COLREGS and Uncertainty


We’re having a “spirited” discussion over on Cruisers Forum about collision avoidance – one of the most controversial topics. It’s here:

If any of you guys wants to see how WAFIs think about some of these things, or even better, if you want to weigh in, have a look (you can register in 2 minutes, and there are a lot of commercial mariners on the forum).

A big part of the discussion is about whether you can safely cross a line of ships moving at 20 knots, in a yacht making 5 knots, if the ships are 1.25nm apart. Obviously suicide – some people even did maneuvering board plots on it – but some on there insist that it’s ok as long as you set up to cross behind one ship with a 180 foot (!) CPA. Others are attempting to explain that a 180 foot CPA is the same as a 0 CPA. Your comments would be very welcome, either here or in the CF thread. The root issue is what is a safe CPA, and why – your opinions would be very welcome and helpful.


It’d be better to have more room.

Cutting close behind one vessel and staying further from the bow of the second is a good technique but it would be prudent to contact both ships by VHF and tell them your plan.

From the point of view of the large vessel the wild card is not knowing what the yacht is up to.


Indeed. But the problem here is the difference in speed. The Relative Motion Line is only 14 degrees from the ship’s course line. So even for a very large Neo Panamax vessel, the yacht will already within 0.7 cables from the ship’s bow when he passes that point on the way to the close crossing behind. I don’t think any of you would allow us to approach you in that configuration – fine on your bow and with a basically 0 CPA.

If there were less difference in speed, you could maneuver by hand to pass close behind, but not here.


Deep-sea ships sailing coastwise in Japan face a similar problem crossing lines of coasters. In Japan there are two-way lines of coasters about a mile apart at about 8-12 kts.

The technique used by a deep-sea vessels to get across is to find via APRA the coaster on the starboard with a near zero CPA. Then then deep-sea comes right and puts that coaster on the port bow. Then as the coaster draws near the deep-sea comes left, always keeping that coaster on the port bow and gradually coming left, passes astern of the coaster.

This works because the much faster deep-sea vessel is in control of the situation. That’s not the case with a vessel at 5 kts.


That sounds right to me.

It’s similar to what I do crossing the English Channel in my yacht. The ships are lined up typically two miles apart, but with the complication that they are not usually exactly in line with each other. I try to pick two ships to pass between which are not going faster than 16 knots. I am typically making 8 to 9 knots myself (17 meter yacht, relatively large and fast). So I’m not exactly “in control of the situation”, but at least I have a fighting chance and the relative motion line isn’t so onerously steep. I line up to the lead ship with a small CPA and squirt across, while keeping a close eye on the ship behind. If I’m stand on (usually the case), I’m careful to call on the radio and explain what I’m doing.


The scenario you describe with the ships one mile apart seem to me to be pushing the limits.

Safer with more room, safer with contact via VHF, safer with AIS so no vessel ID error, safer in the day with good visibility.

Another way a 5 kt yacht could cross is to stop ahead (not dead ahead) of the vessels, say 3 miles ahead, turn so that the large ship will pass astern. This indicates that you do not intend to cross ahead. Then when the yacht is close and about broad on the bow of the large vessel the yacht swings around and heads for the stern of the large vessel.

In this way the yacht is "in control’ of the situation because it is too late for the ship to confuse the situation and turn.

From the point of view of a large vessel, given a close CPA the best situation is the being able to cross astern of a stopped vessel.


My dad was a firefighter and he spend many long nights studying for promotion tests. I liked to read some of the question books as a kid, especially the stuff about mass cuasilties.

But I only remember one question: “a fire truck, ambulance, police car and mail truck get to an intersection at the same time. All are indicating they are responding to an emergency. Who has the right of way?”

The answer was the postal truck… it’s federal.


A postal emergency? Was it a special mail truck with flashing lights and siren?


Yep… they even have their own swat team. The United States Postal Inspection Service is no joke :wink:



Thanks, I never thought of that, and it’s very clever. Making a close CPA less dangerous looking.

The only problem with this is that you will never be able to pass close behind that way. If there’s no defined channel or fairway, we will not be able to predict within a couple of cables were you will be when you pass. Maybe it will be easier when we are stopped – that removes our own variations in course and speed from the CPA error. But choosing the place to stop will still be a challenge, and you’d feel a bit like a sitting duck with no way on. Stopped a couple of cables from where you pass by, and getting under way from 0 after we see you starting to get past – we will pass quite far behind.

Edge of a TSS of course is different.


I had one of these assholes show up at school once and threaten to arrest me unless I told him why someone was shipping me firearms through the mail. I was pretty confused until he put two vintage brass paintball guns on the table that I had been waiting for. He thought they were firearms. When I explained what they were he seemed to insist they could fire shot gun ammunition as well. He didn’t think it was as funny as school security did when I told him to go find a .68 caliber shotgun shell, remove the bolt so you could load it, and then strike the protruding end of the shell with a hammer until it went off. We’ all stand around the corner, brass barrel stock makes good shrapnel after all.

What a fucking clown. More wasted federal money.


One factor in reducing uncertainty is aspect. This is the problem with the 180 foot CPA with a 6 kt yacht. The bridge watch will be watching the aspect and will be unsure of the yacht’s intentions.

Another technique would be steer as if planning to pass in a meeting situation (for example port to port) and then when the large vessel is so close as to be fully committed, again swing towards the stern and pass astern.

From the point of view of the ship I’d prefer to encounter a stopped yacht with his stern towards my planned track. Somewhat reduced uncertainty.


Thanks very much, that’s good to know.

I guess if I am stopped, then you can be sure about my position – allows you to control the crossing.

But if I’m stopped, I’m a sitting duck in case you get it wrong. I guess I would prefer the passing situation. This increases the closing speed, which somewhat sharpens the problem, but a clean pass can be judged pretty well by eye, and since I am still moving, I still have the power to alter course and give you more room if I think we’re too close – unlike the case where I am dead in the water.

Downside is that this would take a lot of room to set up, practically as much room as it would take to simply pass ahead of you.

I guess there are no easy answers.


An agreement via VHF would be preferable to plotting out a 180 foot CPA.


What about this? Say the yacht is turned to show a red light and is stopped right in the gun sights (masthead lights in line). If the large vessel doesn’t alter course and the range is down 200 meters the yacht only has to move 1/2 the beam plus a little margin. Even if the ship puts the wheel hard over seems like the yacht can still beat the ship.

Not saying it’s a good idea but how much risk is the yacht taking? Especially compared to ships in traffic with the wheel hard over?


OK, that makes sense. Thank you.


The problem with lining up for a tight CPA astern is that the larger vessel can not determine the intent of the yacht. This is why there is rule 8(b) Any alteration of course and/or speed to avoid collision shall, if the circumstances of the case admit, be large enough to be readily apparent to another vessel observing visually or by radar; a succession of small alterations of course and/or speed should be avoided.

In most cases the risk of a close CPA can be mitigated with a confirmation of intent via VHF.


I very rarely see vessels reacting at this distance. Id say 5-7 miles is average in my experience. I get nervous using time to CPA, not their distance from me. There are a lot of other factors that play into my comfort level though.


I was talking about large ships (800+ ft) in the open ocean. If they haven’t altered course by 5-7 miles I’d be worried that they’re not planning to and maybe altering course myself. It takes a bit of sea room to turn a huge container ship and when they’re going 20+ kts they close relatively quickly.


Yep I get that. Im always worried with those large ships. Many times they don’t maneuver at all because their definition of risk of collision and mine differ substantially. A lot of times those guys are comfortable a half mile CPA, whereas I’m more comfortable at that 1-1.5 mile CPA. I’m always the smaller vessel and I’ve had quite a few instances of them just completely forgetting their responsibilities altogether.