... they are steering 270.... tide setting both of you down to the south.... your AIS is telling you that you are heading have a CoG of 110..... her AIS is telling you her CoG is 250 so you think she is showing green and passing 1 cable clear down your starboard side when in fact she is showing red and passing down your port side. So what is actually red to red looks like her showing green to your red and passing ahead of you with a cpa of 1 cable ... yes... confusing....



* sorry... couldn't help myself

[bobgarrett]

One further point re AIS.
On a recent rally I noted that one yacht's vessel type on AIS was "pleasure vessel" rather than "sailing vessel". This might not be incorrect strictly speaking but could change the decisions of a ship some distance away.
The owner said it had been set up by a professional. So might be worth checking what your AIS says you are.

[El Pinguino]

I've seen small Class A , passenger ships labeled as 'sailing'.....

Departure port and destination along with cargo info that hasn't been updated is not that unusual either.

Meanwhile hands up all those on the good ship 'ownship' that go to the fuss of reconfiguring their AIS from 'sailing' to 'pleasure vessel' every time they drop the sails and start the motor?

I don't expect to see too many hands.....

[Dockhead]

Originally Posted by El Pinguino

One thing that nobody seems to have mentioned in this thread involves heading and speed.

Few yachts have class A AIS and fewer still would have 'heading' as opposed to CoG input. Most would also be relying on SoG rather than 'water speed'.

So, in using CoG and SoG one is not following 'best practice'

Simply put if relying on CoG and SoG from another ship you do not know her 'heading'.

For much of the time this will have little effect on aspect, cpa, etc, etc, but in areas with considerable tidal streams and when this is combined with ships doing maybe 10 or 12 knots it could have quite ugly consequences..... more so when people fail to look out the windows.


Lookee here...

https://coastalsafety.com/radio-ais-...rd-agency-mca/

but note -

Radio and AIS guidance from UK Maritime Coastguard Agency (MCA)

This article is an abstract from the original and is not a definitive document and should be read as information only


' a) Collision avoidance must be carried out in strict compliance with the COLREG. There is no provision in the COLREG for use of AIS information, therefore, decisions should be taken based primarily on systematic visual and/or radar observations. The availability and display of AIS data similar to one produced by systematic radar target tracking (e.g. automatic radar plotting or tracking aid (ARPA, ATA)) should not be given priority over the latter. AIS target data will only be based on the target vessels’ course and speed over ground whilst for COLREG compliance such data must be based on the vessels’ course and speed through the water.'

This is an extremely good point .

But it's a highly advanced point and way over the heads of many of us, who are -- at best -- concerned with not trying occupy the same spot in the sea with another vessel.

Probably none of us here who has not been through training as a professional seaman has the knowledge to make complex judgements based on aspect -- things like what are the limits of his heading based on which nav lights you see. I'm studying it now myself. It's a whole world mostly unknown to the recreational sailor.

[El Pinguino]

Originally Posted by Dockhead

This is an extremely good point .

But it's a highly advanced point and way over the heads of many of us, who are -- at best -- concerned with not trying occupy the same spot in the sea with another vessel.

Probably none of us here who has not been through training as a professional seaman has the knowledge to make complex judgements based on aspect -- things like what are the limits of his heading based on which nav lights you see. I'm studying it now myself. It's a whole world mostly unknown to the recreational sailor.

Not even remotely advanced... very basic and at the heart of radar based collision avoidance.

Not about how is she heading either... simply about 'is she showing red or green and/or are her steaming lights in line or nearly in line'.

90% of the time the 'heading or cog' issue is neither here nor there... at other times....

Knowing these basics is essential if the recreational sailor is to use either radar or AIS for collision avoidance.

Its why I only use AIS as an early warning device....

[Dockhead]

Originally Posted by El Pinguino

Not even remotely advanced... very basic and at the heart of radar based collision avoidance.

Not about how is she heading either... simply about 'is she showing red or green and/or are her steaming lights in line or nearly in line'.

90% of the time the 'heading or cog' issue is neither here nor there... at other times....

Knowing these basics is essential if the recreational sailor is to use either radar or AIS for collision avoidance.

Its why I only use AIS as an early warning device....

Well, I'm not talking about merely showing red or green or steaming lights in a line -- of course we all know that.

I can do a radar plot by hand, and aspect doesn't enter into it, although you can derive aspect. Relative motion is should be more or less the same whether we reference motion through the water or over ground, the only difference being in case set and drift is not exactly the same between the two vessels.

You're being too modest I think.

[Dockhead]

Originally Posted by El Pinguino

I've seen small Class A , passenger ships labeled as 'sailing'.....

Departure port and destination along with cargo info that hasn't been updated is not that unusual either.

Meanwhile hands up all those on the good ship 'ownship' that go to the fuss of reconfiguring their AIS from 'sailing' to 'pleasure vessel' every time they drop the sails and start the motor?

I don't expect to see too many hands.....

You would never do that -- because Class "B" does not broadcast nav status at all -- Bobgarret is referring to TYPE of vessel, which is a different datum and which is part of the static data.

Don't confuse nav status with TYPE of vessel. Type of vessel doesn't change just because you are using sails or not. Here are possible nav status values:

0 = under way using engine
1 = at anchor
2 = not under command
3 = restricted maneuverability
4 = constrained by her draught
5 = moored
6 = aground
7 = engaged in fishing
8 = under way sailing
9 = reserved for future amendment of navigational status for ships carrying DG, HS, or MP, or IMO hazard or pollutant category C, high-speed craft (HSC)
10 = reserved for future amendment of navigational status for ships carrying dangerous goods (DG), harmful substances (HS) or marine pollutants (MP), or IMO hazard or pollutant category A, wing in ground (WIG)
11 = power-driven vessel towing astern (regional use)
12 = power-driven vessel pushing ahead or towing alongside (regional use)
13 = reserved for future use
14 = AIS-SART (active), MOB-AIS, EPIRB-AIS
15 = undefined = default (also used by AIS-SART, MOB-AIS and EPIRB-AIS under test)

https://help.marinetraffic.com/hc/en...Status-Values-


Nav status is broadcast ONLY by Class "A" sets.



Actually the necessity of putting in destination and changing nav status every time is what kept me from putting in a Class A transponder in my own boat. Too much work if you're short handed, and especially if you're single handed.

Now we can buy the new SOTDMA Class "B" sets and I will change to one of those this winter. Addresses all the crucial problems with Class "B".

[Kenomac]

Originally Posted by Dockhead

You would never do that -- because Class "B" does not broadcast nav status at all -- Bobgarret is referring to TYPE of vessel, which is a different datum and which is part of the static data.

Don't confuse nav status with TYPE of vessel. Type of vessel doesn't change just because you are using sails or not. Here are possible nav status values:

0 = under way using engine
1 = at anchor
2 = not under command
3 = restricted maneuverability
4 = constrained by her draught
5 = moored
6 = aground
7 = engaged in fishing
8 = under way sailing
9 = reserved for future amendment of navigational status for ships carrying DG, HS, or MP, or IMO hazard or pollutant category C, high-speed craft (HSC)
10 = reserved for future amendment of navigational status for ships carrying dangerous goods (DG), harmful substances (HS) or marine pollutants (MP), or IMO hazard or pollutant category A, wing in ground (WIG)
11 = power-driven vessel towing astern (regional use)
12 = power-driven vessel pushing ahead or towing alongside (regional use)
13 = reserved for future use
14 = AIS-SART (active), MOB-AIS, EPIRB-AIS
15 = undefined = default (also used by AIS-SART, MOB-AIS and EPIRB-AIS under test)

https://help.marinetraffic.com/hc/en...Status-Values-


Nav status is broadcast ONLY by Class "A" sets.



Actually the necessity of putting in destination and changing nav status every time is what kept me from putting in a Class A transponder in my own boat. Too much work if you're short handed, and especially if you're single handed.

Now we can buy the new SOTDMA Class "B" sets and I will change to one of those this winter. Addresses all the crucial problems with Class "B".

Which AIS transponder do you currently have and use?