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View Poll Results: Is it safe to pass astern of a fast ship with a 180' CPA in open water
Sure if you have the chutzpah for it 4 6.67%
Sure but I would make it be safe 8 13.33%
I don't know if it would be safe or not 5 8.33%
No not safe at all, Silly to try 27 45.00%
No way at all! Far far too crazy 16 26.67%
Voters: 60. You may not vote on this poll

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Old 07-12-2017, 11:23   #1
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Is it safe to cross astern of a fast ship in open water with a 180 feet CPA

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?
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Old 07-12-2017, 11:32   #2
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Re: Is it safe to cross astern of a fast ship in open water with a 180 CPA

Short answer: yes, it is safe.

What kind of "ship?" Is restricted in its movement (e.g., container) or responsive?

If it were me and the other vessel was not restricted (i.e., a small vessel), I would wait for 10 out of the 12 minutes it would take to close that 1 mile distance and then turn in a way that makes it obvious I was going to go astern. If it was a restricted vessel, I would get on the VHF and let them know that I was aware of their course and speed, and that it was my intent to pass well astern.

Of course, all of these assume that it is the stand-on vessel.

Pete
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Old 07-12-2017, 11:44   #3
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Re: Is it safe to cross astern of a fast ship in open water with a 180 CPA

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Originally Posted by pete33458 View Post
Short answer: yes, it is safe.

What kind of "ship?" Is restricted in its movement (e.g., container) or responsive?

If it were me and the other vessel was not restricted (i.e., a small vessel), I would wait for 10 out of the 12 minutes it would take to close that 1 mile distance and then turn in a way that makes it obvious I was going to go astern. If it was a restricted vessel, I would get on the VHF and let them know that I was aware of their course and speed, and that it was my intent to pass well astern.

Of course, all of these assume that it is the stand-on vessel.

Pete
Hi, Remember that the point is the 180' CPA. You on the boat need to get within 180' of the ship.

What you proposed to do (to wait for 10 minutes) is of course the right thing to do (one of the right things to do).

The question is: Is it safe to get close enough to the ship so as to pass within 180' of it.

Of course COLREGS requires that you and the ship do specific things. And will do them long before you can get within 180' of the ship.

The trick here is to imagine the actions needed to get within 180'....
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Old 07-12-2017, 12:40   #4
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pirate Re: Is it safe to cross astern of a fast ship in open water with a 180 CPA

Depends on the sea state and the size of your boat.. easier to ease off the genoa and close at 4knots.. that should alter the CPA to at least 500'..
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Old 07-12-2017, 12:47   #5
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Re: Is it safe to cross astern of a fast ship in open water with a 180 CPA

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Depends on the sea state and the size of your boat.. easier to ease off the genoa and close at 4knots.. that should alter the CPA to at least 500'..
Of course slowing down to pass at a safe distance astern is a good way to go.

The question is more specific than that. Some think that it is safe to pass 180' behind the ship. That is the specific question.

Can you pass 180' behind a ship doing 20 kts safely....
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Old 07-12-2017, 12:57   #6
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pirate Re: Is it safe to cross astern of a fast ship in open water with a 180 CPA

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Originally Posted by evm1024 View Post
Of course slowing down to pass at a safe distance astern is a good way to go.

The question is more specific than that. Some think that it is safe to pass 180' behind the ship. That is the specific question.

Can you pass 180' behind a ship doing 20 kts safely....
NO.. you'd be to close to his beam and the bow wave is the main threat..
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Old 07-12-2017, 13:06   #7
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Re: Is it safe to cross astern of a fast ship in open water with a 180 CPA

Now that the question is a little clearer to me, I would say no as well, why go within 180 feet?

Pete
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Old 07-12-2017, 13:17   #8
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Re: Is it safe to cross astern of a fast ship in open water with a 180 CPA

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Originally Posted by pete33458 View Post
Now that the question is a little clearer to me, I would say no as well, why go within 180 feet?

Pete
Hard for me to be clear at times. Sorry about that.

180 feet behind the ship doing 20 kts (33 fps)....

I'm not sure if anything can be done but if anyone voted and wanted to change their vote perhaps the mods can do something.
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Old 07-12-2017, 13:22   #9
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Re: Is it safe to cross astern of a fast ship in open water with a 180 feet CPA

For safety you want to be able to be seen at all times by the bridge watch on larger, faster ship. If you can't see their navigating bridge, they can't see you.

So, trying to cut close behind the larger ship may be, in relative terms, "safe", as in is is safer than trying to pass 180 feet in FRONT of the faster ship. However, unless you are confined waters with restricted maneuverability the safest thing to do is to provide plenty of sea room between your vessel and another -- especially when the other is bigger and faster than your vessel.

As a professional merchant ship deck officer for many years, some of the most terrifying incidents I ever had was when a small craft cut in front or behind my ship where I couldn't see them. I usually stopped breathing until I could see the smaller vessel again, because I wouldn't feel the collision if it did happen.
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Old 07-12-2017, 13:27   #10
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Re: Is it safe to cross astern of a fast ship in open water with a 180 feet CPA

If you're talking about a point 180' dead astern, and assuming you won't get swamped by the bow wave or sucked into the screw, and the ship is less than 90' wide, you can theoretically hit that point, but it won't be your closest point of approach.

And of course it is neither prudent nor safe, but I'm not sure if we're talking theory or practice here.
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Old 07-12-2017, 13:36   #11
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Re: Is it safe to cross astern of a fast ship in open water with a 180 feet CPA

Quote:
Originally Posted by evm1024 View Post
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?
Trying to understand the perspective of your question and why anyone on a small slow sailboat, would WANT to pass a large fast ship at a CPA of 180ft or less than 0.3 of a cable?

If it is meant to be a calculation excercise, the reality is that at such a planned and close CPA,
.........the relative speeds and angles are far too tight to insure a safe outcome.

Unless there were last minute extenuating circumstances of multilateral considerations, it would be an unsafe solution to plan for at 4nm away

With your CPA aspect being as you are disappearing at the box ship's stern quarter, it is too easy for that ship to make the wrong sharp avoidance maneuver to bring you in even closer.

The ship's prop wash and squatted wave form displacing huge tonnages of water is just unnecessarily damaging to the small boat
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Old 07-12-2017, 13:54   #12
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Re: Is it safe to cross astern of a fast ship in open water with a 180 feet CPA

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?
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Old 07-12-2017, 14:00   #13
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Re: Is it safe to cross astern of a fast ship in open water with a 180 feet CPA

Quote:
Originally Posted by evm1024 View Post
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?
If the only reason to attempt is to save time or have the "luxury" of not changing course or speed, then no, it is not safe. That's putting convenience over prudence, and giving deference to prudence is often the first pillar of safety.

Is it possible, if it's absolutely necessary? Of course, in good conditions. Would I do it in a deal flat calm sea, clear sky? Maybe. At night in a gale with rain and lightning and no moon? No, I would not unless I had absolutely no other better choice. 180' is a remarkably short distance in bad conditions with poor visibility no matter what your instruments are telling you.

And as Boatie pointed out I would likely be most concerned with the bow wave.
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Old 07-12-2017, 14:14   #14
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pirate Re: Is it safe to cross astern of a fast ship in open water with a 180 feet CPA

Don't remember the exact details but a few years back a 36ft yacht was capsized and four or five lives lost when it was swamped by the wake of a Cross Channel ferry E of the Isle of Wight..
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Old 07-12-2017, 14:17   #15
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Re: Is it safe to cross astern of a fast ship in open water with a 180 feet CPA

Let's assume that you are the give-way vessel.

The COLREGS require that you make a clear and obvious change in course and speed early enough so that the stand-on vessel knows what you are doing and doesn't have to worry about taking evasive action of his own.

Under the premise you describe, it is very hard to see how the captain of the larger vessel can be sure of your intentions. You are in clear violation of Rule 8(b):

(b) Any alteration of course 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 or speed should be avoided.

This is similar to a sailboat approaching me who has decided that he can clear my stern by 5 feet. Maybe he can, but I have no way of knowing what his intent is, or even if he sees me. That forces ME to take evasive action, which might make things worse.

So, is it safe to pass 180 feet astern of a larger vessel? Yes... usually. However it is UNSAFE (and illegal!) to do so in such a way that the large vessel can not be sure of your intent.
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