Checking potential collision course?

ancientsailor said:
“It’s all very well in practice, but it doesn’t work in theory” – I use it in practice and proved it won’t work in theory.
Click to expand...

Agreed. It isn't a mathematically accurate method but it's a close enough approximation and generally works.

What needs to be established is - under what circumstances does it not give a 'good enough' answer? If the ship-shore distance is a lot less than boat-ship distance the relative movement will be no use as a guide but at what ratio of distance does it become usable: 1:1, 5:1, 10:1 ?
 
ancientsailor said:
Unfortunately I quoted an earlier post which quoted from when the ship was heading east ... all has changed now she's heading west!
Click to expand...

many thanks for this post, I was struggling having refered to my earlier posts, to keep up, even though snowleopard had said he had changed it.

Are senior moments contagious?

Presumably this does not affect the bearings ( are these called relative)?
 
scotty123 said:
many thanks for this post, I was struggling having refered to my earlier posts, to keep up, even though snowleopard had said he had changed it.

Are senior moments contagious?

Presumably this does not affect the bearings ( are these called relative)?
Click to expand...

The collision regs refer to compass bearings .. which apply in all circumstances.

A relative bearing is measured from a point on the vessel:
  • "on the port bow"
  • "two points abaft the starboard beam" pretty old-fashioned now
  • "red 20 degrees" (on the port side, 20 degrees measured from right ahead)

The second animation http://www.sailskills.co.uk/Webdemo_ColRegs/sailskills_sample_animations_compass_bearing.html
shows how two vessels collide even as the relative bearings change - the compass bearing does not. I wrote the animation and it was not "flogged" in any way. Both vessels go at a constant rate and keep the same turn rates ... well, within the limits of the software I used

I don't think senior moments are catching ... what was this all about?
 
ancientsailor said:
The collision regs refer to compass bearings .. which apply in all circumstances.

A relative bearing is measured from a point on the vessel:
  • "on the port bow"
  • "two points abaft the starboard beam" pretty old-fashioned now
  • "red 20 degrees" (on the port side, 20 degrees measured from right ahead)

The second animation http://www.sailskills.co.uk/Webdemo_ColRegs/sailskills_sample_animations_compass_bearing.html
shows how two vessels collide even as the relative bearings change - the compass bearing does not. I wrote the animation and it was not "flogged" in any way. Both vessels go at a constant rate and keep the same turn rates ... well, within the limits of the software I used

I don't think senior moments are catching ... what was this all about?
Click to expand...

"I don't think senior moments are catching ... what was this all about?"

Something snowleopard said in an earlier post.
 
I have just modelled this graphically using vectors and the proposition is valid, with or without tide. Distance of the shore is irrelevant theoretically. I shall endeavour to model mathematically, but the equations are much more complex than the simplistic ones used so far.
 
jimi said:
I have just modelled this graphically using vectors and the proposition is valid, with or without tide. Distance of the shore is irrelevant theoretically. I shall endeavour to model mathematically, but the equations are much more complex than the simplistic ones used so far.
Click to expand...

Shall be most interested to see - look forward to it.

I simply used plane triangles, moving the vessels on their courses specified in the scenario, recalculating the bearings between them and the land - well the spreadsheet did!
 
Concede defeat, theotetically vessels can be on a collision course and both perceive the other vessel to be moving forward against the background.

In my defence, it is just a rule of thumb that is used to make an intial appraisal of whether to take early action or not. In theory the actual "pivot point" on the land is 30% different distance wise. On far away land 30% will not be a significant and may well be perceived as the same point. At close quarters 30% is quite different and one ought to be using other sensory equipment and tools to detect relative courses of craft to detect if collision is imminent.

So all in all, happy conclusion, we're all right. ;-)
 
jimi said:
Concede defeat, theotetically vessels can be on a collision course and both perceive the other vessel to be moving forward against the background.

So all in all, happy conclusion, we're all right. ;-)
Click to expand...

Fireball gave a clear example earlier (post 66) of a circumstance where it does not work.

So, in my mind, Fireball is just a bit more right than some of you! :p
 
Fireball's example works if you are talking about a transit (i.e. two fixed reference points in line), but it also works it you are talking about a single reference point on land (such as the top of the hill or, in Fireball's case, the ferry terminal in Cowes). I think Jimi's last post acknowledges that.

If the other boat is moving ahead relative to the ferry terminal it may indicate that you with miss each other, but it doesn't guarantee it in the way that changing relative bearings do.

(If the other object is fixed, then the moving relative to background method does work in all circumstances where you maintain a constant heading).
 
Last edited:
Angele said:
If the other boat is moving ahead relative to the ferry terminal it may indicate that you with miss each other, but it doesn't guarantee it in the way that changing relative bearings do.
Click to expand...

If the other boat is moving ahead relative to the ferry terminal you WILL go behind it, unless you or he subsequently change course or speed, in which case you have to reassess.
 
If the other boat is moving ahead relative to the ferry terminal you WILL go behind it, unless you or he subsequently change course or speed, in which case you have to reassess.
Click to expand...

No! Not necessarily. Take Fireball's example and draw it on a chart and prove it for yourself.

Jimi is a convert to the cause because, I think, he did just that.
 
Actually I'm not a covert!

takin a compass bearing on an vessel and determining if the bearing is constant is eaxactly an example of what we are talking about, the only difference is that the magnetic pole is positioned quite some distance away.. but its still a fixed point on earth.. my altered argument would be that if there was sufficient accuracy to steer abolutely to a fixed compas bearing then the vessels would miss each other (think about it , to collide the lines of reference must be parallel at different times) a fixed compass bearing indicates a pivot point around the magnetic pole and therefore the lnes cannot be parallel).

Therefore a fixed bearing is a rule of thumb to see if risk of collision exists.

Likewise the speed of movement of a vessel against background land is also a very useful rule of thumb to see if risk of collision exists.


The key words are "rule of thumb" "risk".

Neither is an absolute guide.

Sorry for argiung it badly but rapidly scribbled whist up to ears at work!
 
Angele said:
Fireball gave a clear example earlier (post 66) of a circumstance where it does not work.

So, in my mind, Fireball is just a bit more right than some of you! :p
Click to expand...

Oi - don't go dragging me back into it - I still can't picture what they're describing - but if it works for them then great ...

Personally I tend to use relative bearing to the target - visually rather than with a compass! - I find that always works ...
 
In Fireball's example, I think it is a reasonable approximation to ignore the change in compass bearing from sailing less than one mile west as infinitessimally small. Certainly adequate when one considers that the other boat is also not infinitessimally small such that you will hit one end or the other.

Anyway, taking Fireball's example a bit further. (Apologies, but positions are only approximate because working from a small chartlet in Reeds and not a full size paper chart).

End of ferry pier: 50deg 45.9min N, 01deg 17.5min W
Other boat: 50 deg 46.1min N, 01 deg 17.5min W (i.e. at the entrance to Cowes, 0.2nm due north of the pier).
Your boat 50deg 47.1min N, 01 deg 17.5min W (i.e. precisely 1 nm north of other boat, and therefore 1.2nm from ferry terminal.

If SOG is 5kts in 2kts of ebb tide and lets say our course is 45 deg from the true wind (i.e. we are sailing southwesterly). Can't be bothered to work out what our COG is, but for every 0.68nm through the water, we ahve sailed 1.0mile over land,which can be broken down into a VMG component of 0.88 nm and a southerly component of 0.48nm. The other boat is doing the same, but his 0.48nm component is to the north.

After 0.2hours (12 mins), the two boats are now just 0.04nm apart. The bearing of the other boat has remained constant. The collision is imminent and yet the Cowes ferry pier is nowbehind our beam (remember we are heading SWly) - previously it was ahead the beam.

So, it looks like the other boat is "overtaking" the pier.

CRUNCH!!
 
fireball said:
Oi - don't go dragging me back into it - I still can't picture what they're describing - but if it works for them then great ...

Personally I tend to use relative bearing to the target - visually rather than with a compass! - I find that always works ...
Click to expand...

Sorry Fireball.:o
 
Angele said:
In Fireball's example, I think it is a reasonable approximation to ignore the change in compass bearing from sailing less than one mile west as infinitessimally small. Certainly adequate when one considers that the other boat is also not infinitessimally small such that you will hit one end or the other.

Anyway, taking Fireball's example a bit further. (Apologies, but positions are only approximate because working from a small chartlet in Reeds and not a full size paper chart).

End of ferry pier: 50deg 45.9min N, 01deg 17.5min W
Other boat: 50 deg 46.1min N, 01 deg 17.5min W (i.e. at the entrance to Cowes, 0.2nm due north of the pier).
Your boat 50deg 47.1min N, 01 deg 17.5min W (i.e. precisely 1 nm north of other boat, and therefore 1.2nm from ferry terminal.

If SOG is 5kts in 2kts of ebb tide and lets say our course is 45 deg from the true wind (i.e. we are sailing southwesterly). Can't be bothered to work out what our COG is, but for every 0.68nm through the water, we ahve sailed 1.0mile over land,which can be broken down into a VMG component of 0.88 nm and a southerly component of 0.48nm. The other boat is doing the same, but his 0.48nm component is to the north.

After 0.2hours (12 mins), the two boats are now just 0.04nm apart. The bearing of the other boat has remained constant. The collision is imminent and yet the Cowes ferry pier is nowbehind our beam (remember we are heading SWly) - previously it was ahead the beam.

So, it looks like the other boat is "overtaking" the pier.

CRUNCH!!
Click to expand...

In this case , I suspect most people would have worked out that risk of collision existed sometime before Crunch ( If you'd used on rules of thumb and commonsense )
 
You must log in or register to reply here.

Share this page

Top