collision avoidance

[KayZee]

Two questions for seat of the pants/rule of thumb collision avoidance disregarding, for the moment, the use of AIS or Radar. One I don't have and the other is almost never on.


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?


2. Do you prefer to use a hand bearing compass and use magnetic bearing? Is there a drawback to this method?


appreciate the help,


Kurt

[JoeRobertJr]

Using any point on your boat to form a range assumes that your head isn't moving and your boat isn't turning. I have done this kind of "eyeballing" it, and that may be good enough if the other boat is far enough away and the bearing drift is significant.
Using a hand bearing compass should give much more accurate results even if you are yawing in the waves, or a gust pushes your heading off a few degrees for a few seconds.

[tkeithlu]

Fixed bearing with range decreasing means a collison.

If you are on a fixed heading, then a pelorus does the job. If he stays on that relative bearing and gets bigger, it's time to change course.

[Jim Cate]

Using the HBC is way better in all cases since it's accuracy does not depend upon your boat maintaining a constant course. There are no drawbacks that I am aware of, and I've used this technique for decades.

Jim

[KayZee]

Originally Posted by Jim Cate

Using the HBC is way better in all cases since it's accuracy does not depend upon your boat maintaining a constant course.

Jim

This confuses me. Maybe the physics of it eludes me no matter how much I think about it. If we're measuring the relative bearing to me, how can it not matter is my course changes while taking bearing with a HBC? I tried to look at a few articles before coming here to get laughed at but they all seemed to be long on what to do and be short on the why.



This in no way means I'm ignoring what seems to be solid advice only that I'll feel more secure with more understanding/learning.


Thanks,


Kurt
svCarol Marie

[hlev00]

The advise is about while taking a bearing. Your body can do a fair job of keeping the HBC on target while the boat moves around under you. The assumption is that your vessel is more or less continuing on the same course. If you are turning and the bearing stays the same the target is changing course too. Or there hasn’t been enough time between bearings.

[chrisr]

a lot depends on your level of experience

i personally find that provided i am standing each time in pretty much the same spot on my boat, i can judge a steady bearing sufficiently by eye (this respect : if i don't see a significant change, i assume it's steady)

but until you've had a lot of practice, the HBC is a great aid and certainly better than trying to make a transit from 2 objects onboard. this will be heavily affected by changes in own ship heading.

cheers,

[Stu Jackson]

Originally Posted by KayZee

This confuses me. Maybe the physics of it eludes me no matter how much I think about it. If we're measuring the relative bearing to me, how can it not matter is my course changes while taking bearing with a HBC? I tried to look at a few articles before coming here to get laughed at but they all seemed to be long on what to do and be short on the why.



This in no way means I'm ignoring what seems to be solid advice only that I'll feel more secure with more understanding/learning.


Thanks,


Kurt
svCarol Marie

Kurt, the concept is simple, and here's the WHY?:


YOU are considered to be in ONE PLACE AT A TIME AND NOT MOVING AROUND ON YOUR BOAT.


The front and back of your boat IS MOVING, because no boat can maintain exactly the same heading at all times, right?


Does that give you the answer?

[goboatingnow]

Originally Posted by KayZee

This confuses me. Maybe the physics of it eludes me no matter how much I think about it. If we're measuring the relative bearing to me, how can it not matter is my course changes while taking bearing with a HBC? I tried to look at a few articles before coming here to get laughed at but they all seemed to be long on what to do and be short on the why.



This in no way means I'm ignoring what seems to be solid advice only that I'll feel more secure with more understanding/learning.


Thanks,


Kurt
svCarol Marie

The simple answer being that the assumption is your boat is turning around yiu as the centre of rotation

With practice , I can use my eye and a stanchion to judge of things might get dodgy

[Jim Cate]

Originally Posted by KayZee

This confuses me. Maybe the physics of it eludes me no matter how much I think about it. If we're measuring the relative bearing to me, how can it not matter is my course changes while taking bearing with a HBC? I tried to look at a few articles before coming here to get laughed at but they all seemed to be long on what to do and be short on the why.



This in no way means I'm ignoring what seems to be solid advice only that I'll feel more secure with more understanding/learning.


Thanks,


Kurt
svCarol Marie

Kurt, one way to consider it is that if the bearing to a distant boat or rock or whatever stays constant, and the distance is diminishing, eventually you will collide with it. It matters not which way your boat is pointing or whether your shroud or other bit stays aligned with the target... a constant bearing and diminishing distance can have but one outcome! Remember that your boat may not be moving in the direction in which it is pointed, and that the target may be moving in any direction or not at all... but if its bearing stays fixed and the distance diminishes it means that you are moving directly towards it.

I find it hard to explain otherwise... perhaps some one can do better!



Jim

[Nauticus]

I've read all 10 posts. This one is the only one to make sense to my old stubborn brain.
Thanks Jim!

Tim

Originally Posted by Jim Cate

Kurt, one way to consider it is that if the bearing to a distant boat or rock or whatever stays constant, and the distance is diminishing, eventually you will collide with it. It matters not which way your boat is pointing or whether your shroud or other bit stays aligned with the target... a constant bearing and diminishing distance can have but one outcome! Remember that your boat may not be moving in the direction in which it is pointed, and that the target may be moving in any direction or not at all... but if its bearing stays fixed and the distance diminishes it means that you are moving directly towards it.

I find it hard to explain otherwise... perhaps some one can do better!



Jim

[Uricanejack]

Relative motion takes a little bit of getting used to.
The idea is you perceive motion the way you see it as relative to you.

Stand beside a highway and the cars driving along it appear to fly past at high speed.
You are stationary and your perceived motion is both relative and true.

Jump in one of the cars drive along the highway, if it’s busy the car in the lanes beside you appears to stay in the same place.
You are both going in the same direction at 60 miles an hour
At the same time you just get a brief glimpse of the person standing beside the highway

Now your perception and reality are quite different.

On a boat or an airplane it’s the same just on a different scale.
5 knots or 500 knots it works the same.

When you look around you the centre of the world you perceive is where you are siting.

If you sail past a rock. You can see the rock is a rock, you know it’s not moving.
But you see it in front of you and it appears to move backwards while in reality you are moving forward.

Change the rock to a boat.

If the boat is anchored or not moving it will appear to move backwards as you sail past it just like the rock.

Or if the boat is sailing along at the same course and speed as you it will appear to stay exactly the same place.

It’s realities motion would be stationary. In true motion it’s just like the cars on the highway you are both moving.

Unfortunately boats go every which way at all sorts of speeds.

The HBC works because it always points the same way. Regardless of what you do with the boat. (Don’t worry about deviation yet)

So long as you steer a steady course at a steady speed. Your compass will still point at North and your course will be in line with the front of the boat.
Speed doesn’t matter, it will point the same if you are at anchor, doing 1 knot 10 knots or a 100 knots.

So if you look around and see another boat or boats.

You take a bearing with the compass.

If the bearing stays the same, it could be the other boat is going at exactly the same speed in the same direction as you are going in your boat.

Just like the car on the highway. This does happen sometimes,

more often it’s converging or diverging.

If the boat gets smaller it’s moving away from you or diverging and will not be a problem.

If the boat is on a steady bearing and getting bigger it is converging or getting closer. If both boats do nothing they will eventually collide.

A pellourus has been mentioned, pellourus is not recommended, why not? because a pellourus is not independent of the boat.
If your steering is not very precise, the pellourus bearing will change.
A compas bearing will not change if you steering is not very precise.

Few of us can steer very accurately on a small boat. Our course tends to average out to where we want to go.

The compass is the recommended method according to the rules.
Radar plotting is also acceptable according to the

Lining up a Stanton works roughly but is not approved. Just like the pellourus.

Or watch the land in the back ground.
If the boat stays in the same place and does not appear to move against the land.
The boat is on a steady bearing.

If the other boat appears to be gaining on the land in the back ground, it’s going to cross ahead.
If the boat is dropping back on the land in the background it’s going to cross behind you.

Someone will come along soon and say this doesn’t work,

Don’t worry about them it works just fine except for some specific occasions when currents might be different.

It also doesn’t work if there is no land in the back ground. So use the compas if there is no land.

[StuM]

Originally Posted by Jim Cate

the target may be moving in any direction or not at all. ... but if its bearing stays fixed and the distance diminishes it means that you are moving directly towards it.

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.

[StuM]

Originally Posted by Uricanejack

Or watch the land in the back ground.
If the boat stays in the same place and does not appear to move against the land.
The boat is on a steady bearing.

If the other boat appears to be gaining on the land in the back ground, it’s going to cross ahead.
If the boat is dropping back on the land in the background it’s going to cross behind you.

Someone will come along soon and say this doesn’t work,

Don’t worry about them it works just fine except for some specific occasions when currents might be different.

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.

[Stu Jackson]

Tim,


Not a joke:


Take two thimbles and your phone, and put them on your desk.


Pretend one thimble is you on your boat, the other thimble is another boat, and ther phone is a big rock / land.


Now, using either one or both of your hands, move either or both of the thimbles.


Watch what happens.




Good luck.