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Old 13-07-2013, 11:00   #886
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Re: Freighters vs. Sailboats

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Originally Posted by Richard5 View Post
On radar I see he's just now coming to the 24 nm ring. I begin to plot his relative bearing. Six minutes later(1/10 of a minute, helps with math to compute distance) there is no change. I prepare to tack towards Anacapa Is. Another few minutes go by and no apparent change in RB so I tack to new course of about 80* off original heading...
You realize that a steady bearing at 24 miles means nothing? The rate of bearing change at a long distance is very slow; you can't assume a risk of collision exists based on that. Bearings taken by radar are even more suspect.

When you said you came to a new course - was that northerly or southerly? I'm trying to picture which side of the channel this happened on?
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Old 13-07-2013, 11:32   #887
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Quote:
Originally Posted by Lodesman View Post

You realize that a steady bearing at 24 miles means nothing? The rate of bearing change at a long distance is very slow; you can't assume a risk of collision exists based on that. Bearings taken by radar are even more suspect.

When you said you came to a new course - was that northerly or southerly? I'm trying to picture which side of the channel this happened on?
I was qualified to do radar nav and formation maneuvering on a 119,000 ton 620 ft ship. I can tell you with certainty that CBDR (Constant Bearing Decreasing Range) looks the same at 20nm as it does at 20 yards. CBDR at 20nm is not a cause for action but CBDR at 20kyds (10nm) is cause for calling the other guy bridge-to-bridge and questioning his intentions. CBDR is CBDR no matter what the range and the true bearing shift (in a CBDR situation) is zero degrees per minute at every range right up to the point of impact.
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Old 13-07-2013, 12:08   #888
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Re: Freighters vs. sailboats

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Would you send your kids for sailing lessons at this Yacht Club ......

Yes! as soon as they finish their swimming lessons.
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Old 13-07-2013, 13:12   #889
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Re: Freighters vs. Sailboats

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Originally Posted by CaptainDana View Post
I was qualified to do radar nav and formation maneuvering on a 119,000 ton 620 ft ship. I can tell you with certainty that CBDR (Constant Bearing Decreasing Range) looks the same at 20nm as it does at 20 yards. CBDR at 20nm is not a cause for action but CBDR at 20kyds (10nm) is cause for calling the other guy bridge-to-bridge and questioning his intentions. CBDR is CBDR no matter what the range and the true bearing shift (in a CBDR situation) is zero degrees per minute at every range right up to the point of impact.
Sorry, that's just not correct. True, in theory a collision course will show a CBDR at any distance, but so will a comfortable miss until you get close enough for the bearings to change significantly.

Example: You are sailing at 5 kts, and a 20-kt ship is approaching head-on. You happen to be on parallel paths, with a one-mile separation, but you don't know that yet.

You first sight him on radar, at a range of 25 miles. The angle off your bow is 2.29 degrees.
One minute later the bearing is 2.33 degrees
At five minutes the bearing is 2.5 degrees. Can you reliably measure this? I can't.
At ten minutes the bearing is 2.75 degrees. I still can't see a change, and we are now at a range of 21 miles.
At 20 minutes the range is 17 miles and the bearing has increased to 3.42 degrees. I doubt if I could reliably ell the difference.
30 minutes: range 15 miles, bearing 3.9 degrees. This is probably my first eyeball sighting.
45 minutes: range 8.4 miles, bearing 6.8 degrees. OK, perhaps we pass comfortably? That's about a 3 degree change in bearing in fifteen minutes, but it's bumpy and my compass is bouncing around.
50 minutes: range 6.3 miles, bearing 13.5 degrees. We're good.
60 minutes: range 1 mile, bearing 90 degrees. Closest Point of Approach.

At 50 minutes, had the bearing remained unchanged we would now have ten minutes to get out of the way. If we turn 90 degrees at 5 kts we can get about 0.8 miles clearance.

So CBDR works reasonably well at likely eyeball ranges, but at 20 miles you will not be able to tell until quite some time has passed. Yes, in practice you may very well alter course before my 50-minute mark, but you can't even tell which way to turn (ignoring COLREGS) for quite some time.

An yes, I used arcsins (and a spreadsheet) to do this math. I could have used arctans, but it's all triangles, trig, squares, and roots, anyway.
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Old 13-07-2013, 14:41   #890
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Re: Freighters vs. Sailboats

Here is a chart showing the change of bearing as you approach on the parallel course I described above, bearing in degrees vs time in minutes:


If you reduce the CPA from one mile to 1/2 mile, it now takes 28 minutes from the initial 20-mile contact for the bearing to change by one degree. In 48 minutes the bearing has increased by five degrees. TCPA remains at 60 minutes. Increase the CPA to two miles and we now see a 5-degree bearing change at 31 minutes.

On the water this number-crunching is not really useful. But it should help us understand why it's not always so obvious, especially at long ranges, how an approach will evolve. The good news is that, assuming no big ship course changes, we should still have time to figure it all out and do the right thing.
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Old 13-07-2013, 15:16   #891
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Re: Freighters vs. Sailboats

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Originally Posted by cappy208 View Post
@Richard5 You described an interesting meeting. The part I liked best was: "I altered at 18 nm."

You altering at 18 miles is useless when the ship is running in a channel, is making course changes while meeting other traffic, and dodging yachts.

I don't know the nautical charts there. Are there any traffic schemes in the sound?

Too many times, yachts call up ships and have NO freaking idea they are even IN a TSS or an actual channel.
But I did not say that.

The "channel" in this case has also been called the "canal". It is a body of water between the mainland and several islands that lie offshore between 11 nm (east end, Anacapa Is) and approx 24-30 nm for the other islands. This body of water is not a sound, bight, or other. Google the nav charts, that could probably answer your questions better. Yes, there are TSS but you know ship traffic is not required to be bound by the defined lanes.

As for running in a channel, you did notice I did say I dared not to come closer to the island for fear of wash rocks? I've seen merchant vessels spin pirouettes but I never thought they would cut that close to shore while at speed. (>25kt).

Exiting to the east of the Santa Barbara "channel", traffic is able to steer to the south once clear of the last island to the east (Anacapa Is). Thence they fairly point towards Pt Fermin on the approach to LA/Long Beach harbor. Passage through the "channel" is west-east but the port is to the south. This means traffic will make a course change somewhere in the vicinity of Anacapa Is. The mainland is to the north in the channel so it is warranted to stick closer to the islands than to the mainland. Further east there is a line of offshore oil rigs. But some skippers sure cut that dogleg course change. I hadn't anticipated he would come so close to the rocks.
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Old 13-07-2013, 15:39   #892
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Re: Freighters vs. Sailboats

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Originally Posted by Lodesman View Post
You realize that a steady bearing at 24 miles means nothing? The rate of bearing change at a long distance is very slow; you can't assume a risk of collision exists based on that. Bearings taken by radar are even more suspect.

When you said you came to a new course - was that northerly or southerly? I'm trying to picture which side of the channel this happened on?
I begin plotting as soon as able. In this case, because I was aware I could expect a solution, I began to pay attention. I agree that I should not maneuver based upon a far off single bearing. But that plot is the start and we'll watch how it unfolds. It's not as if I'm, Oh no, a ship I better turn!

I see the story in my post was somewhat truncated. But I did try to convey that as we got closer there was no change. About 10-12nm I made the tack. I figured we were inside one nm from wash rocks at the crossing. I checked and rechecked and made ready for a tack to a new heading. In hindsight I guess I should have stayed on the southerly boundary of the lane.

My new heading bore off to the south while north of Anacapa Is in a fog. Horz vis was about 1/4 nm but quite bright (indicative of a not very deep fog layer)
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Old 13-07-2013, 15:45   #893
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Re: Freighters vs. Sailboats

Feel free to use your charts, adjust your course 20 miles out, always assume you are the give way vessel when something larger than a rubber ducky appears over the horizon.......

I can only hope the others here that cruise extensively or make a living from the sea are as truly amazed as I am....
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Old 13-07-2013, 15:50   #894
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Re: Freighters vs. Sailboats

When I was still in the SF Bay area, if I wanted to sail south from Sausalito to the City front and seen a large ocean going vessel travelling north past Alcatraz several miles away, I knew he was going to follow the shipping channel and turn in my direction. Ten minutes later he was going to be where I was going to be, so I did a 180* and sailed back toward Sausalito for five minutes before doing a second 180* to continue to the City front.

What I've found useful on open ocean passages, for collision avoidance with ship traffic, is the VHF.

Talk to the person on the bridge of the approaching vessel. Decide what each of you plans on doing. Works everytime.
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Old 13-07-2013, 16:13   #895
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Re: Freighters vs. Sailboats

Comms on the V don't always yield the desired result. And on the open water don't expect them to yield when you are under sail or even actively involved in fishing operations. Too bad I lost a series of pictures but it was a car carrier steaming straight through a fleet of fishing vessels in international waters.

Probably the best communications I had with a large vessel was a NOAA R/V. Even then the skipper was perplexed why I called since we weren't crossing. I was concerned if he was in tow of anything from his stern where I would be passing. Golly I suppose I could have looked for signals in his rigging but I was just a dumb kid. The only diff now is I'm older, right?
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Old 13-07-2013, 17:40   #896
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Re: Freighters vs. Sailboats

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I was qualified to do radar nav and formation maneuvering on a 119,000 ton 620 ft ship.
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Old 13-07-2013, 19:58   #897
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Re: Freighters vs. Sailboats

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Yes, there are TSS but you know ship traffic is not required to be bound by the defined lanes.
Not quite right.

Rule 10(d)(i):
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A vessel shall not use an inshore traffic zone when she can safely use the appropriate traffic lane within the adjacent traffic separation scheme. However, vessels of less than 20 meters in length, sailing vessels and vessels engaged in fishing may use the inshore traffic zone.
Sounds like you were between a rock and a hard place, and this guy cut the corner, contrary to the rules. I would guess you didn't paint on his radar. Is there a VTS in place there?
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Old 13-07-2013, 20:49   #898
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Re: Freighters vs. Sailboats

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Originally Posted by cappy208 View Post
Regarding AIS, if you have it, and know how to scroll through the info, it GIVES you cpa and Tccpa. What more do you need?

The topic about using AIS to determine if you are in the 'cone of death' is superfluous. If you HAVE AIS you already have superior information at your fingertips.
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amazingly.... EVERY TIME I sail...my speed stays EXACTLY the same until the freighter passes.
Is CPA and TCPA from your AIS so great, if your ownship data on which it is derived, is so sketchy?

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How about instead of posting negative comments, you look at it closely and see where the table may be useful and we will all benefit .
Honestly, I don't think I'd go to your table to figure out if risk of collision exists. That's not to say I can't see of some utility. Perhaps it would be a good instructional tool - obviously some people can't wrap their heads around relative motion and when risk of collision can exist. Or perhaps it could be used to show newbies that they don't have too much to worry about being a slow vessel out on the marine superhighway - seeing that the danger zone in front of a fast freighter is a surprisingly narrow piece of the pie.

If you want to extend the range at which you can determine risk of collision perhaps your table has a use. As I said earlier and Paul convincingly demonstrated, you can't trust that a steady bearing at long range means risk of collision exists. While AIS gives you CPA and TCPA, I suspect that those figures bounce around quite a bit, and can't really be relied upon at long range. You can put reasonable faith in the accuracy and steadiness of a freighter's course and speed reported on AIS. That and the compass bearing to the freighter gives you all the information you need to enter your table and determine risk of collision. I think. You might want to test it in the real world and get back to us.
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Old 13-07-2013, 21:42   #899
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While AIS gives you CPA and TCPA, I suspect that those figures bounce around quite a bit, and can't really be relied upon at long range. You can put reasonable faith in the accuracy and steadiness of a freighter's course and speed reported on AIS.
That sounds like a contradiction. - aren't CPA and TCPA derived from course and speed reported on AIS and your own GPS course and speed, if you have reasonable faith in the data then why not have reasonable faith in the CPA? It's just the box doing the work for you.
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Old 13-07-2013, 22:47   #900
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Re: Freighters vs. Sailboats

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That sounds like a contradiction. - aren't CPA and TCPA derived from course and speed reported on AIS and your own GPS course and speed, if you have reasonable faith in the data then why not have reasonable faith in the CPA? It's just the box doing the work for you.
Lodesman may want to answer this, but I have reasonable faith in the freighter's reported course and speed. However, my own jumps around when in a seaway. The CPA/TCPA are calculated using the position, course, and speed of both vessels, so it can be pretty erratic at times. In my NavMonPc software I can change the filtering for my SOG and COG, which can smooth the CPA calculations somewhat. Other AIS systems may have similar filtering options. I still have to keep an eye on things as our vessels approach, especially if it's a close one.
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