collision avoidance

[Jim Cate]

Originally Posted by StuM

That's only true if the target is not moving.

If the target is moving (the more common case under discussion) and the bearing stays fixed and the distance diminishes, you are moving directly towards a certain location ahead of you that he is also moving towards and you will both reach that location at the same time.

Quite so, Stu, and well expressed.

I knew some one would do it better than I did!

Jim

[NPCampbell]

Originally Posted by KayZee

1. When using a relative bearing with a fixed point on your boat, is it important to create a range/transit with two objects, for example, a stanchion and a shroud, or is it enough to observe from a single object with a fixed head position?

I don't think you'll be able to get far enough between any two objects on your boat to make a difference with an HBC measurement.

If my math is correct ...

Let's say another vessel is 1000 yards away. 1 arc minute @ 1000 yards is 10 inches. So if you were able to take 2 measurements on your boat, 10 feet apart (120 inches), that would be a difference of 120/10 = 12 arc minutes = .2 degrees. I am assuming .2 degrees is way less than a handheld compass can accurately measure. The farther the vessel is away, the higher the accuracy of the compass would have to be .

[Uricanejack]

Originally Posted by StuM

Do not believe this. It is false and dangerous!

Think about it.

When you look at the land on your beam, any given point is always moving astern i.e. the bearing to it is changing.

If a boat remains in a constant position relative to a point on the land, the bearing to it must also be changing in the same way. He is "falling back" and will pass behind you.

If he appears to be moving ahead of the background, he may well be on a constant bearing from you and on a collision course. Whether it is a potential collision or not is dependent on the relative distances between you, him and the land and your relative speeds. You just can't tell what the outcome will be with any degree of certainty.

If it’s dangerous, I wonder how I managed to survive the last 40 years. More to the point I wonder why I haven’t killed anyone.

Or maybe it works. Well enough.

[TrentePieds]

Kurt:

Let's not get too scientific about this. SPURIOUS accuracy is the enemy of the sailorman :-)

Sit at your tiller or stand at your wheel. In the distance you see a ruddy great lump of SOMETHING. Doesn't really matter at this point exactly what that something is. From where you sit (or stand), the lump appears in alignment with a stanchion. 30 seconds later you look at the lump again, and (scenario #1) it is now AFT of the stanchion. Oho! You are gonna go in front of it! (scenario #2) The lump now appears FORWARD of the stanchion. Oho! You are gonna go behind it. This is true whether the lump is a ruddy great lump of rock or a 50K ton freighter moving at 22 knots!

Now scenario #3: The lump STILL appears over top of the stanchion. Oho! You are gonna smack into it! So what to do? Just alter course, of course!. Lumps of rock NEVER do that. Freighters may or may not, but why force the issue since you are so nimble, and the poor sod earning his keep on the water is not? Treat all freighters as rocks and don't go near them :-)!

We could get all scientific and calculate the "Closet Point of Approach", the CPA. Doing that's fun if you have nothing better to do, but for MOST Sunday sailors, doing so is never necessary.

So then: If you turn toward the lump it will appear to move FORWARD of the stanchion. If you turn away from it the lump will appear to move AFT of the stanchion. At this point, that's really all you need to know. Why is that? It's because YOUR speed is, let us be generous, a mere 12 knots. That means (roughly) that you move (6,000 x 12)feet = 72,000 feet per hour. That equates to (72,000/3,600) = 20 feet per second, there being 60 x 60 = 3,600 seconds in a hour. That means further, that in the time twixt you first sighting and your second sighting of the lump you'll have moved a whole (20 x 30) = 600 feet, i.e. something less that 15 boat lengths assuming that you are in a 45-footer!

If you haven't seen the lump till it's within 15 boat lengths of you, it ain't your HBC technique you need to worry about :-) !!

You can alter course by 45 or more degrees in half that distance, and alter course betimes is what you do! In most cases, though not all, turn towards the lump.

The HBC is a nifty little device, but I prefer to use my trusty binoculars with a compass in one reticle because I never have to go looking for it. It sits in its holder on my binnacle next to the steering compass. It comes in handy for complying with COLREG #5!

What about converting from "magnetic" to "true", then? I don't bother when doing pilotage. We have 23ºs of variation in my waters. I'll be lucky, if the sea is lumpy, to steer to such a degree of accuracy, and in any event the binoc deviation corresponds closely to the deviation of the steering compass. Like I said, SPURIOUS accuracy is the enemy of the sailorman.

Common sense and forehandedness is the friend of the sailorman :-)

Safe sailing, and all the best :-)

TrentePieds

[StuM]

Originally Posted by Uricanejack

If it’s dangerous, I wonder how I managed to survive the last 40 years. More to the point I wonder why I haven’t killed anyone.

Or maybe it works. Well enough.

Or maybe you don't understand simple trigonometry and vectors but have been lucky.

[StuM]

Originally Posted by TrentePieds

What about converting from "magnetic" to "true", then?

In the context of collision avoidance and bearings to another vessel,it doesn't matter whether you work in degrees mag, degrees true, grads, mills, compass points or radians nor which direction your origin lies. If the number doesn't change risk of collision exists.

[requiem]

Originally Posted by TrentePieds

Why is that? It's because YOUR speed is, let us be generous, a mere 12 knots. That means (roughly) that you move (6,000 x 12)feet = 72,000 feet per hour. That equates to (72,000/3,600) = 20 feet per second, there being 60 x 60 = 3,600 seconds in a hour.

Speaking of simplicity, it's good to remember you can take your speed in knots and halve it to get the value in m/s. That's because the 1852 meters in a nautical mile is conveniently about half the number of seconds in an hour.

[requiem]

Originally Posted by Uricanejack

If it’s dangerous, I wonder how I managed to survive the last 40 years. More to the point I wonder why I haven’t killed anyone.

Or maybe it works. Well enough.

I believe Stu is technically correct (the best kind of correct), but the method does work, likely based on unstated assumptions that work for sail. The common scenario would involve sailing boats, which generally don't move particularly fast, and picking a point on the horizon rather than nearby.

Say you're doing 12 knots (6 m/s, probably a safe upper limit for most sailing boats), and the point on the horizon is at least 5 km away. After 30 seconds you'll have moved 180 m, and the point on the horizon will have changed bearing by anywhere from 0 to 2 degrees* depending on its relative bearing. That's already within the margin of error of a hand compass. Using a speed of 6 knots, or an even more distant object such as a far-off mountain, the change in bearing becomes even more negligible.

So for a quick assessment it works well, so long as you are using a truly distant reference and can see clear movement against it.

* 180/5 = 36 mils, and ~18 mils make a degree.

[Dockhead]

Originally Posted by Jim Cate

Quite so, Stu, and well expressed.

I knew some one would do it better than I did!

Jim

You and Stu are both right. Jim -- what you said is correct if you are talking about RELATIVE motion (which is the kind of motion you can perceive directly). Stu is right in terms of absolute motion.


To the OP: The practical answer is this --

1. Use a stanchion or shroud by all means for a quick, down and dirty read on a vessel which is still far off. As others have said, it's much less accurate (because unlike a hand bearing compass, it does not account for your own boat's deviations of course). HOWEVER, it's plenty accurate enough for one purpose -- if the other vessel is passing far ahead or behind, then using this method you can see a big change of bearing. So it's a quick screen when you have a lot of different ships around and you want to know which ones require more attention. Note: "which is still far off".

2. Hand bearing compass used with skill is accurate enough to do actual collision avoidance with. If the stanchion method does not eliminate a target as passing well ahead or behind, then start taking bearings with HBC. If the bearing is steady, then at the right moment (early if you are give-way, after giving the other vessel an appropriate chance if you are stand-on), make an appropriate maneuver and observe to see that the bearing is now clearly changing.

The beauty if an actual radar plot (or AIS reading) is that you can see not only that the bearing is changing, but you can see whether or not your maneuver will produce an acceptable CPA.

[Pizzazz]

All hand/head/shroud/hand compass bearings on a small boat are wildly inaccurate as they measure the bearing relative to the heading of the boat which changes constantly, even under autopilot control. You need the bearings off your course off the GPS and the other guy’s course from the AIS. Alternatively, use your radar.

There is another complication that if you hand steer, you will subconsciously start to track the other boat (especially in t open sea where other visual markers do not exist). You can check this by hand steering for half an hour then measuring the CPA, then let the autopilot steer for half an hour and measure the CPA again. 9/10 you will be further away under autopilot. It is just human nature to flock together

[Jim Cate]

All hand/head/shroud/hand compass bearings on a small boat are wildly inaccurate as they measure the bearing relative to the heading of the boat

This is not true of the HBC bearing, and that is why it is a better means of evaluating collision potential. If you read the various posts upthread discussing HBC use, this should become clear.

Jim

[Manateeman]

Radar, AIS, night vision, expensive binoculars...all nice to have.
Everybody at some point will find themselves in a confusing situation.
Pick up the radio.
State your vessel type, speed, course and current position.
Wait.
Believe me. No one who is a licensed professional Merchant Mariner wants to run you over. We have more equipment and experience. You have probably been plotted or observed long before a bad situation is at hand.
I can’t tell you the times I’ve wished a small vessel would just tell me their intentions on the radio.
It’s just this simple.
It’s why professionals radio each other to form an agreement on meeting, crossing or overtaking each other.
You might be asked to show a light on your sails or flick on and off your lights so we know for sure which vessel we are talking to.
Most professional captains will give you information on local issues or weather if they have time. Be courteous. Learn the rules and be willing to give way because they might have some issue on board their vessel.
Communication is the most important factor in avoiding collisions.
Please don’t be embarrassed to ask for help or to say you are confused.
The radio is the most important anti collision tool you have.
Please answer if you are called. Please call if you see me.
We can then confirm who is who and who is going where.
It’s just that simple.
Happy trails to you.
Captain Mark and his Merchant Mariner Manatees

[Lodesman]

Originally Posted by Pizzazz

All hand/head/shroud/hand compass bearings on a small boat are wildly inaccurate as they measure the bearing relative to the heading of the boat which changes constantly,

Hand bearing compass doesn't measure relative bearing - it's kind of in the name - y'know "compass". Since it's fixed to the Earth's magnetic field, it's usually pretty steady.

For the OP, the smart people who came up with the Rules, put it right in there:

7(d)In determining if risk of collision exists the following considerations shall be among those taken into account:
(i) such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change,

[StuM]

Originally Posted by Pizzazz

All hand/head/shroud/hand compass bearings on a small boat are wildly inaccurate as they measure the bearing relative to the heading of the boat which changes constantly, even under autopilot control. You need the bearings off your course off the GPS and the other guy’s course from the AIS. Alternatively, use your radar.

Not so. Have you every used a HBC or binoculars with a built in compass?

A compass "measures the bearing" relative to magnetic north*

With a decent HBC or binoculars with built in compass, you should be able to get a consistent bearing to less that 2° every time even if your heading is bouncing around.



* +/- deviation which can be ignored as it will be effectively a constant for a certain direction if you don't move around the boat while taking successive readings and the boat is not swinging through huge angles.
** this footnote added for the other pedants around here

[hlev00]

You can also use this collision avoidance technique for monitoring thunderstorm cells or squalls. Get a bearing on the center of the cell - your choice of binnacle, or HBC, or even shroud/stanchion. Then keep taking them occasionally and assess if the bearing is changing. Of course thunderstorms are very dynamic so you may be missing one, only to have another develop and slam into you. The "center of the storm" may not be terribly obvious either but from time to time I find this technique helpful if I don't have radar to pick up the cell.