Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[Kelkara]

Just for fun, and going back to CPAs and cones of uncertainty... I wrote a quick Excel macro to plot the position of an approaching ship on a radar-like display ... including uncertainty in the observations of the ship's position/course. (The table shows values for the median course)

This shows the possible relative positions that the ship may move to over the next 30 minutes. Obviously as the ship gets closer it will start to become apparent which course the ship is on, and whether a risk of collision exists.

The red dots are possible positions of the ship with less than 10 degrees change in bearing from the initial sighting. i.e. appearing to be on a collision course.

In this example (a simple 90 degree crossing) the ship is first sighted 5 miles away on a bearing of 069. The uncertainty in my observations of the ship means that the CPA might be over 1nm or a very near miss ... from the sailboats point of view I can't tell yet ...

Even in the best case scenario with just over 1nm CPA the ship is less than 10 minutes away before I am likely to determine that there is no risk of collision, and in the worst case I won't know that it's going to miss until less than 1 minute.

If the ship doesn't alter its course, the question is: when should I decide to alter my course ... if I'm under sail and stand-on? or under power and give-way?

If the ship is on the median course ... After 8 minutes with the ship just 3 miles away I still can't tell if we're on a collision course or not, so I make a 45deg turn to starboard (second pic) and after another 3 minutes it's clear that the CPA will be a bit less than 1nm. Alternatively at this point the ship finally sees me and makes a 10deg turn to starboard (3rd pic), and now the CPA is likely a little over 1nm.

Just a fun way to try and visualise the original premise of the thread.

[El Pinguino]

Originally Posted by weavis

Yep. My lawyer would.

So, did he get you off?

I've posted this before but it is still worth a read

https://www.atsb.gov.au/media/25004/mair12_001.pdf

I have made my views on tri-lights clear enough - with photos - in another thread.

[weavis]

Originally Posted by El Pinguino

So, did he get you off?

I've posted this before but it is still worth a read

https://www.atsb.gov.au/media/25004/mair12_001.pdf

I have made my views on tri-lights clear enough - with photos - in another thread.

post 987

[Dockhead]

Originally Posted by El Pinguino

So, did he get you off?

I've posted this before but it is still worth a read

https://www.atsb.gov.au/media/25004/mair12_001.pdf

I have made my views on tri-lights clear enough - with photos - in another thread.

You're talking about this thread? http://www.cruisersforum.com/forums/...ur-191729.html

Excellent observations which all of us should read.

As to the Lysaght/Gunter case you linked to:

This is case on point to the tricolor light discussion, and actually one of the cases I was thinking of earlier.

In this case, a yacht actually got run down by a ship because the ship's OOW was confused by incorrect nav lights. The yacht was not visible on radar, the red sector of the tricolor was not operating, and the yacht was apparently displaying an anchor light (probably to make up for the burned out bulb in the red sector -- this particular tricolor had three separate bulbs in it).

The ship's bridge crew maneuvered based on the information they received from the yacht's nav lights -- the white light they saw appeared to be a stern light, so they thought they were seeing stern aspect. In fact the yacht was presenting a port aspect, and by the time this could be perceived, it was too late to prevent the collision.

I think we can take away from this case something like the following:

1. Radar doesn't always trump nav lights -- plastic boats often do not show up.

2. Professionals sometimes rely on nav lights to make maneuvering decisions.

3. False information from incorrect nav lights does lead to collisions in the real world.

[weavis]

Originally Posted by Dockhead

You're talking about this thread? http://www.cruisersforum.com/forums/...ur-191729.html

Excellent observations which all of us should read.

As to the Lysaght/Gunter case you linked to:

This is case on point to the tricolor light discussion, and actually one of the cases I was thinking of earlier.

In this case, a yacht actually got run down by a ship because the ship's OOW was confused by incorrect nav lights. The yacht was not visible on radar, the red sector of the tricolor was not operating, and the yacht was apparently displaying an anchor light (probably to make up for the burned out bulb in the red sector -- this particular tricolor had three separate bulbs in it).

The ship's bridge crew maneuvered based on the information they received from the yacht's nav lights -- the white light they saw appeared to be a stern light, so they thought they were seeing stern aspect. In fact the yacht was presenting a port aspect, and by the time this could be perceived, it was too late to prevent the collision.

I think we can take away from this case something like the following:

1. Radar doesn't always trump nav lights -- plastic boats often do not show up.

2. Professionals sometimes rely on nav lights to make maneuvering decisions.

3. False information from incorrect nav lights does lead to collisions in the real world.

....Saving and posting to my lawyer....

[Dockhead]

Originally Posted by Kelkara

Just for fun, and going back to CPAs and cones of uncertainty... I wrote a quick Excel macro to plot the position of an approaching ship on a radar-like display ... including uncertainty in the observations of the ship's position/course. (The table shows values for the median course)

This shows the possible relative positions that the ship may move to over the next 30 minutes. Obviously as the ship gets closer it will start to become apparent which course the ship is on, and whether a risk of collision exists.

The red dots are possible positions of the ship with less than 10 degrees change in bearing from the initial sighting. i.e. appearing to be on a collision course.

In this example (a simple 90 degree crossing) the ship is first sighted 5 miles away on a bearing of 069. The uncertainty in my observations of the ship means that the CPA might be over 1nm or a very near miss ... from the sailboats point of view I can't tell yet ...

Even in the best case scenario with just over 1nm CPA the ship is less than 10 minutes away before I am likely to determine that there is no risk of collision, and in the worst case I won't know that it's going to miss until less than 1 minute.

If the ship doesn't alter its course, the question is: when should I decide to alter my course ... if I'm under sail and stand-on? or under power and give-way?

If the ship is on the median course ... After 8 minutes with the ship just 3 miles away I still can't tell if we're on a collision course or not, so I make a 45deg turn to starboard (second pic) and after another 3 minutes it's clear that the CPA will be a bit less than 1nm. Alternatively at this point the ship finally sees me and makes a 10deg turn to starboard (3rd pic), and now the CPA is likely a little over 1nm.

Just a fun way to try and visualise the original premise of the thread.

This is really useful and excellent work, which really illustrates the principle better than a 1000 words!!

One thing which would be really interesting to do with this, would be to analyze the distance at which you can no longer avoid a collision without a drastic maneuver, and the point at which you can no longer avoid a collision at all. This will change with different relative speeds and different crossing angles, but I think if we graph it, we can see that there's a point where the maneuver required becomes very large, very fast.


A crucially important principle of collision avoidance is that in order to pass safely with another vessel, you have to maneuver to stay out of not just the dimensions of his hull, but the dimensions of where he may be according to whatever information you possess about his current and future position, and your own. That was kind of the whole premise of this thread. Kudos to Kelkara, for coming up with this way to show it graphically

[ramblinrod]

Originally Posted by Dockhead

You're talking about this thread? http://www.cruisersforum.com/forums/...ur-191729.html

Excellent observations which all of us should read.

As to the Lysaght/Gunter case you linked to:

This is case on point to the tricolor light discussion, and actually one of the cases I was thinking of earlier.

In this case, a yacht actually got run down by a ship because the ship's OOW was confused by incorrect nav lights. The yacht was not visible on radar, the red sector of the tricolor was not operating, and the yacht was apparently displaying an anchor light (probably to make up for the burned out bulb in the red sector -- this particular tricolor had three separate bulbs in it).

The ship's bridge crew maneuvered based on the information they received from the yacht's nav lights -- the white light they saw appeared to be a stern light, so they thought they were seeing stern aspect. In fact the yacht was presenting a port aspect, and by the time this could be perceived, it was too late to prevent the collision.

I think we can take away from this case something like the following:

1. Radar doesn't always trump nav lights -- plastic boats often do not show up.

2. Professionals sometimes rely on nav lights to make maneuvering decisions.

3. False information from incorrect nav lights does lead to collisions in the real world.

First of all, the photos provided (excellent) show that from the elevation of the bridge from a large ship, a trilight (shown properly) may be confused with shore lights and deck level nav lights may be more easily seen.

On the other hand, to vessels with a lower viewing angle like a motor yacht, the deck mounted lights may be confused with shore lights. Additionally in any significant wave, they may be seen very intermittently when the vessel observed is in a trough, so a mast head mounted trilight would be more effective.

What to do? (

Perhaps showing both would be better for all circumstances listed above.

Blasphemy.

Of your conclusions drawn above.

1. Nobody ever said it did. This should be obvious to anyone. IMHO, the yacht owner was a fool for not hoisting the radar reflector for the conditions they were in.

2. Captain Obvious.

3. Captain Obvious. However it is foolish to draw solid conclusions from a single case. Nevertheless, I am sure if one reviewed a number of cases the conclusion would be drawn and sound. (See first sentence.)

Additionally, there is nothing in the report saying that the anchor light was on. It was declared it could have been on (because the yacht was equipped with one that was capable of being on, but the owner declared it was not on. In all liklely it was not, and the white light observed on the bridge was the trilight faded lens. (If they in fact saw a white light and weren't lying through their teeth, but it is certainly plausible they did see a trilgiht sidelight because of the faded lens, so I would suspect that.)

However, had the yacht been displaying a proper trightlight, the accident may still have occurred, but more blame would be placed on the ship.

Had the yacht been displaying both a trilight and deck mounted nav lights, it should been visible to the ship. (It would have been better for the yacht to have been confused as NUC or RAM or fishing, than a sternlight.)

So while I don't condone showing deckmounted sidelights and a trilight together, Pings photos and this case illustrate circumstances where doing so could prevent collisions.

I know some are claiming that it violates Colregs under the "could be confused" provision, but I'm just sayin' that it what I get from these particular photos and this particular case we are currently considering.

So I still hold my original position that it is unlikely that one would confuse a sailboat showing deck mounted sidelights and a trilight simultaneously, with something else, that would have any significant consequence (of course there is a very remote possibility it could, but as a risk, extremely low), whereas sailing with a trilight whose sidelights actually show white, certainly would seem to be very risky.

This is one of the reasons why I poo-poo not unstepping the mast every off season, (in Ontario). As well as reducing working on the rig and UV exposure and stress on the running rigging, it also enables one to easily inspect the nav lights, antennas, cables, etc.

If one is not inclined to unstep the mast for the winter, they should be going up in the bosuns chair every spring to inspect EVERYTHING before the first voyage. (I don't think I have ever seen this on any winter mast-up boat I have ever seen at any marina or yacht club.)

What is very interesting to me in this case study, is that there was agreement on the bridge that the white light was constant bearing, when in fact, it sounds like it wasn't. This really could not be blamed on the yaw of the sailboat, because over time, the bearing would vary regardless.

Of course it is really difficult to know what happened for sure, but I would certainly suggest that the sailboats nav lights should have been in proper working condition (false on about 30 - 40% of the boats I inspect) and the radar reflector should have been hoisted.

I think the investigator raised an excellent point, that under the circumstances, the bridge should have been manned better.

And this is what I meant in previous posts about expectations of ships and rec vessels. While the same Colregs apply to all, there are other rules that apply to commercial vessels, and a higher standard of training, experience, and seamanship expected.

Lastly, I suspect if the ship took appropriate timely action, the collision should have been avoided regardless.

Ultimately, as I see it, factors on both the yacht and ship contributed to the collision.

[evm1024]

Come on folks!

When there is a collision it is always both parties fault. Investigations only show how to assign percentages of fault. (I'll throw in - with a rare exception every now and again - like a sub surfacing under you)

If you think that you need to have sidelights-stern light and a tricolor to be seen then install brighter lights.

Visualize rather than seek an exception to prove your case.

In open water there are no shore lights to blend in with navigational lights.

[ramblinrod]

Originally Posted by ramblinrod

First of all, the photos provided (excellent) show that from the elevation of the bridge from a large ship, a trilight (shown properly) may be confused with shore lights and deck level nav lights may be more easily seen.

On the other hand, to vessels with a lower viewing angle like a motor yacht, the deck mounted lights may be confused with shore lights. Additionally in any significant wave, they may be seen very intermittently when the vessel observed is in a trough, so a mast head mounted trilight would be more effective.

What to do? (

Perhaps showing both would be better for all circumstances listed above.

Blasphemy.

Of your conclusions drawn above.

1. Nobody ever said it did. This should be obvious to anyone. IMHO, the yacht owner was a fool for not hoisting the radar reflector for the conditions they were in.

2. Captain Obvious.

3. Captain Obvious. However it is foolish to draw solid conclusions from a single case. Nevertheless, I am sure if one reviewed a number of cases the conclusion would be drawn and sound. (See first sentence.)

Additionally, there is nothing in the report saying that the anchor light was on. It was declared it could have been on (because the yacht was equipped with one that was capable of being on, but the owner declared it was not on. In all liklely it was not, and the white light observed on the bridge was the trilight faded lens. (If they in fact saw a white light and weren't lying through their teeth, but it is certainly plausible they did see a trilgiht sidelight because of the faded lens, so I would suspect that.)

However, had the yacht been displaying a proper trightlight, the accident may still have occurred, but more blame would be placed on the ship.

Had the yacht been displaying both a trilight and deck mounted nav lights, it should been visible to the ship. (It would have been better for the yacht to have been confused as NUC or RAM or fishing, than a sternlight.)

So while I don't condone showing deckmounted sidelights and a trilight together, Pings photos and this case illustrate circumstances where doing so could prevent collisions.

I know some are claiming that it violates Colregs under the "could be confused" provision, but I'm just sayin' that it what I get from these particular photos and this particular case we are currently considering.

So I still hold my original position that it is unlikely that one would confuse a sailboat showing deck mounted sidelights and a trilight simultaneously, with something else, that would have any significant consequence (of course there is a very remote possibility it could, but as a risk, extremely low), whereas sailing with a trilight whose sidelights actually show white, certainly would seem to be very risky.

This is one of the reasons why I poo-poo not unstepping the mast every off season, (in Ontario). As well as reducing working on the rig and UV exposure and stress on the running rigging, it also enables one to easily inspect the nav lights, antennas, cables, etc.

If one is not inclined to unstep the mast for the winter, they should be going up in the bosuns chair every spring to inspect EVERYTHING before the first voyage. (I don't think I have ever seen this on any winter mast-up boat I have ever seen at any marina or yacht club.)

What is very interesting to me in this case study, is that there was agreement on the bridge that the white light was constant bearing, when in fact, it sounds like it wasn't. This really could not be blamed on the yaw of the sailboat, because over time, the bearing would vary regardless.

Of course it is really difficult to know what happened for sure, but I would certainly suggest that the sailboats nav lights should have been in proper working condition (false on about 30 - 40% of the boats I inspect) and the radar reflector should have been hoisted.

I think the investigator raised an excellent point, that under the circumstances, the bridge should have been manned better.

And this is what I meant in previous posts about expectations of ships and rec vessels. While the same Colregs apply to all, there are other rules that apply to commercial vessels, and a higher standard of training, experience, and seamanship expected.

Lastly, I suspect if the ship took appropriate timely action, the collision should have been avoided regardless.

Ultimately, as I see it, factors on both the yacht and ship contributed to the collision.

And ultimately, IMHO even though the yacht puled some boneheaded stunts, the ship should be more to blame, if for no other reason, than the expectation of a higher level of competence expected, to avoid mowing down a sailboat, even under these conditions.

[ramblinrod]

Originally Posted by evm1024

In open water there are no shore lights to blend in with navigational lights.

Did anyone say there was?

I don't recall the exact figure, but it was something like rec boats are within sight of shore 97% of the time they are away from the mooring.

So who's focusing on the exception?

[evm1024]

Originally Posted by ramblinrod

First of all, the photos provided (excellent) show that from the elevation of the bridge from a large ship, a trilight (shown properly) may be confused with shore lights and deck level nav lights may be more easily seen.

On the other hand, to vessels with a lower viewing angle like a motor yacht, the deck mounted lights may be confused with shore lights. Additionally in any significant wave, they may be seen very intermittently when the vessel observed is in a trough, so a mast head mounted trilight would be more effective.

What to do? (

Perhaps showing both would be better for all circumstances listed above.

Blasphemy.

SNIP!

Also, Care to address this?

If you think that you need to have sidelights-stern light and a tricolor to be seen then install brighter lights.

[ramblinrod]

Originally Posted by evm1024

Also, Care to address this?

Huh?

If lights are being confused with shore lights, how is installing brighter lights going to help?

They'll just look like brighter shore lights.

Nav lights are nav lights. You buy them at a marine store. They indicate on the package if they meet Colregs.

BTW, beware using LEDs from Ebay. They may not meet nav light requirements.

I bought my trilight and anchor light LEDs from a chandlery and paid a lot of money for them (Lunasea brand IIRC). I generally have the brightest anchor light in the anchorage.

I also have a little blue LED that shows from under my masthead TV antenna.

I sure hope nobody confuses me with a snowplow.

[ramblinrod]

Originally Posted by Uricanejack

Just my opinion. The old Mk1 eyeball is still the best quickest and most reliable way to determine risk of collision. It will not give precision re distance. Wow that looks big and its getting bigger fast.
There are quite a few relatively big fast vessels where we sail.

I recommend you put the shields up before that kind of crazy talk! ;-)

[evm1024]

The motivation for displaying deck level navigation lights and a mast head tricolor is not to mitigate any confusion with shore lights. Shore lights only entered into the discussion a short while ago.

The motivation for displaying both is to be seen - more is better.

I suggest that if someone is worried about being seen with your existing nav lights then you would be better served upgrading the lights to a better/brighter set. Rather than running deck level lights and tricolor.

Better set you say. Ya, going from one of those dinky acorn type to a series 41 aquasignal LED would represent an upgrade.

Going from a set that is intended for vessels less than 12 meters to a set for vessels over 12 but less than 50 would be a good upgrade. Or to one where the lights were intended for a vessel over 50 meters....

[fahtcha]

The recreational motorboats where I tend to sail seem to solve this issue by passing 10 feet off my boat's bow. A bit inconvenient for me but there doesn't seem to be much risk of collision.