Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[transmitterdan]

Originally Posted by Quebramar

One issue I'm facing is how to get to clarify where Vessels (mine and the individual target identified) will be at CPA. My B class transponder does not provide this info and I am wondering whether a simple (...) trig formula could help figure out. That would help assess my next steps, for I'm fine crossing 3 cables astern but not less than a full NM ahead.

Snip...

Happy to hear from more experienced sailors how to interpret where vessels will cross at CPA

OpenCPN will give you an estimated position of the other vessels at their CPA if it is within your AIS guard ring which you can set at 1NM. It draws a line from your boat to the point of CPA. I don't know what other chart plotters or MFDs do this. Even with such an estimate the location of the other ship at the CPA will keep changing as each vessel's course and speed change. So you have to pay attention as the situation evolves.

So far as I know all plotters place the guard ring around your vessel. I have thought about whether it would be better to place the guard ring around all other nearby vessels instead. Thus it would then be feasible to have a guard ring that can be something other than circular (larger radius ahead and smaller astern). I think that makes more sense to most people. But it would require a very powerful computer to keep up with all those rings in a dense traffic situation.

[El Pinguino]

That is what the Sperry PAD system did back in the dreamtime/early 80's... interesting kit.. but had a few drawbacks such as allowing people - with a less than perfect understanding of what the rools required - to make dodgy decisions....

[nigel1]

Originally Posted by Dockhead

I have heard, but can't confirm, that certain Vesper free standing AIS displays can do this also.

I can confirm that this is correct, I have a Vesper Watchmate WMX850, and that has the option to show a graphical display of the CPA, a very useful function.

[rramsey]

I might be wrong but AIS seems to miss something. It doen't tell me if the CPA is ahead or astern of the other vessel. So I don't know if I am supposed to slow down or speed up.

Had a difficult passing on the North Sea once. I was crossing a lane with five vessels coming from starboard and three from port. Which one was I going to pass where and would slowing down or speeding up resolve my puzzle? I mean, slowing down might actually cause more trouble then less. AIS didn't tell me, nor did AIS on OpenCPN.

Am I missing something?

[transmitterdan]

But I don't think knowing the estimated position of two vessels at CPA will always clearly show whether my vessel will pass ahead or behind of the other. For that it needs to show the location when my vessel crosses the path of the other. That may or may not be the CPA.

That's why I have wished for the ability to set a CPA range based on the other vessel and not mine. I don't care if I am within 1NM astern of a big ship but for sure don't want to be 1NM ahead of her in a crossing.

[Dockhead]

Originally Posted by transmitterdan

OpenCPN will give you an estimated position of the other vessels at their CPA if it is within your AIS guard ring which you can set at 1NM. It draws a line from your boat to the point of CPA. I don't know what other chart plotters or MFDs do this. Even with such an estimate the location of the other ship at the CPA will keep changing as each vessel's course and speed change. So you have to pay attention as the situation evolves.

So far as I know all plotters place the guard ring around your vessel. I have thought about whether it would be better to place the guard ring around all other nearby vessels instead. Thus it would then be feasible to have a guard ring that can be something other than circular (larger radius ahead and smaller astern). I think that makes more sense to most people. But it would require a very powerful computer to keep up with all those rings in a dense traffic situation.

You don't need a guard ring for this function -- just use "Show Target CPA". This is especially useful because you can show and hide whichever targets you choose -- so that you are displaying only those crossings you are concerned about. It is incredibly useful -- it makes it vastly easier to visualize a way through a complicated multi-target mess.

Another thing I forgot to mention in my previous post on this --

The projected COG lines of your own and other vessels. If you set them for the same value, then you can see how you are crossing. When I'm single handed and unable to get at OpenCPN at my nav table, I often adjust the projected COG lines from my default position of 10 minutes, to help me visualize crossings. On my regular Zeus plotter at the helm.

[Dockhead]

Originally Posted by rramsey

I might be wrong but AIS seems to miss something. It doen't tell me if the CPA is ahead or astern of the other vessel. So I don't know if I am supposed to slow down or speed up.

Had a difficult passing on the North Sea once. I was crossing a lane with five vessels coming from starboard and three from port. Which one was I going to pass where and would slowing down or speeding up resolve my puzzle? I mean, slowing down might actually cause more trouble then less. AIS didn't tell me, nor did AIS on OpenCPN.

Am I missing something?

Greetings -- just passed by Ijmuiden a few days ago on my way from Amsterdam to Dover.


OpenCPN does this brilliantly, but you need to activate the function "Show Target CPA". It will then draw a graphic of how you are crossing. It is so good that I find it practically indispensable.

And you are absolutely right about this being essential information. The commercial plotter makers should come up with something to do this on their plotters. I find the AIS display on my Zeus plotters to be rather poor.

[skipmac]

Checked the Vesper website and it appears that the Watchmate will show graphics of the CPA including the positions of both vessels at CPA so you can see whether the crossing is ahead or aft of the ship.

Link to the Vesper page with a screen diagram on this feature.

https://www2.vespermarine.com/vision...ing-situation/

The Vesper does output NMEA2000 (and wifi) so wondering if that information will show on my Garmin plotter in the cockpit if I link it to my network.

[nigel1]

Originally Posted by skipmac

Checked the Vesper website and it appears that the Watchmate will show graphics of the CPA including the positions of both vessels at CPA so you can see whether the crossing is ahead or aft of the ship.

Link to the Vesper page with a screen diagram on this feature.

https://www2.vespermarine.com/vision...ing-situation/

The Vesper does output NMEA2000 (and wifi) so wondering if that information will show on my Garmin plotter in the cockpit if I link it to my network.

It does not pass on that graphical CPA to the CP as far as I know, only the targets. I have the Raymarine e120 hooked up to the Vesper AIS, and the Raymarine display will generate a relative vector from the target which will show if you will pass ahead or astern.
The new Raymarine software also has a collision avoidance function, which basically indicates where NOT to steer to to maintain a set CPA.

LightHouse II R15 - Key Features | Raymarine

[bcn]

Originally Posted by Dockhead

OpenCPN does this brilliantly, but you need to activate the function "Show Target CPA". It will then draw a graphic of how you are crossing. It is so good that I find it practically indispensable.

Activated from the contextual menu: right click the target and select. On a tablet the equivalent is a loooong tab

[ramblinrod]

Originally Posted by Dockhead

This is a big drawback of the AIS display on recreational plotters! You are absolutely right -- you must know HOW the vessels are crossing, in order to know which way to turn. In a multi-vessel scenario this is absolutely essential information.

There are three ways I know to get this crucial information:

1. Track the bearings of your targets in a notebook. Slow and inefficient but used to be my primary method.

2. Do standard radar plots, and/or use true motion and trails on the radar screen.

3. Use OpenCPN!


OpenCPN has a brilliant function -- right click on an AIS target, then choose "show target CPA". Then it shows you graphically the geometry of the crossing -- incredibly useful.

I have heard, but can't confirm, that certain Vesper free standing AIS displays can do this also.


Concerning passing close by ships under sail -- not only is this foolish because of the uncertainty of your relative positions due to the "cone of uncertainty" we have been discussing, but large ships also have a profound influence on the local wind, on their windward sides as well as leeward sides. Not a good situation to be in, having figured that 180 feet is plenty of room, to find yourself suddenly becalmed!

If crossing perpendicular astern, and suddenly becalmed, so what? It just makes the distance greater.

[ramblinrod]

Originally Posted by transmitterdan

I have thought about whether it would be better to place the guard ring around all other nearby vessels instead. Thus it would then be feasible to have a guard ring that can be something other than circular (larger radius ahead and smaller astern). I think that makes more sense to most people. But it would require a very powerful computer to keep up with all those rings in a dense traffic situation.

Exactly, as I have been stating all along, a fixed CPA of 1 nm is simply ridiculous.

For crossing a tanker perpendicular.

A) Across bows at 20 knots, 1nm (or more if possible).

B) Across bows at 5 knots, 1/2 nm is plenty (but I still shoot for astern).

C) Astern at 20 knots, 1/4 nm is plenty.

D) Astern at 5 knots, 1/10 nm is plenty.

For crossing a 30-60 ft yacht, these distances can be reduced substantially.

In a race, < 1 mm (without contact) is sufficient, more across bows is better; < 1 mm (without contact) is ideal, when crossing astern.

[nigel1]

Originally Posted by ramblinrod

Exactly, as I have been stating all along, a fixed CPA of 1 nm is simply ridiculous.

For crossing a tanker perpendicular.

A) Across bows at 20 knots, 1nm (or more if possible).

B) Across bows at 5 knots, 1/2 nm is plenty (but I still shoot for astern).

C) Astern at 20 knots, 1/4 nm is plenty.

D) Astern at 5 knots, 1/10 nm is plenty.

For crossing a 30-60 ft yacht, these distances can be reduced substantially.

In a race, < 1 mm (without contact) is sufficient, more across bows is better; < 1 mm (without contact) is ideal, when crossing astern.

My standing orders posted on a 6000t anchor handling tug require the OOW to keep a minimum CPA of 1 mile at sea outside of restricted waters, if not, they are to call me.
So at 0300, another boat attempting to shoot the stern by a cable would result in me getting called out of my pit.

So, when advocating these minimal CPA's take into account the watchkeeper's on the other vessel.

[Nani Kai]

Originally Posted by Dockhead

Combined speed is 30 knots, so if no one alters course, we will get run down and crushed to smithereens in 10 minutes, during which the ship will travel four miles and we will travel one mile. The place of our death will be one mile from our starting position.

Certainly every capable and alert sailor knows that if the bearing of an approaching vessel remains the same while the distance between you is decreasing you are on a collision course. And we all know the Colregs require us to take substantial and unambiguous action early to avoid collisions. We all keep watch with our MKI eyeballs and whatever mechanical and electronic means we have available. Kudos to us all.

What DockHead is saying is that's not always enough. What he so clearly points out is that a very large vessel traveling at high speed represents a much larger danger to a smaller, slower vessel than one might imagine from 5nm out.

Have you never found yourself in closer quarters than you want to be with another vessel due to some lapse in attention or exhaustion or misinterpretation of data or distraction? DockHead is pointing out that should you find yourself in a compromising situation with another vessel 5nm away you may think you have all the time in the world when actually you may be a single nm and 10 mins away from collision.

I appreciated his presentation as well as others comments regarding collision avoidance. I learn new stuff all the time from this forum. It's like sitting around on the aft deck swapping lies and swatting flies with a crusty bunch of old sea dogs without the smell of diesel, old pipe tobacco and rancid fish oil.

[Paul Elliott]

Originally Posted by rramsey

I might be wrong but AIS seems to miss something. It doen't tell me if the CPA is ahead or astern of the other vessel. So I don't know if I am supposed to slow down or speed up.

Had a difficult passing on the North Sea once. I was crossing a lane with five vessels coming from starboard and three from port. Which one was I going to pass where and would slowing down or speeding up resolve my puzzle? I mean, slowing down might actually cause more trouble then less. AIS didn't tell me, nor did AIS on OpenCPN.

Am I missing something?

OpenCPN has at this point become very powerful, and I use it, but I still use NavMonPc for AIS monitoring. NavMonPc lets you select an AIS target and it can then automatically extend both the target's and your own course/speed vectors to the CPA. You can visually deduce whether you will cross ahead or astern of the other vessel. NavMonPc also has flexible alarm parameters that include basic guard zone circles as well as CPA / TCPA alarms.

NavMonPc does nothing special for multiple target CPA processing (other than the basic target / vector display).

NavMonPc is no longer the only program out there with these capabilities, and it's getting a bit long in the tooth (I wrote it), but it's free and many people still use it.