Cruisers & Sailing Forums (https://www.cruisersforum.com/forums/)
-   Seamanship & Boat Handling (https://www.cruisersforum.com/forums/f90/)
-   -   Collision Avoidance, Cones of Uncertainty, and Appropriate CPA (https://www.cruisersforum.com/forums/f90/collision-avoidance-cones-of-uncertainty-and-appropriate-cpa-189919.html)

Pelagic 24-08-2017 09:35

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by wjhutchings (Post 2463129)
Agreed it is a good thread, however Boatman61 has it in one, I spent 30 odd years on those things yachties love to hate, merchant ships, from deck cadet to Master, and the best bit of anti collision information I was given was pick your spot on the bridge and stay there, line vessels up with a window frame whatever, stanchion is just as good, if the other vessel move off the bearing towards the bow it is crossing, if moves aft you are crossing if stays on the bearing you are on collision course. Once you have ascertained this then get serious and look at CPA's etc. Remember this you can cross astern of a vessel in perfect safety once you see its stern it's separation distance is increasing, but never never try cutting across the bow of a ship, one because they loose sight of you very easily under the bow and if something goes wrong whilst attempting this manoeuvre then you will be run down

All wise words WJ but that only works when you have good visibility......
knowing how to plot radar targets in restricted visibility is another matter as is the measure of a safe CPA.

Paul Elliott 24-08-2017 09:38

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by wjhutchings (Post 2463129)
[...] line vessels up with a window frame whatever, stanchion is just as good, if the other vessel move off the bearing towards the bow it is crossing, if moves aft you are crossing if stays on the bearing you are on collision course. [...]

When we are at sea and the boat is corkscrewing around with seas on the quarter I really can't reliably use the stanchion trick. This is why I love my hand bearing compass. In these conditions, AIS calculated CPA can also be difficult since your COG and speed are skewing so much (which is part of that "cone of uncertainty"), but with the proper filtering / smoothing you can still get useful numbers.

Quebramar 24-08-2017 09:43

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
[emoji106]

wjhutchings 24-08-2017 09:44

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Pelagic (Post 2463177)
All wise words WJ but that only works when you have good visibility......
knowing how to plot radar targets in restricted visibility is another matter as is the measure of a safe CPA.

My advice to anyone in a small vessel especially if they have wooden or fibre glass hulls is unless you are caught at sea in bad visibility is to stay in port it is much safer, no matter what anyone tells you about radar reflectors unless the weather is flat calm there is good chance your radar return will be lost in the clutter, AIS might be Ok but radar is the main collision avoidance tool used in poor visibility.

Clivevon 24-08-2017 10:57

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Good thread. The thing about different opinions in the end is that everyone can sail their own boat their own way. But it is good to hear what others do. My offshore sailing is in the central English Channel, including crossing overnight to or from France. This has got a lot less stressful since I fitted an AIS Transponder & a plotter at the helm which can integrate AIS & Radar and gives me vectors, CPA & TCPA. The shipping lanes are always busy. My philosophy is to stay out of their way wherever possible, and maintain a minimum of 1 mile CPA, especially at night, except where I am passing astern closer (say half a mile) so as to have the maximum distance in front of the next ship. Given the way CPA can flick around in a seaway, I dont otherwise like anything less than a mile and I dont like crossing the bows anyway. During the day and good visibility I'm happy to go a bit closer, but basically I just like to stay out of their way and make any course changes absolutely clear. Musnt forget all those vessels out there without AIS transponders, though.

skipmac 24-08-2017 12:08

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Wish every boater would read this thread. Wouldn't hurt if more commercial captains read it as well so they would have a better idea of what's going on with the small boats around them.

For what it's worth, my thoughts, opinions and comments on the issue.

- Planning for a 180' miss of a ship in open ocean (or anywhere else for that matter unless the ship is anchored or tied to a pier) is totally nuts.

- a 1 degree course change is basically nothing and worthless in a crossing situation.

- For all the reasons stated, passing astern of a ship is much to be preferred to crossing the bow.

- Tracking a ship as soon as it's visible by checking for a constant bearing has worked for me but without more accurate course information it is often very difficult to tell if the ship changed course at the limits of my sighting the vessel to increase the CPA.

- Emergency avoidance. DISCLAIMER!!!!! I am in no way advocating this or anything like it. Just did some math of my own on the minimums to save your posterior if you end up in an life threatening collision situation.

NOTE: For simplicity most numbers are rounded to the nearest whole number.

Assuming a ship traveling at 24 kts that comes down to 13 meters per second (41 ft/second). A sailboat at 6 kts will be about 3 m/sec (10 ft/sec). Assuming the ship's beam of 60 m, sailboat length 12-13 m and a couple of meters for clearance, call it 100 m it would take the sailboat about 33 seconds to cross the bow of the ship. In 33 seconds the ship would travel about 430 m (1400 ft).

So if a sailor realized at the last second that a 1 degree course change didn't make enough room to miss a ship, he/she is running downwind with poles and preventers rigged so a 180 wasn't feasible, he/she would have about 430 m or a quarter mile to clear a ship's bow. That's significantly more than 180'.

ramblinrod 24-08-2017 12:32

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by bobgarrett (Post 2462927)
Some excellent thoughts on this topic, and we all must bear in mind that ship collisions are rare.
But why is that? Because of the colregs and the professional application of them.
Like others I will look at handheld compass bearings as well as AIS. However, those who think they have avoided a low CPA by a minor course correction might learn otherwise if they had also watched their AIS and seen a ROT figure for the ship a few miles away. I would contend that it may well have been the ship that avoided the collision not you.

I think you are considering only the most extreme "head on" and "high speed" situation. In many other cases, one degree may be plenty.

By the letter of the law, one making a turn to avoid collision, should make a big turn, so that if the other vessel is watching, they may see it. But I assure you a large vessel with 100 small vessels buzzing about within 1 nm, is not so concerned with the one 5 miles away, and if that skipper makes a slight turn to stay out of the cluster, they will be thankful (if they even knew what you did, which is highly unlikely).

I liken this to sailing through the various clusters of sailing dinghies outside Kingston On. Their "area of awareness" (my term) is about 200 ft. When we approach the area, we plot a course to avoid about
1 mile away. But in reality, conditions change due to the erratic movement of the smaller vessels, and we likely change course 5 times over that mile, as new situations unfold.

I don't imagine this is much different than what goes on in the bridge of a vessel much larger than ours, except they prolly assume my area of awarenesss is about 1 mile.

rramsey 24-08-2017 13:38

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Probably time for some recapping. Is the following right?

1. 1 degree change is not enough. Does not create enough distance, is not clear for the other vessel. Some disagree.

2. 1nm minimal CPA seems to be the consensus, although some feel this is way too much

3. Passing behind the other vessel seems to be the consensus

4. Choose a course and stick to it so tankers can anticipate 5-10nm ahead.

5. The difference in speed and size means that most of the avoiding needs to be done by the tanker

6. Stopping to wait is a bad idea. You lose maneuverability and it might be anticipated by the tanker

7. Using the stanchion bearing method is effective for assessing wether CPA will be astern of over the bow of the tanker

Anyone else?

StuM 24-08-2017 14:28

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by ramblinrod (Post 2463295)
I think you are considering only the most extreme "head on" and "high speed" situation. In many other cases, one degree may be plenty.

How many sailors (or sailboats) can hold a course to 1 consistently?

conachair 24-08-2017 23:33

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by rramsey (Post 2463335)

7. Using the stanchion bearing method is effective for assessing wether CPA will be astern of over the bow of the tanker

Not very accurate though, maybe fine in relatively benign conditions when the CPA will be large, hand bearing compass is much more accurate. Or even better ais.

Olddave 25-08-2017 00:02

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by conachair (Post 2463651)
Not very accurate though, maybe fine in relatively benign conditions when the CPA will be large, hand bearing compass is much more accurate. Or even better ais.

That method of determining whether you are on a collision course or not, has been taught to commercial crew and masters since day dot.
It is very accurate and one of its best features is that the person on watch does not need to move or do anything more than start monitoring the other vessel.
As everyone should know, you just line up some part of your vessel with the other vessel and if it does not change its position, you are on a collision course!
If you complicate such a simple method by looking through binocs you might find some people start paying attention much later because it's too rough or too wet or who knows? KISS!
Dave

bcn 25-08-2017 00:41

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Paul Elliott (Post 2463180)
When we are at sea and the boat is corkscrewing around with seas on the quarter I really can't reliably use the stanchion trick.

Observing a minute or so the yaw angles will average and we try to observe a tendency - are the maximums and minimums (bow of the other vessels in relation to the stanchion/window frame) moving in a direction?

With AIS on a small boat you have similar problem getting a stable CPA if you don't dampen your heading and COG vectors a lot.

Nota bene: these are not arguments against using these valuable techniques and tools.

Olddave 25-08-2017 00:51

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Continuing on from my previous post.
For the benefit of those new to watchkeeping, It's probably worth adding that when you begin to monitor another vessels bearing, you should also begin to consider who is the stand on vessel and who should give way.
If you are required to give way it should be done early and by a substantial change of course or speed or both. There are occasions where slowing is appropriate.
If you are the stand-on vessel and have any concerns about the situation, it is entirely appropriate to communicate with the other vessel. Always be wary of talking to the wrong vessel though! AIS is a great help because you often have the other vessels name, and they should respond when called by name.
With contact made you should find ships will agree to give way to your sailing vessel if they are able. They sometimes may ask you to maintain you current course and speed, and in this situation you should remind them that you are a sailing vessel and at the mercy of wind and tide. That is one of the main reasons we have the right of way.
Dave

TheThunderbird 25-08-2017 00:53

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Best binoculars have a night compass w light

A sailboat is basically STILL as compared with any powered ship. Thus, aiming at passing on its bow is nonsense

If anyone passes X feet off the stern of a big ship, he/she has been X+-50' at its bow before. Never do it! Unless it is a great distance

I ASK crew to have 2 judgements, basically to wake me up as soon as lights appear (night watch)

main clue is from nav lights ,GREEN TO GREEN, RED TO RED.

Far more effective than calculating CPA on an AIS...

DH is right, but.. His calculations are son of the instrument driven approach of current sailors... (sorry DH)

conachair 25-08-2017 01:01

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Olddave (Post 2463656)
That method of determining whether you are on a collision course or not, has been taught to commercial crew and masters since day dot.
It is very accurate and one of its best features is that the person on watch does not need to move or do anything more than start monitoring the other vessel.
As everyone should know, you just line up some part of your vessel with the other vessel and if it does not change its position, you are on a collision course!
If you complicate such a simple method by looking through binocs you might find some people start paying attention much later because it's too rough or too wet or who knows? KISS!
Dave

Maybe on a big ship with a heading which is stable, on a small vessel getting kicked around in a seaway and a windvane/autopilot not keeping an exact course it just isn't very accurate. Fine for a lot of targets a lot of the time but accuracy can't be relied on on a small vessel. If the the target doesn't open up quickly it's better using a hand bearing compass.

IRPCS (rule 7,D, i ) Risk of collision.
(a) Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there as any doubt such risk shall be deemed to exist.

(b) Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observations of detected objects.

(c) Assumptions shall not be made on the basis of scanty information, especially scanty radar information.

(d) In determining if risk of collision exists the following considerations shall be among those taken into account.

(i) such risk shall be deemed to exist it the compass bearing of an approaching vessel does not appreciably change.

Dockhead 25-08-2017 01:29

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by conachair (Post 2463651)
Not very accurate though, maybe fine in relatively benign conditions when the CPA will be large, hand bearing compass is much more accurate. Or even better ais.

The stanchion method is really, really useful for quick elimination of targets which are no risk so that you can identify the targets you need to work on.

This is very powerful, and I use it continuously when I'm single handing. It's so easy that it even beats getting up to look at the plotter screen to see what AIS says.

But it's not accurate enough in any kind of sea condition to distinguish a couple miles CPA from a zero CPA, some distance away. So once you have a target that appears to be on a constant bearing from the way it appears against your stanchions, you have to drill deeper to understand whether you need to worry or not. A hand bearing compass will get you a lot further, but AIS will give you as nearly exact information as it is possible to have.


One piece of information which was virtually impossible to have before AIS is the answer to the question of whether he has seen you and did he maneuver. How many crossings have been botched totally by sailors who panic at 3 or 4 miles out, thinking they are on a collision course, and make some maneuver when they should have been holding course and speed, some maneuver which in 50% of the cases will put them under the bow of the approaching ship (that's the "choice words" scenario in Nigel's post above). Because they couldn't see that the ship was going to pass with a 1 mile CPA, and assumed they weren't seen and needed to do something.

conachair 25-08-2017 01:35

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Dockhead (Post 2463670)
The stanchion method is really, really useful for quick elimination of targets which are no risk so that you can identify the targets you need to work on.

This is very powerful, and I use it continuously when I'm single handing. It's so easy that it even beats getting up to look at the plotter screen to see what AIS says.

But it's not accurate enough in any kind of sea condition to distinguish a couple miles CPA from a zero CPA, some distance away. So once you have a target that appears to be on a constant bearing from the way it appears against your stanchions, you have to drill deeper to understand whether you need to worry or not. A hand bearing compass will get you a lot further, but AIS will give you as nearly exact information as it is possible to have.

:thumb:

I have a few clothes pegs dotted around the cockpit for quick check on the fishing boat etc way over there not transmitting AIS, one on sprayhood & one on life lines for a quick check to see if attention is needed :cool: Still needs a fairly constant heading which with some seas running can be all over the place

PS Just came across this which will at some point get printed & laminated up as a quick check when dog tired at dawn single handed. Again ;)

https://www.skysailtraining.co.uk/col..._flowchart.pdf

Dockhead 25-08-2017 02:22

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by rramsey (Post 2463335)
Probably time for some recapping. Is the following right?

1. 1 degree change is not enough. Does not create enough distance, is not clear for the other vessel. Some disagree.

2. 1nm minimal CPA seems to be the consensus, although some feel this is way too much

3. Passing behind the other vessel seems to be the consensus

4. Choose a course and stick to it so tankers can anticipate 5-10nm ahead.

5. The difference in speed and size means that most of the avoiding needs to be done by the tanker

6. Stopping to wait is a bad idea. You lose maneuverability and it might be anticipated by the tanker

7. Using the stanchion bearing method is effective for assessing wether CPA will be astern of over the bow of the tanker

Anyone else?

Generally all correct, more or less, but don't forget the most important one!

8. Follow the Rules!


Other comments:

Stopping can be a good idea or a bad one depending on circumstances. If you have place to stop which is definitely out of traffic then it's just fine.

The "tanker" -- at sea speed and in open water -- has more power to resolve a dangerous crossing, because of his speed. However, contrary to what some people have posted here, the procedure is not determined by some fantasies about who is more "maneuverable" in a given situation, it is determined by the Rules. So you give way to him when you are required. Your speed simply means that you cannot leave your maneuver until a few miles off.

And don't forget Rules 9 and 10. "Not impeding", contrary to what many believe, does not change anyone's stand-on or give-way status. If you are required to "not impede", you must maneuver prior to the risk of collision ever arising. So stay out of channels and fairways in harbors and approaches if there is any commercial traffic, and cross TSS's only when you will not thereby force a ship to maneuver.

Dockhead 25-08-2017 03:57

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Quebramar (Post 2462995)
I'm regularly crossing the Channel close to its busiest/scariest area North of Sandettie where TSS's from Antwerp, Rotterdam and Scotland merge to get to Dover Straight. It is a very busy area and what makes it scary is that when you've crossed one lane, you cannot expect to immediately cross the other one in opposite direction, making the 'look left... look right' method very insufficient.

AIS is clearly my friend in such a case, and clearly at a 10NM distance from me so I can assess (not especially take action immediately). That is the area where it happened to me to heave-to, waiting for the complex multi-target situation to clarify and possibly resolve.

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.

For those in doubt of manoeuvrability of sailboats in this kind of situation, may I point to the video of the collision of R. Wilson on YouTube : with full sail (genny) the boat seemed to have (surprisingly as per the skipper) stalled right in front of the tanker...

In such place I mentioned above, I'm always ready to crank the engine if need be in order to shorten the time in the TSS's... safety first.

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


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!

StuM 25-08-2017 04:22

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Olddave (Post 2463656)
That method of determining whether you are on a collision course or not, has been taught to commercial crew and masters since day dot.
It is very accurate and one of its best features is that the person on watch does not need to move or do anything more than start monitoring the other vessel.
As everyone should know, you just line up some part of your vessel with the other vessel and if it does not change its position, you are on a collision course!
If you complicate such a simple method by looking through binocs you might find some people start paying attention much later because it's too rough or too wet or who knows? KISS!
Dave

That may work well on a stable platform like a commercial ship, but not so well in a small sailboat in anything more than a gentle sea state. Especially if you can't keep your head in a stable position relative to the stanchion of whatever.

Thanks, but I'll continue to use a hand bearing compass (there's probably a good reason why COLREGs refers to a "compass bearing", not just a "bearing")

As for "it is very accurate", it's also worth keeping in mind Rule 7.d.(ii):
"such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range."

transmitterdan 25-08-2017 04:41

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Quebramar (Post 2462995)
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 25-08-2017 04:50

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
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 25-08-2017 05:19

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Dockhead (Post 2463748)
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 25-08-2017 05:40

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
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 25-08-2017 05:40

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
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 25-08-2017 05:44

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by transmitterdan (Post 2463772)
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 25-08-2017 05:50

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by rramsey (Post 2463809)
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 25-08-2017 05:59

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
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 25-08-2017 06:33

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by skipmac (Post 2463829)
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 25-08-2017 06:36

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Dockhead (Post 2463821)
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 25-08-2017 07:11

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Dockhead (Post 2463748)
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 25-08-2017 08:54

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by transmitterdan (Post 2463772)
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 25-08-2017 09:00

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by ramblinrod (Post 2463925)
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 25-08-2017 09:12

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Dockhead (Post 2462454)

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. :thumb:

Paul Elliott 25-08-2017 09:19

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by rramsey (Post 2463809)
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.

Dockhead 25-08-2017 09:29

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by ramblinrod (Post 2463862)
If crossing perpendicular astern, and suddenly becalmed, so what? It just makes the distance greater.

Indeed! Passing astern of a vessel which is much faster than you is quite safe for this reason. The old salts like to say that if you can see the transom (or stern light) of a faster vessel, then you are absolutely safe, and that is true.

If you are in open water and you set up to cross astern from an appropriate distance -- 5 miles or more -- you can still get in trouble, though, if you end up going faster than you planned or other errors kick in, so you still need some kind of margin of error. But I agree with you that it is much less than if you pass ahead. A few cables is generally safe enough as long as you stay alert and ready to correct if something goes off (and I'm sure you'd be doing that anyway right?).

But as a matter of courtesy, at least, it is still better to stay a mile away from ships in open water even if passing behind. As some of the commercial mariners in here have said -- standing orders will likely require the OOW to get the master out of his bed for any pass closer than 1 mile.

rramsey 26-08-2017 06:07

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
I have been looking into another (possibly) important factor. Apart from propelling the vessel, a propellor also sucks water in front of it and pushes it out at it's back. That means that, in front of the propellor, the water level is lowered and behind it it is raised. I image it looks like a ball in a stretched sheet.

To see the effect for slow cruising large vessels, see youtube:
https://www.youtube.com/watch?v=1BisTlNcwDk
https://www.youtube.com/watch?v=8sEdgHH9F10

There are videos on youtube of ships and jet skis being "sucked" into large vessels. Actually, it is not so much "sucking" as is is "falling", somewhat like riding doen a wave. Imagine again the sheet with a ball in the middle (the way gravity is explained on TV) and you will see that any area in front of the propellor slopes into the propellor. Th closer you get, the steeper the slope.

That means that, als you get closer to the vessel, you fall towards it more and more. Now I guesstimate that, as you get further away, the effect drops squared. But it also means that, if you get close, you'r in a **** load of trouble. And many videos on Youtube seem to confirm that. It's nearly impossible to escape if you pass the "event horizon".

That means that missing a 400ft beam ship is just not enough. Passing it at 50ft is going to set you off down the slope! Happy skiing!

rramsey 26-08-2017 07:13

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
another nice video demonstrating this effect of a ship passing at really low speed:
https://www.youtube.com/watch?v=93mCiEDKMLw

Lodesman 26-08-2017 09:26

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by JPA Cate (Post 2462618)
Dockhead,

Trig may be basic math to you, but I never had anything beyond high school algebra. Now, my guess is that many, maybe even most of the men here have that knowledge, but unless you propose a Trig for Dummies class here on CF, can you show us maths ignoramuses another way to do this, please?

Google "radian rule". You can estimate what course you need to take to achieve a desired passing distance, based on the information you get from radar, AIS or reasonable estimations. For example, you spot a small coastal freighter about 5 miles ahead and take a compass bearing. As it closes to about 3 miles, with the bearing steady, you've determined (either from the electronics or estimating the range and timing the closing rate) that the TCPA (time to CPA) will be 10 mins. If your speed is 6 kts, then you know that CPA position is 1 mile ahead of you (10 mins at 6 kts= 1nm). You can plot an imaginary X in the ocean at that point. If he maintains course and speed, the freighter will be at that X in 10 mins. Radian rule tells us that if you were to alter course be 6 you would open the distance on that X by one-tenth of your current distance from it. So 1/10 of 1nm is 1 cable or 200 yards. If you wanted to avoid the ship by 500 yards, you would need to change your course by 15. If you wanted 1/2 nm separation, you would need to turn 30, and so forth.

Dockhead 27-08-2017 07:29

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Lodesman (Post 2464655)
Google "radian rule". You can estimate what course you need to take to achieve a desired passing distance, based on the information you get from radar, AIS or reasonable estimations. For example, you spot a small coastal freighter about 5 miles ahead and take a compass bearing. As it closes to about 3 miles, with the bearing steady, you've determined (either from the electronics or estimating the range and timing the closing rate) that the TCPA (time to CPA) will be 10 mins. If your speed is 6 kts, then you know that CPA position is 1 mile ahead of you (10 mins at 6 kts= 1nm). You can plot an imaginary X in the ocean at that point. If he maintains course and speed, the freighter will be at that X in 10 mins. Radian rule tells us that if you were to alter course be 6 you would open the distance on that X by one-tenth of your current distance from it. So 1/10 of 1nm is 1 cable or 200 yards. If you wanted to avoid the ship by 500 yards, you would need to change your course by 15. If you wanted 1/2 nm separation, you would need to turn 30, and so forth.

Thanks very much. All these traditional skills are really useful. All but lost in this age of video game navigation.


All times are GMT -7. The time now is 06:11.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2021, vBulletin Solutions, Inc.


ShowCase vBulletin Plugins by Drive Thru Online, Inc.