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

[Dsanduril]

Maybe we need to focus on the simple 'is it safe' part of the question rather than the 180'. As asked the answer to the question must be 'no', because as asked the question assumes nothing more than average skill and knowledge on the part of both mariners. And having seen the average worldwide, including on massive ships, no, it is not safe.

Had the question asked 'is it safe for a skilled/knowledgeable mariner' then the answer might be 'it can be safe'. Reduced ad absurdum we can look at the Blue Angels and their ilk - they do maneuvers with much tighter tolerances, at much higher speeds, in 3D every day. And 99.9(9?)% of the time they live through it with no damage. But they are also very skilled at their craft, and practice the tight maneuvering every day. And every once-in-a-while they die in the attempt. I wouldn't ask the average pilot to attempt anything even remotely that risky or complicated.

Take a look at another very sad thread here where the husband went overboard and the wife could not handle the boat to get back to them. Certainly not safe for her to attempt this maneuver, and probably not him. Reading from the outside (which is really speculation), they didn't practice for such events, and many (most?) mariners don't. Those who practice maneuvering, plotting, close quarters work, etc. could probably make this a safe crossing, but for the rest of us...

[kish]

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?

Total madness to even think of approaching so close to a ship, any ship..
180 feet from where?
180 feet is too near the accuracy limit of (+/- 10 meters or 33 feet) of gps.
Where is the gps located on the ship reporting its' AIS transmission (bow or stern)and the update period of the AIS transmission for a ship moving at 20kis another factor.

This guy tried it, crossing ahead, with disastrous result, but no fatalities.

https://www.youtube.com/watch?v=_tUoUxzt9sI

[Don C L]

Yeah I would like to ask how many have been (intentionally) within even 500 feet of a tanker doing 20 knots in open sea or not? (NOT that it is something to brag about) ....and why are they conducting a race in a shipping channel?
just wondering.

[Paul Elliott]

Originally Posted by Don C L

and why are they conducting a race in a shipping channel?
just wondering.

In San Francisco, any race that crosses or leaves the bay will involve several shipping channels. If a racer gets five blasts on the horn, they are usually disqualified. If they survive.

[kish]

Originally Posted by Don C L

Yeah I would like to ask how many have been (intentionally) within even 500 feet of a tanker doing 20 knots in open sea or not? (NOT that it is something to brag about) ....and why are they conducting a race in a shipping channel?
just wondering.

Southampton on England's south coast is a very busy commercial port and its waters are home to several Cowes yacht clubs, which have run regattas since 1826.

The speed and density of commercial shipping there, would in modern times prohibit running regattas in restricted commercial waters, but the British believe in tradition and carry on regardless.
Sacred Cowes!!

[hpeer]

Originally Posted by Don C L

Yeah I would like to ask how many have been (intentionally) within even 500 feet of a tanker doing 20 knots in open sea or not? (NOT that it is something to brag about) ....and why are they conducting a race in a shipping channel?
just wondering.

Not open sea but I’ve been passed or have passed many freighters in confined waters. Overtaken by a auto carrier in the C&D canal, he was doing 13 knots and our separation was probably under 100’. Also passed by a freighter coming into Halifax in pea soup. I went out of the channel and over some ledges to be safe. Radar, AIS, Harbor Control directing us. I could hear and feel him but didn’t see him until much latter in the inner Harbor where the fog cleared. That was probably within 200’ and he was doing about 12 knots.

In open sea I strive to keep at least 1nm separation.

[markpierce]

Originally Posted by Paul Elliott

In San Francisco, any race that crosses or leaves the bay will involve several shipping channels. If a racer gets five blasts on the horn, they are usually disqualified. If they survive.

Fortunately, ships generally travel at 12 knots or less there (San Francisco Bay/Delta/Estuary = confined waters) making their wakes inconsequential. High-speed ferries, some recreational boats, and some tugs are another matter. Easy to deal with ships there from a ColReg perspective because they're always the stand-on vessel and can take no or little attempt to avoid a collision if you place your boat in harm's way.

[Dockhead]

Originally Posted by kish

Southampton on England's south coast is a very busy commercial port and its waters are home to several Cowes yacht clubs, which have run regattas since 1826.

The speed and density of commercial shipping there, would in modern times prohibit running regattas in restricted commercial waters, but the British believe in tradition and carry on regardless.
Sacred Cowes!!

This is correct, and well analyzed.

That accident occurred during Cowes Week -- the world's longest-running and pre-eminent yacht racing event, and the prototype for Kiel Week and innumerable other regattas around the world. Cowes is also the home of the Fastnet and other RORC races, the Round the Island Race, and is where the America's Cup race originated. Yacht racing has something like sacred status here.

[valhalla360]

Originally Posted by evm1024

One other way to look at this is to ask:

In what conditions would it BE SAFE to attempt crossing aft of a ship going 20 kts with a CPA of 180' in a boat going 5 kts at right angles to each other in open waters?

Why is still the critical question. Given the variability in speed and heading over the time frame, to purposely to so in open water is stupidity.

I did not specify if it was a sailboat or power boat because it does not change the safety of the crossing.

Certainlly it changes it. Under power, you can more reliably control your speed and heading. With a sailboat, a big container ship,
might blanket the wind and leave you with little or no control.

I did not specify who was stand on because it does not change the safety of the crossing.

Yes, it does change the safety as it changes the expectations to take action (though challenging a ship for right of way is again stuipd)

I did not specify the sea state, weather, fog etc because it does not change whether the crossing is safe or not (if it is unsafe in ideal conditions). If it is unsafe in clear weather, calm seas then adding in bad weather only increases the risk. Safety is the absence of risk by definition.

It certainly changes the safety. In fog, its much easier to misjudge the crossing. Safety is not the absence of risk but risk kept to acceptable levels.

I did not specify why anyone would attempt the crossing because it does not change the safety of the crossing.

While it does not change the safety, it does change what would be an acceptable risk.
- On a clear sunny day out for a daysail, the risk may be unacceptable.
- If a madman has rigged a bomb set to trigger if your heading changes or your speed drops below 5kts, it's the "safe" course of action. (that's about the best I could come up with for a reason)

I did not specify the color of the boat.....

All of my did not's above were written with a light hand with no intent to harm or chastise anyone.

Sorry but to get a firm answer requires many more details if you want a good answer...other than for 99.99999% of scenarios, just don't do something so stupid in the first place and it's a non-issue.

[evm1024]

Originally Posted by valhalla360

Sorry but to get a firm answer requires many more details if you want a good answer...other than for 99.99999% of scenarios, just don't do something so stupid in the first place and it's a non-issue.

Mike,

I see that you think that it is not safe.

Given the variability in speed and heading over the time frame, to purposely to so in open water is stupidity.

I'm not sure where you are coming from with your answers in red. Could you explain them in a bit more detail.

It certainly changes the safety. In fog, its much easier to misjudge the crossing. Safety is not the absence of risk but risk kept to acceptable levels.

noun, plural safeties.
1.
the state of being safe; freedom from the occurrence or risk of injury, danger, or loss.

How does the boat and the ship detect each other in a fog? Oh, you assumed that they both have radar, That the ship radar actually gets an acceptable return when painting the boat and that the boats radar is on, working, someone looking at it.

And none of that has anything to do with the question.

Point or order:

What is it you are trying to achieve with your posts in this thread? What are we missing?

It is a simple question that can be answered yes or no. Is it safe to attempt to pass under the stern of a ship (5kt, 20kt, 180' cpa, etc as described)?

If you think that it can be done safely please describe how you would do it.

[waterman46]

Do not get that close if wind direction is such that ship may blanket you, causing loss of control just as you enter his bow waves or wake turbulance. Let ship know, when about 1 mile off, that you intend to pass astern, by either radio contact and/or luffing up to slow and give visual indication of such.

[valhalla360]

Originally Posted by evm1024

Mike,

I'm not sure where you are coming from with your answers in red. Could you explain them in a bit more detail.

You've made up a situation where speed, headings and position is known with perfect accuracy. Reality is, visually, you will be lucky to assess the positions to within a 1/4 mile. Even with radar, you might be off by 500'. So there is no way to start the maneuver knowing you will pass within 180' of the ship. If you are really doing 5.1kts, you likely wind up hitting the ship broadside.

noun, plural safeties.
1.
the state of being safe; freedom from the occurrence or risk of injury, danger, or loss.

If you go with this definition, motoring on a calm sunny day in deep water with no other boats around is not safe as a meteor could fall from the sky and sink the boat.

How does the boat and the ship detect each other in a fog? Oh, you assumed that they both have radar, That the ship radar actually gets an acceptable return when painting the boat and that the boats radar is on, working, someone looking at it.

No it could be a light fog or haze where you can just barely make out the ship but even if you had radar, you can't assume you don't miss something.

And none of that has anything to do with the question.

Point or order:

What is it you are trying to achieve with your posts in this thread? What are we missing?

It is a simple question that can be answered yes or no. Is it safe to attempt to pass under the stern of a ship (5kt, 20kt, 180' cpa, etc as described)?

If you think that it can be done safely please describe how you would do it.

The math problem is easy but without a context, the question makes no sense. It's like saying:

Is it safe to stand in the middle of the road with a semi bearing down on you at 70mph and you step out of the way at the last second?

If you manage to step out of the way, it's perfectly safe but you can't count on timing it perfectly and without a reason to do so, it's just a silly question.

You suggested there was a history to the question. Can you share what the history is?

[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?

And that's really the heart of the matter --

There are no means available which can distinguish a 180 foot CPA from a collision course, not even AIS is accurate enough. So we even can't set it up. You might theoretically set up what is really a collision course and then try to steer off down his side when you get really close, and then dart in behind, but this would be an insane kamikadze maneuver, depending mostly on luck to not end up under his bows, because by the time you get close enough to really see how you're crossing, it will be too late for you to move very far, to move out from under his bows.

They key to visualizing this is understanding that at this speed difference, the relative motion line is only 14 degrees off his course line. He will bearing down almost on your beam and you will have very little control over the crossing. You are more like a sitting duck.

It would be completely different if his speed is the same as yours. Then he will be well ahead of your beam as you approach, and you will be able to see his quarter. From this angle, you have a lot of control over the crossing, and if you set it up ahead with AIS, then you will be able to correct as you get closer.

[evm1024]

Originally Posted by Dockhead

And that's really the heart of the matter --

There are no means available which can distinguish a 180 foot CPA from a collision course, not even AIS is accurate enough. So we even can't set it up. You might theoretically set up what is really a collision course and then try to steer off down his side when you get really close, and then dart in behind, but this would be an insane kamikadze maneuver, depending mostly on luck to not end up under his bows, because by the time you get close enough to really see how you're crossing, it will be too late for you to move very far, to move out from under his bows.

They key to visualizing this is understanding that at this speed difference, the relative motion line is only 14 degrees off his course line. He will bearing down almost on your beam and you will have very little control over the crossing. You are more like a sitting duck.

It would be completely different if his speed is the same as yours. Then he will be well ahead of your beam as you approach, and you will be able to see his quarter. From this angle, you have a lot of control over the crossing, and if you set it up ahead with AIS, then you will be able to correct as you get closer.

Truly the central core of the matter. The speed differential is the killer.

I'm working on a perl script that does model the crossing. I was going to just let it sweep through variations in speed and heading and get the collisions that could happen.

But, thinking more about it I may make it interactive. Start with the crossing setup as described, let the code pick a random variance in speed and angle for the ship.

Then every 15 seconds you the boat driver gets to see the angle to the ship and perhaps speed of the ship and then input a course correction and/or speed correction to the boat.

I wonder how many collisions there would be? I wonder how often one coule get to that mystical 180' CPA.

Slowly working on it.

[Dsanduril]

Originally Posted by Dockhead

...It would be completely different if his speed is the same as yours....

Originally Posted by evm1024

Truly the central core of the matter. The speed differential is the killer....

Is the speed difference really the crux of the question? If he is going our speed (5 knots) then all things proceed sedately, and the equation changes considerably.

OTOH, if we are both doing 20 knots (no speed difference) then I, as a small boat, am more maneuverable, but I am also on a plane and have less reaction time. So what happens when I take my 20-30-40-50 foot boat, at 20 knots on a plane, right under his counter and hit the bow wave and then the wake? Think I'd much rather take on that task at 5 knots than at 20. So, is the difference really that critical, or is it simply that even one vessel at 'high' speed makes the situation less safe?