Checking potential collision course?

bedouin said:
For the method to work the distance has to be sufficiently large that the bearing can be considered to be be "constant" over the time being measured
Click to expand...
Bingo. By George he's got it!
That is why I said the scale was (almost) irrelevant -- because the only bit of my diagram that might change is the distance to the landmark (buoy, on my diagram).

And it's why the method works for snap decisions taken at the last minute, such as between racing boats or in very confined waters such as the Solent. It's also why the method does not work for the more carefully considered decisions taken at longer range in more open water and in ship/yacht encounters.

Is it my imagination, or have we worked our way back round to where we were about 150 posts ago?
 
Last edited:
timbartlett said:
Bingo. By George he's got it!
That is why I said the scale was (almost) irrelevant -- because the only bit of my diagram that might change is the distance to the landmark (buoy, on my diagram).

And it's why the method works for snap decisions taken at the last minute, such as between racing boats or in very confined waters such as the Solent. It's also why the method does not work for the more carefully considered decisions taken at longer range in more open water and in ship/yacht encounters.

Is it my imagination, or have we worked our way back round to where we were about 150 posts ago?
Click to expand...

Jings, I'm glad you're starting to see the light, suggest you examine a few more scenarios using my spreadsheet, and you might start to see even more light!

BY the way, I agree categorically that the use of this method in open waters does not work... principally because there is no land on the background.

I disagree though that it does not work with ship yacht encounters, it does generally but you've got to check both stem and stern.

What is also worth checking out is how at bearings only start to change significantly when the range closes BOTH COMPASS and alignment with background.. in fact this method is probably more accurate in CERTAIN CIRCUMSTANCES than trying to use a hand bearing compass in a small craft in choppy conditions.

But I rather suspect that cetain minds a closed to new ideas partic when entrenched..


Anyway, fairwinds and may all your bumps be fendered!
 
jimi said:
.... But I rather suspect that cetain minds a closed to new ideas partic when entrenched...
Click to expand...

In practical situations the reality is likely to be that both methods are used (even by "minds a closed"). In my own case, on the linked post at the start, I had a flawed understanding in where I assumed that the land / stanchion transit method was infallible.

What this post highlights to me is that there is always a risk in following what could be called a Standard Practice where the theory is not understood.
 
Am I missing something here?

Tim is talking about a fixed MARK on the shore.

Jimi is talking about the LAND HORIZON.

These are totally different scenarios.
 
Skysail said:
Am I missing something here?

Tim is talking about a fixed MARK on the shore.

Jimi is talking about the LAND HORIZON.

These are totally different scenarios.
Click to expand...
Not really. The "moving against the land" method relies on the fact that the land is not absolutely uniform, because if it were, you would have no way of knowing whether the other vessel had "moved" relative to it or not. In other words, your "land horizon" (??) is made up of lots of landmarks all joined together -- dips, hills, hollows, changes of colour and texture, trees, buildings and so on.
 
Well I suspect anyway that this thread has raised awareness of a potentially useful Rule of Thumb for a lot of people who will experiment with its usefulness or otherwise.

I look forward to the future discussion when positive feedback is voiced. There will , of course , be no negative feedback cos all the people for whom it failed to work will have been squashed by collision, and thus be unable to voice their concerns!
 
Ho hum - just arrived back at the keyboard after 5 days away, and it's still going!

TESTIMONIAL

It works for me, and in 40 something years sailing I have yet to hit another moving vessel (did clobber a parked one, once, but that was a different kettle of wasps altogether).

End of testimonial

Now, here's an interesting slant. You WANT to hit something - a mooring buoy say. There's a tide running. What do the YM text books say? Steer a course and speed which holds the buoy constant against the background and you will fetch it. If it moves against the background you'll miss it.

Pipe that in your smoke and stick it.
 
Indeed, and in the case of the buoy, they're quite right, and there's a very good reason why. The buoy is fixed to the sea bed, fixed to the land, and not moving. Another boat is not fixed to the sea bed, and is moving quite independently of anything on the shore.
 
Joker said:
There are several answers to that.
Click to expand...

The prime one being that the method works. But, like a bumblebee, you seem unable to fly. Let me try, one last time, to teach you!

For two moving objects to collide, they must be on a constant bearing. We agree, I'm sure.

One way - there are others - of seeing whether a bearing is constant is to establish a transit between one object - let's call it the target – and another, which we'll call the shore. I'm sure you'll accept that a transit is effectively a bearing - albeit one which, in this example, you don't write down

If the target doesn't move against the shore at the moment of observation, then the bearing is constant and a collision course exists. If it does move against the shore then the bearing is changing and there is no collision risk. Agreed?

The target is reassessed a little later (because things change). Sensible seamanship - you'd do the same with a hand bearing compass, I'm sure.

This time you establish a transit with a different piece of shore because you and the target have moved, and both may have changed speed and course, but the land has stayed stock still.

The same criterion applies - movement against that instant's shore transit - no collision danger; no movement, collision danger exists. Yes?

The monitoring continues at intervals until the target is outside of any possible collision danger which, in my method, is when the target's bearing is changing rapidly against the land or - if constant - is moving away from you.

My method has another benefit. You can monitor multiple targets without needing to shoot bearings, note them down, and remember which target they were shot against when you repeat the exercise. Doing the same with an HBC is a recipe for confusion, and that creates danger.

(Incidentally, I accept there may be the odd circumstance - say a high speed motorboat speeding along the beach - where the method is fallible, but those examples make themselves very plain early one and alternative methods can be used.)
 
Last edited:
timbartlett said:
Probably luck. It obviously has absolutely nothing to do with the way you assess risk of collision.
Click to expand...

Well I've been awfie lucky as well... as have most of the people I've sailed with!

BTW , Tim, for a so called expert in navigation, my perception is that you either have rather a limited ability to grasp some fundamentals of trigonometry and geometry. Tried a few times but "pearls before swine" comes to mind.
 
Twister_Ken said:
Now, here's an interesting slant. You WANT to hit something - a mooring buoy say. There's a tide running. What do the YM text books say? Steer a course and speed which holds the buoy constant against the background and you will fetch it. If it moves against the background you'll miss it.

Pipe that in your smoke and stick it.
Click to expand...
That approach will always work against a fixed object - as it ensures you're approaching on a straight course across the ground.
It will not always work if you're trying to hit a moving object - where as taking a bearing - either using a compass or just using the experience of your eye - and ensuring that is constant will get you to the target every time.

Smoke that in your stick and pipe it! :p
 
Twister_Ken said:
T<snip> at the moment of observation
Click to expand...
How long is a moment?
If you use a camera it looks like nothing is moving at all.
If you use a video camera and take say - 10 seconds of footage then the other boat could well be moving against the background and still be a risk of collision.

Proudly stating that you've used it for 40 years and not hit anything doesn't say anything about the method - only that when a collision course has existed either you or the other boat has taken action to avoid impact. If you're suggesting that you've only used the method above then I guess there are quite a few irritated skippers who have had to give way when you haven't ... but I would like to think that your eye is keener than that and you don't only use shore transits to judge collision course.

I will try it when I'm next on the water - but I still contend that whilst this method can work, it is not infallible and needs to be used with the understanding of its limiting factors.

I can state (and you can call this a testimonial if you like) I have observed my boat on a collision course with another similar vessel when the background was moving quite quickly. The situation was with racing dinghies in light wind but a strong current - even "a moment" would've proved the discussed theory wrong for this instance.
 
FB, the key to understanding the theory is COG .. collisions occur over the ground simultaneously to above it in the water
 
You must log in or register to reply here.

Share this page

Top