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

[valhalla360]

To intentionally do it is stupid. So far no scenario has been provided where it makes sense to try and do this.

If things go wrong and you end up 180' off the stern...Depending on a wide variety of variables, it can vary from a bit nerve wracking to death & destruction.

[Dockhead]

Originally Posted by valhalla360

To intentionally do it is stupid. So far no scenario has been provided where it makes sense to try and do this.

If things go wrong and you end up 180' off the stern...Depending on a wide variety of variables, it can vary from a bit nerve wracking to death & destruction.

Yes. But note also that if completely depends on the relative speed. If the ship is moving slower than you are, then it is completely safe so long as you keep a careful watch. You can pass even 100' from the stern of a ship moving slower than you are, or at anchor.

It gets harder and harder, the greater the difference in speed. At the same speed as you, the ship will be half way between dead ahead and directly abeam -- so 45 degrees ahead of your beam. You can easily correct to point behind him, when and as necessary. Once you get close, you will still be quite far away from his course line, so still easy to correct. You will be able to see his quarter. But the faster he is going, the further back he will be coming from, and the closer you will get to his course line earlier in the crossing. At a 4:1 difference in speed like in this scenario, he will be coming at you from almost abeam of you, and when you start to get close, it will look like you are going to get run down. So you have less and less control, and can see less and less well how the crossing is developing. You won't see his quarter by the time you're very close and you can't tell the difference visually between a 180 foot pass and coming right under his bows.

[valhalla360]

Originally Posted by Dockhead

Yes. But note also that if completely depends on the relative speed. If the ship is moving slower than you are, then it is completely safe so long as you keep a careful watch. You can pass even 100' from the stern of a ship moving slower than you are, or at anchor.

The premise was a 20kt ship against a 5kt sailboat. There is no scenario I can think of where it's a good idea to try to get within 180' of the stern.

Of course, I wouldn't agree with the idea that it's safe as long as you are faster. Saw a ship going up the Savanna River at around 10kts but fighting the current. A 90' sports fisher came out of a side channel on plane and proceeded to stuff the bow after launching off the wake. I would guess they were a similar distance behind the ship. They said they were fine but the helmsman had smacked his face on he wheel and they were idling away when we turned up the channel.

[micah719]

The question is too vague.

Are the vessels under sail or under power?

Where is the wind and tide?

Which vessel is the stand-on vessel?

The only certain fact from the question is that they are on a collision course, and the only conclusion is the vessel to starboard has right of way and that the other must change course or speed.

Without further info, if I were on the slower vessel I would alter course 45 degrees towards the present bearing of the faster vessel.

In 9 minutes the ship will be dead ahead 0.75nm.

In 12 minutes the ship will be at the crossing of the original courses, I will be 0.3 nm off his beam, from where a Mk13 torpedo would hit in 5.3 seconds. I have lost 3 minutes 36 seconds off my original course, and 2216lb of cargo I'd rather not explain to customs.

[hamburking]

NO, it is not safe. Your margin of safety is too small. You can't count on the other vessel maintaining their speed or course. And they can't count on you. You are on a collision course...take action. A change of course of just a couple degrees is all it takes.

You are looking to avoid a catastrophe. I am looking to avoid the possibility of a catastrophe. Understand the difference.

On open water, give them at least a mile. Mid ocean, 5 miles is better.

[evm1024]

Originally Posted by valhalla360

The premise was a 20kt ship against a 5kt sailboat. There is no scenario I can think of where it's a good idea to try to get within 180' of the stern.

Of course, I wouldn't agree with the idea that it's safe as long as you are faster. Saw a ship going up the Savanna River at around 10kts but fighting the current. A 90' sports fisher came out of a side channel on plane and proceeded to stuff the bow after launching off the wake. I would guess they were a similar distance behind the ship. They said they were fine but the helmsman had smacked his face on he wheel and they were idling away when we turned up the channel.

Actually:

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

We do not know if the boat is under sail or under power.

The question is designed to get us to think about what it actually takes to get close to a ship going 4 time faster than we are.

[newhaul]

Originally Posted by evm1024

Actually:

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

We do not know if the boat is under sail or under power.

The question is designed to get us to think about what it actually takes to get close to a ship going 4 time faster than we are.

to actually intentionally get that close to a ship in that given scenario takes just two things.

)1balls the size of an elephant
2) a good measure of stupidity thrown in .

[Don C L]

Like a moth to a flame I am drawn again to reading these threads and scratching my head. Now as I understand it we are out in open water and the supertanker is going 20 knots. Sure sailing around in LA harbor there is no issue, but discussing how close you can get to a ship doing 20 knots and be safe.. with a CPA of 180' maybe? is, well, how can one say this politely? Odd?
I regularly have to cross a busy shipping lane. Unless I check on AIS, and the figures are right, I won't know their speed. Of course if the approaching ship's bearing is not changing, then I will be slowing down so that the bearing IS changing. I stay out of their way until I am sure they will pass well ahead of me, as in like a half mile, at least... which, since they are going 20 knots means I only will have to wait a few minutes! My concern is often I need to get across the shipping lane before that ship that is behind it on the horizon comes down on me.
As far as what it takes to get within 180' of a ship doing 20 kts and I am dong 5 kts (flat sea, no wind or current) I am sure there is a (fairly) simple mathematical solution to that. I don't know what it is, but my curiosity is piqued.

[Pete7]

Originally Posted by markpierce

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.

I disagree, I don't think you will have the opportunity to do this in Stu's example, which I have copied into this post.

You as the stand on vessel and therefore have an obligation, if you attempt to do something else it is likely to cause absolute confusion on board the ships bridge. After all at 4 miles the Ships Captain has probably already decided what he is going to do about the little yacht he can see in the distance. Assuming no other traffic or restrictions you describe in your later post No 71, then the Captain has probably ordered a turn to Starboard to pass astern of you, which is in the process of being carried out. Its just you haven't seen it yet. The Captain will assume you will continue towards the top of the image as the stand on vessel.

Should you alter course half way you could easily end up right in the new track of the ship and give the Captain no alternative. You could end up 180 ft in front on the bows never mind astern of the ship.

Pete

[evm1024]

Originally Posted by Don C L

Like a moth to a flame I am drawn again to reading these threads and scratching my head. Now as I understand it we are out in open water and the supertanker is going 20 knots. Sure sailing around in LA harbor there is no issue, but discussing how close you can get to a ship doing 20 knots and be safe.. with a CPA of 180' maybe? is, well, how can one say this politely? Odd?
I regularly have to cross a busy shipping lane. Unless I check on AIS, and the figures are right, I won't know their speed. Of course if the approaching ship's bearing is not changing, then I will be slowing down so that the bearing IS changing. I stay out of their way until I am sure they will pass well ahead of me, as in like a half mile, at least... which, since they are going 20 knots means I only will have to wait a few minutes! My concern is often I need to get across the shipping lane before that ship that is behind it on the horizon comes down on me.
As far as what it takes to get within 180' of a ship doing 20 kts and I am dong 5 kts (flat sea, no wind or current) I am sure there is a (fairly) simple mathematical solution to that. I don't know what it is, but my curiosity is piqued.

You are on your way to seeing the full impact of this attempted crossing.

With a 180' CPA planned (even assuming that you are on rails):

At 4 miles from the ship the ship appears to be around 14 degrees forward of your beam.
At 2 miles from the ship the ship appears to be around 14 degrees forward of your beam.
At 1 mile from the ship the ship appears to be around 14 degrees forward of your beam.
At 1 mile from the ship the ship appears to be about 14 degrees forward of your beam. (less than 3 minutes to a 180' CPA)

Now if we change it so that instead of a planned 180' CPA we have a bit of uncertainties (like we are not on rails etc) so that the actual CPA is 0. Which is to say we will hit the ship. Notice the change in aspect of the ship:

At 4 miles from the ship the ship appears to be around 14 degrees forward of your beam.
At 2 miles from the ship the ship appears to be around 14 degrees forward of your beam.
At 1 mile from the ship the ship appears to be around 14 degrees forward of your beam.
At 1 mile from the ship the ship appears to be about 14 degrees forward of your beam. (less than 3 minutes to impact)


The point being that even with the ship about 1 mile away you still cannot see the place in the featureless ocean 1 mile ahead of the ship and 0.25 mile ahead of the boat where they will "meet". The ship is still about 14 degrees ahead of your beam.

Look at another way. When you are a ship length (600') away from the path of the side of the ship the ship is still 0.34 nm away. (2330 feet).

[StuM]

Originally Posted by hamburking

You are looking to avoid a catastrophe. I am looking to avoid the possibility of a catastrophe. Understand the difference.

This!

[StuM]

I find it amazing that 8 people (at this stage) think they can "make it be safe"

[Scott Berg]

Originally Posted by evm1024

The crossing scenario is concocted but specific for a reason.



You are in open waters and doing 5 kts, there is a ship doing 20 kts slightly more than 4 miles away bearing 14 degrees forward of your beam. The bearing is not changing.



Is it safe to attempt to cross behind the the ship with a CPA of 180 feet?

As others have said, NO!

[Don C L]

Originally Posted by evm1024

Look at another way. When you are a ship length (600') away from the path of the side of the ship the ship is still 0.34 nm away. (2330 feet).

Yes, I see, the triangle is shrinking... but remind me again, unless I am a really neurotic racer and there is no one at the helm of said ship, why have I set such a course? When I was very much younger and a lot more stupid I used to see how close I could get to the stern of a passing ship, such as this scenario implies... I probably really pissed off a couple of captains... I still never got near 180'.

probably shouldn't admit that

[30yearslater]

Originally Posted by hamburking

NO, it is not safe. Your margin of safety is too small. You can't count on the other vessel maintaining their speed or course. And they can't count on you. You are on a collision course...take action. A change of course of just a couple degrees is all it takes.

You are looking to avoid a catastrophe. I am looking to avoid the possibility of a catastrophe. Understand the difference.

On open water, give them at least a mile. Mid ocean, 5 miles is better.

Bingo! My sentiments exactly. React early and dramatically before horns start blowing and people start getting panicky. If the relative bearing isn't "substantially" changing, risk of collision is deemed to exist. The Colregs don't specify at what distance this situation occurs but a prudent mariner avoids the possibility of catastrophe.