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'.

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

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.

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?

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

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

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

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.

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

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

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

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.

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

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)

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

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.

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

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.

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

: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

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

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.

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate 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!

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

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."