Is it safe to cross astern of a fast ship in open water with a 180 feet CPA

[Dockhead]

Originally Posted by Kelkara

Ignoring the size of the ship and consequences of its waves etc ... I just thought I'd comment on what is involved in the game of chicken that will be needed to even get that close ...

The first image is how this situation will appear on radar with a 180' CPA, the second is with a 0' CPA collision, showing the ship's relative position every minute. The red dots show when the bearing of the ship is less than 5.5 degrees change from the initial sighting. Even by the time the ship is just one mile away the bearing has changed by just 1 degree. How confident will the skipper of either boat or ship be that they can tell the difference between these two scenarios?

Until the last minute the bearing change over the previous 10 minutes will be less than 5 degrees, by which time it is far too late for the ship to do anything ... so it is now all down to the sailboat ... supposing in this final minute you decide that in fact you misjudged and this is actually a collision course ... the scenario we are now considering is that there is a ship 0.3 miles away from you bearing down on you at 20 knots, you have less than one minute to get out of the way ... Is this a safe scenario? Or would you have bailed out much earlier?

A lot of people have spent a lot of time on this question, I think this is the best work done by any of us.

One of the many things I learned from this puzzle is how hard it is for some people to visualize what such a crossing would look like.

At this difference of speed, a crossing that close is actually a collision course (as Captain Bill pointed out -- since you cannot have any information accurate to 180 feet) with the ship approaching almost on your beam. As you say -- a game of chicken. You count on being able to see it and correct any error later in the crossing, but you can NEVER actually see it -- once you get a perceptible bearing change (and that was a brilliant idea to analyze that ), by that time, different parts of the ship will have perceptibly different bearings anyway.

So in the end, you will either be lucky and fall short, and never get anywhere near that close, or you will be unlucky and overshoot a little bit, and end up very possibly right under his bows, with very likely no time to get out of his way.

The question was whether or not it is safe to do it -- actually the question should better be whether it is safe or not to ATTEMPT it. And of course, it is not.

We are also ignoring the question of what will the ship do? But this is an important question. No ship making full sea speed in open water is going to allow you to approach him on such a course. If you somehow manage to get within a couple of miles on such a course, he will alter course. Let's hope safely. But if he doesn't see you until a few cables off, and tries something desperate, you could be in a world of hurt.

So I would have liked to see another poll choice: "Can't be done and reckless to try."

[Dockhead]

Originally Posted by newhaul

Can't say in till I have more information . What kind and of ship? what is his hull shape?
Here we have all manner of ship from catamaran fast ferries to navy aircraft carriers. All of which present their own unique set of variables. The fast ferry I would have no issue even 100 feet from her . The carrier I would want to be at least 1500 ft away from her. So the answer I would give wasn't in the poll and that is " it depends "

Certainly -- if you're talking about a fast cat ferry like we have over here -- making 38 to 45 knots? -- you have no influence on the crossing anyway, in a boat making 5 knots (or even 7 or 8 knots). You are standing still.

In the Solent, we have the Red Jet ferries making 4 crossings every hour during busy times. I learned to just ignore them with time. They steer around us like playing a video game, and if they get it wrong, there is nothing we can do about it.

[newhaul]

Originally Posted by Dockhead

Certainly -- if you're talking about a fast cat ferry like we have over here -- making 38 to 45 knots? -- you have no influence on the crossing anyway, in a boat making 5 knots (or even 7 or 8 knots). You are standing still.

In the Solent, we have the Red Jet ferries making 4 crossings every hour during busy times. I learned to just ignore them with time. They steer around us like playing a video game, and if they get it wrong, there is nothing we can do about it.

well there is their maneuverability but I was actually more think of the wakes and bow waves produced.

[Dockhead]

Originally Posted by newhaul

well there is their maneuverability but I was actually more think of the wakes and bow waves produced.

Right -- I see, and that's a good point.

But concerning "maneuverability" -- it's their SPEED which is the main thing, not their maximum ROT. The greater the difference in speed, the less influence the slow vessel has on how the crossing will go.

[evm1024]

Originally Posted by Dockhead

SNIP


So I would have liked to see another poll choice: "Can't be done and reckless to try."

Very true. I did not include that option because the actual option would be "can't be done by design, only by blind luck and reckless (or not i.e. collision) to try".

It is interesting to see the various reactions to the question. A number of people have not visualized the actuality of the crossing yet. They start with the ship already at close quarters and think about how they would cut under the stern.

But many also do see due to the variables (cones of uncertainties) that they at 4 miles out (from the ship) have to attempt a collision course hoping that they will be able to cut behind the ships stern.

Many also realize that to attempt this should and would scare the crap out of them. It is in no way comparable to running parallel to a ship in a crowed harbor and then darting behind its stern at the proper moment.

We are basically blind in attempting this crossing.

[Dockhead]

Originally Posted by evm1024

Very true. I did not include that option because the actual option would be "can't be done by design, only by blind luck and reckless (or not i.e. collision) to try".

It is interesting to see the various reactions to the question. A number of people have not visualized the actuality of the crossing yet. They start with the ship already at close quarters and think about how they would cut under the stern.

But many also do see due to the variables (cones of uncertainties) that they at 4 miles out have to attempt a collision course hoping that they will be able to cut behind the ships stern.

Many also realize that to attempt this should and would scare the crap out of them. It is in no way comparable to running parallel to a ship in a crowed harbor and then darting behind its stern at the proper moment.

We are basically blind in attempting this crossing.

I agree. It's a very interesting puzzle which smokes out how well people can visualize this. Thanks to the guy who invented it

I think a common problem people have with visualizing this problem is that they universalize their experience in pilotage waters, where you can see where the ship is going, and just head for that. Run down its side outside of the channel, for example, and dart across the stern after he passes. Open water brings completely different problems.

[Dockhead]

Originally Posted by Captain Bill

I'm assuming your 180ft cpa was determined by AIS as MARPA isn't going to get you that good of a plot and actually neither is AIS. Even AIS is giving you a distance from his AIS GPS antenna. Do you know where on a 1000 ft ship the antenna is located. Unless it's located on the tip of the stern (I doubt they ever are) you might well be running into some point on their hull rather than 180ft astern.

The heart of the matter

[markpierce]

Originally Posted by evm1024

I just thought I would take this poll on crossing a fast moving ship in open water.

The question is:

You are in a boat doing 5kts and there is a ship doing 20 kts about 4 miles away.

Your crossing angle is 90 degrees and the boat is about 1 nm from the path of the ship. The ship is 4 nm from the boats path.

Do you think it is safe to attempt to pass astern of the ship with a 180' CPA?

Your question is very confusing, so I'm not answering the poll. Is the boat four miles away from the ship or one mile? ... is the boat on a course 90 degrees to intersect the ship's course? Regardless, if the ship was a mile away going 20 knots and the boat is going 5 knots directly headed for the ship 90 degrees to its path, the boat will pass miles behind the ship as one would cross the ship's course at least ten minutes behind the ship.

I favor crossing behind a ship if there is any doubt of safely (with a wide margin) crossing in front.

[TheThunderbird]

That is the type of Qs very much raised after the introduction of AIS
I feel like it is similar to giving a gun to blind people.

No offence intended, please, but l can't better express my mind.

PS being a structural engineer, l invite to apply a Safety Factor of 7-10X to any calculation, people's life being involved...

Enough of it, please.
IT GIVES BADLY ADVISED PEOPLE THE FEELING THAT THIS IS A GOOD WAY OF REASONING AT SEA.

[StuM]

Originally Posted by markpierce

Your question is very confusing, so I'm not answering the poll. Is the boat four miles away from the ship or one mile? ... is the boat on a course 90 degrees to intersect the ship's course?

Seems crystal clear to me. At the initial point in time, the vessels paths differ by 90° (i.e. they are on perpendicular tracks.) The slow vessel is one mile from the TRACK of the fast vessel. The fast vessel is 4 miles from the TRACK of the slow vessel.

[evm1024]

Originally Posted by markpierce

Your question is very confusing, so I'm not answering the poll. Is the boat four miles away from the ship or one mile? ... is the boat on a course 90 degrees to intersect the ship's course? Regardless, if the ship was a mile away going 20 knots and the boat is going 5 knots directly headed for the ship 90 degrees to its path, the boat will pass miles behind the ship as one would cross the ship's course at least ten minutes behind the ship.

I favor crossing behind a ship if there is any doubt of safely (with a wide margin) crossing in front.

There is a right triangle. The 90 degree corner is the point where you and the ship will be closest. The base of the triangle is 4 miles long and represents the path of the ship. The adjacent side is it your ( the boats ) path it is 1 mile long.

The ship is doing 20 kts and will travel the 4 miles to the 90 degree corner in the same time it will take the boat to travel its 1 mile path.

The hypotenuse of the triangle is the distance between the boat and the ship. It is just a bit more than 4 miles. (4.12 miles)

From the ship the boat appears about 14 degrees off the bow a little more than 4 (4.12) nm away.

From the boat the ship appears around 14 degrees forward of the beam about 4 miles away. We would not say that the ship was 76 degrees off the bow...

Does this help?

The key here is not the exact geometry but rather can you get close to a ship going 4 times your speed and predictable pass under its stern with a CPA of 180'.

[rhubstuff]

A lot of things that people think are "safe" are also stupid. Go ahead and pass close behind a large, fast ship if you: 1. Don't mind taking 3 or 4 green waves over the bow. 2. Like to scare the crap out of professional sailors for no reason (whether or not you are the stand-on vessel. And 3. Like to help ruin the reputation of reputable, experienced sailors that follow the rules and practice courtesy at sea. And if its a naval vessel its a good way to get a burst of .50 cal. fire across your bow. What happens after that is not up to you.

[Cheechako]

I'm not really sure what this question is really about. I have passed close astern of large freighters many times on Puget Sound. They probably weren't doing quite 20 knots in the Sound but it's a non issue. The only large wave produced by these boats seems to be at the bow and dissipates very quickly.
Here's a smaller 200-300 ft ferry wake traveling ~25 knots.
https://www.shutterstock.com/video/c...n-seattle.html

[Dave_S]

You guys who are saying this is Ok must be imagining some other ship than me. I see a 1000' container ship.

I don't believe anyone would intentionally set a course to pass 180' behind a container ship travelling at 20kn at 90° to it's course.

This would mean when you are 430' from its line you are aiming at the bow of a 200' tall, half a million tons travelling at 20kn.

Nah, not happening.

[markpierce]

Originally Posted by StuM

Seems crystal clear to me. At the initial point in time, the vessels paths differ by 90° (i.e. they are on perpendicular tracks.) The slow vessel is one mile from the TRACK of the fast vessel. The fast vessel is 4 miles from the TRACK of the slow vessel.

Thanks for the clarification. ... In that situation, I'd continue on course until the ship is two miles of intersection. If collision remains imminent, I'd likely turn 90 degrees to starboard to parallel the ship's course and then turn onto original course after ship passes.