collision avoidance

[KayZee]

Originally Posted by TrentePieds

My previous (#19) laid out the basics. In practice I am often in the situation you say you are, and what I do is this:


TP

There's was a lot of good stuff in the original 3 pages while I was away from the board. I did read them but it was too much to unpack to reply specifically.


Thank you for thoughtful replies.


Kurt

[Uricanejack]

Originally Posted by KayZee

A lot of words and I greatly appreciate the time it took to write them out. I do I have a nice old pair of Fujinons with the compass. The talk about aspect is VERY helpful though I'm still puzzling on the math of the compass bearings being greater or less than 180 degrees. I will re-read it a few more times.



Thank you for staying on topic of trying to help me understand the methods helpful to me.


Kurt

One of these things where a picture is worth a thousand words.
Trying to explain it in words not so easy, so following the words not easy.
Unfortunately my computer skills are limited.

I will try again with the 180. Hopefully this helps.

the 180 is not the compass bearing or your heading.

The 180 is just the reciprocal to your heading.

pick any course or heading doesn’t matter.

How about east or 090

If you see a vessel right ahead, You can only his bow equal amount of each side, or at night both side light ps and Mast lights in line.

It’s head on he is going the other way or the opposite direction.

He must be on a reciprocal heading of west or 270 which is 180 degrees opposite.

his aspect is head on right ahead. Not good, you should both alter to starboard.
Even if the other guy does nothing. But you alter 10 or 20 degrees to starboard.
You will start to see more of his red side. And he will appear to be 10 or 20 degrees to port of your bow.
His bearing will now start to change
And you will soon loos sight of his green side and light
He is passing clear astern.

Try drawing a couple of boat shapes with a 360/2 for and aft so 180 on Port side 180 on the starboard side.
mark by a protractor, 122.5 degrees on either side which equals the sector for a side light
draw a red and green arc for each side Port and starboard.

If you have two of those.

Put them on a table.

Leave one which is your boat pointing the same way.

Move the other one around either side point it in different directions.

It will start to make sense.

[StuM]

Originally Posted by KayZee

I'm fixated on RB's because that's what I was taught that mattered both by sight and when tracking on radar with a bearing line.

The problem is that there is a lot of confusion over the words "relative" and "bearing" and the phrase is frequently misunderstood/misused ( as we have seen in several posts in this thread)

"relative bearing" actually means:

"the number of degrees off your bow (or track) to the object"
i.e "his relative bearing is 30° to port"

When trying to identify an object or course to steer etc, this is appropriate (bearing relative to your heading) and is the only time that the phrase "relative bearing" is correct.

However some people also use the term to mean
"the direction (as a a true or magnetic bearing) to the object from your current position at the current time"
i.e "his relative bearing is 175°M and two minutes ago it was 172°M"

When talking about visually determining whether risk of collision exist, this is the correct "bearing" to use, but, use of the word "relative' in this case is incorrect.

[requiem]

Perhaps some diagrams? (Assume heading and course are the same.)



Note here that the relative bearing alone of these vessels doesn't determine their aspect; their heading is also needed. Of the five ships shown here on your starboard bow, two show green, two show red, and one shows white.

[Uricanejack]

Thanks
Good illustration, I had to look at for a bit, to figure it out.

[Dockhead]

Originally Posted by StuM

The problem is that there is a lot of confusion over the words "relative" and "bearing" and the phrase is frequently misunderstood/misused ( as we have seen in several posts in this thread)

"relative bearing" actually means:

"the number of degrees off your bow (or track) to the object"
i.e "his relative bearing is 30° to port"

When trying to identify an object or course to steer etc, this is appropriate (bearing relative to your heading) and is the only time that the phrase "relative bearing" is correct.

However some people also use the term to mean
"the direction (as a a true or magnetic bearing) to the object from your current position at the current time"
i.e "his relative bearing is 175°M and two minutes ago it was 172°M"

When talking about visually determining whether risk of collision exist, this is the correct "bearing" to use, but, use of the word "relative' in this case is incorrect.

Note that relative bearing - what you get with a stanchion - gives you information about risk of collision ONLY to the extent your own heading is constant If you are on a collision course, your relative bearing can be changing, but you are no less on a collision course. That's why we use compass bearings in collision avoidance, not relative bearings.

[StuM]

Originally Posted by Dockhead

Note that relative bearing - what you get with a stanchion - gives you information about risk of collision ONLY to the extent your own heading is constant If you are on a collision course, your relative bearing can be changing, but you are no less on a collision course. That's why we use compass bearings in collision avoidance, not relative bearings.

I seemed to remember Cockcroft saying something about it too
Page 46 in my version:


Risk of collision shall be deemed to exist if the compass bearing of
an approaching vessel is not appreciably changing, not the relative
bearing. The relative bearing will be affected by changes of heading.
Sighting an approaching vessel against components of the ship's
structure may give a rough indication of whether there is risk of
collision and may provide sufficient basis for deciding whether to
make a bold alteration to pass astern of a vessel being overtaken or
crossing from the starboard side. Such bearings, however, must
always be related to the ship's heading and may be affected by slight
changes in the observer's position unless careful transits are taken.

[Lodesman]

Originally Posted by KayZee

I'm fixated on RB's because that's what I was taught that mattered both by sight and when tracking on radar with a bearing line.

Hopefully you see by now that you've been taught wrong. Particularly when traversing a swell such that your heading is slewing back and forth - it's impossible to use relative bearing with any accuracy. With the true (or magnetic/compass) bearing it doesn't matter what your heading is doing, and better yet, after you make a bold alteration of course - you can immediately look down the bearing of the other vessel to watch if it changes, without having to go through mathematic gymnastics - (he's 17º on my stbd bow, I turn stbd 45º, that means he's 45-17º on the port bow...) I don't even know how you'd do that by stanchion.

With regard to radar - in an ideal world we'd all have gyro-stabilized radars and all bearings would be referenced to true; but in small boats our radars tend to be relative to ship's head. If your's is compass-stabilized, I encourage you to use it in the North-up configuration and take all bearings as Compass, this way you can easily compare and relate bearings take visually with the radar. This is not to say that radar is useless in Head-up - your bearings will be relative, but the way we use radar tends to be less dependent on instantaneous measurement of bearing (as it is with a visual bearing), and more with an averaged view. You might also be conflating relative motion with relative bearing. With radar, the significant factor for determining risk of collision is the relative motion(RM) lines of the contacts. If the RM vector is pointing at own-ship (centre of screen) then risk of collision exists - given that in this case the RM is radial, the bearing (relative, compass or true) is steady.

[KayZee]

Originally Posted by Lodesman

. With radar, the significant factor for determining risk of collision is the relative motion(RM) lines of the contacts. If the RM vector is pointing at own-ship (centre of screen) then risk of collision exists - given that in this case the RM is radial, the bearing (relative, compass or true) is steady.

THERE it is! Relative MOTION not relative bearing! Trying to remember what I read in books 30 years ago is not always reliable.



You've all been very patient and generous with your time and knowledge. I've learned a lot which in the end is what I came to do.


Kurt

[KayZee]

Originally Posted by Uricanejack

Thanks
Good illustration, I had to look at for a bit, to figure it out.

Me too on both counts.



Kurt

[limte]

In regard to relying on AIS for collision avoidance, it is important to note that all vessels do not carry AIS, and that vessels that do carry AIS cannot always be relied upon to have it operable.
Several years ago off of the south coast of Nova Scotia, in very poor visibility, but at a moment in which the fog diminished slightly, we spotted a large freighter several hundred yards away. The freighter did not show up on our AIS. It did however show up on our radar screen. Apparently, the freighter did not have operable AIS, even though this was considered a relatively high traffic area.
In poor visibility, AIS may not be necessarily be considered an adequate replacement for functional radar. Also, without a MARPA function in a radar unit, or the ability to quickly plot radar courses, it is not an easy task to determine whether or not another vessel may present a collision risk.
As an example, at night a vessel appearing to be getting closer on a radar screen may suggest the risk of a head-on-collision. In this case one would perhaps be looking for red and/or green bow lights in assessing risk. The actual case may be that one is in an overtaking situation, and should instead be looking for a stern light.


Regards,
Bob S

[merrydolphin]

I have always used a good hand bearing compass. When reasonable calm use my Fujinon compass binocs. Othwise a. Quality hockey puck bearing compass.

A rough guide can be something like a an upright part of the boat IF your head stays in the same place.

Sailed the oceans for decades this way before modern aids like radar and AIS. And it is indeed still the ultimate authority on collision. Your eyeballs and on board computer, your brain! All thought there have been times when after being up for days. That neither are working all that well!

[Bill_Giles]

The compass has two advantages: one is that you can hand over the watch to someone else with just a number instead of saying “if you sit here he should be just over that station”. This is easily confused. The other is that it is easier to keep track of more than one target.

However, it is easier with the relative bearing to see whether the target is going to pass ahead or behind you (I prefer that he passes ahead).

The other thing is that if you are going to change course do it early and positively as you don’t want him to do the same at the same time. In very general terms I steer to pass his stern.

[jonathanv]

I have a question on a similar note. I was on night watch in the summer and at one point I was looking at a vessel on a constant bearing but I could not determine whether the distance was changing. He was likely far away and on a parallel course but it still made me somewhat nervous. At night, with only a single navigation light it can be hard to judge how far an object is.

How do you deal with those cases?

[Kelkara]

Originally Posted by jonathanv

I have a question on a similar note. I was on night watch in the summer and at one point I was looking at a vessel on a constant bearing but I could not determine whether the distance was changing. He was likely far away and on a parallel course but it still made me somewhat nervous. At night, with only a single navigation light it can be hard to judge how far an object is.

How do you deal with those cases?

Assume that you ARE on a collision course and make a move; immediately if you're give way, a bit later if you're stand-on ... or turn on the radar and cheat.