Checking potential collision course?

[fireball]

I accept that if lining up the other vessel, with a fixed point to get a transit, wouldn't work because that both would be past it too quickly & there wouldn't then be any transit, but doesn't comparing the other vessel with a land background (moving background) give an indication as to whether the other vessl is slower or faster?

Lining up with a fixed point or background is a transit ...

I don't understand what the significance of the other vessels speed is - it doesn't matter if he's travelling at 6 or 26 knots - if his actual bearing to you stays the same then you're going to hit!

Using land as an aid to bearing can work - but (IMHO) only in certain circumstances - and where current is a significant element of both vessels COG it won't (again IMHO).

 

[jimi]

Fieball, you're missing the point entirely .. ground vectors of vessels relative to ground.
I cannot see how it does not work. I've used the method in the solent when going into the tide with a SOG of 1.5knots and 5 knots of tide, it still worked.

 

[webcraft]

Fieball, you're missing the point entirely .. ground vectors of vessels relative to ground.
I cannot see how it does not work. I've used the method in the solent when going into the tide with a SOG of 1.5knots and 5 knots of tide, it still worked.

How do you know it worked (apart from the obvious fact that you didn't hit anything)?

- W

 

[ChrisE]

6 to 15 knots is a significant speed compared to the current (except at peak springs where the 6 knots isn't so significant!) - The whole point is that IT DOES NOT take into account the tide - it disregards it as being insignificant - I haven't got the time or inclination to draw it out - but I have experienced it first hand.

I don't doubt you have but as someone said at the start this works for most situations and I have to take issue with you on the tide. I spend most of my time around Hurst Narrows in the FB and it is a rare day when:

a. I don't have to assess the liklihood of collision with a yot or stinkie when drifting with the tide and
b. the tide run is less than 4 knots.

And any more jokes about the speed of my yot and I'll ram with the FB...

 

[Twister_Ken]

Fireball, I think you're missing the point. Which is that you are observing the other vessel's progress (or lack thereof) against the land. You are not trying to sight it against a fixed point on the land. Those who talk of transits in this situation are misleading us.

As to boat speed vs current speed, I raced in the Solent and Channel for over 25 years in springs and neaps, in drifters and in hoolies, and it works in all of them. And we never hit anything.

Can I suggest you try it some time!

 

[fireball]

Fieball, you're missing the point entirely .. ground vectors of vessels relative to ground.
I cannot see how it does not work. I've used the method in the solent when going into the tide with a SOG of 1.5knots and 5 knots of tide, it still worked.

ok - 2 minutes and I'll give an example how I see it ...

In a true west wind, you're beating westwards off Cowes against another indentical yacht who is on the opposite tack. There isn't much wind and boat speed is 3 knots.
You have a current of 2 knots (ebb) going west. Both boats SOG is around 5 knots.
At Cowes you're about 1 mile apart and you are directly north from the other vessel and Cowes - you can't see the ferry terminal (well you can, but it is obscured by the other yacht) - 0.7 miles later through the water and you'll have travelled another ~0.5miles west - you're still directly north of the other boat because by some miracle you're both travelling at the same speed and identically affected by the current.
Now if you look past the other yacht the background is Gurnard ... however, because you're on opposite tacks and doing the same speed you're still on a collision course. - your relative bearing to the other vessel (180°) has not changed.

 

[fireball]

Fireball, I think you're missing the point. Which is that you are observing the other vessel's progress (or lack thereof) against the land. You are not trying to sight it against a fixed point on the land. Those who talk of transits in this situation are misleading us.

from Jimi's

Personally speaking, whenever there is land in front I use it to judge if i'm going to pas in front or behind. If the vessel concerned is moing forward against the land I'll pass behind, moving backwards I'll pass in front, steady the collision course. This has worked without fail for me. Obviously the drawback is that there's got to be land behind the vessel therefore no use in open sea.

So - you're not talking about transiting the vessel against the land! Ok ... been arguing cross purpose then ... !!
I still can't quite see how you're working this though ...

 

[YELLOW BOAT]

Unless I'm misunderstanding I think there's made a classical fault!
If two vessels are near enough to be at risk of a collision the will be in the same ( tide or else) current! Of course on a river or other very local current it can be different but I'm talking now about sea , several miles outside.
If the two vessels are in the same current there is no difference to the same situation without any current.
Imagine both vessels are moveless ( no sail, no engine , no wind) than they are driftng with the current at the same speed and direction. Therefore the distance and relative direction will not change.
So drifting with the current not necessarely leads to a collision because its the same as lying still and the other ship moving!
Ad

 

[bedouin]

Surely the only issue here is when this is an appropriate method?

The advocates are recommending this as a good method for assessing potential collision with a vessel of similar size (e.g. in racing conditions). That implies close quarters and short term and under those circumstances it seems very sensible - better in fact than a hand bearing compass.

For larger vessels, and greater distances (therefore times) you need to be more aware of the limitations.

 

[jimi]

from Jimi's

So - you're not talking about transiting the vessel against the land! Ok ... been arguing cross purpose then ... !!
...

absolutely .. its all about relative motion

 

[ChrisE]

absolutely .. its all about relative motion

Yes absolutely, although I have real trouble when looking at jimi's bendytoy from the vantage point of my ocean going greyhound to perceive any motion at all!

 

[ancientsailor]

The situation here is a ship travelling East at 20 knots and a yacht going North at 5 knots. They are on a converging course such that the yacht will pass 1/2 mile ahead of the ship. The table shows the bearings of the ship at various distances.

Dist_Bearing
5.6__77
4.6__77
3.6__78
2.5__79
1.5__81
1.3__81
1.1__82
0.9__84
0.7__86
0.5__90

... Until the ship is within 1 mile, the change in bearing is just 5°

I’ve just got side-tracked by this thread into putting all the figures into a spreadsheet for the example above (I really should have been doing something else – I must get out more).

Assuming the point on land is six miles off at the start of the figures (try it any earlier and the ship’s further away than the land) gives the result below (there are some rounding errors but less than a degree).

The results show that the (compass) bearing of the:

• ship changes very slowly – less than 15 degrees in 15 minutes. It’s going to be close
• land changes slightly faster – but not hugely compared to the ship. It’s going to be close, but it looks like the ship will pass close ahead – WRONG, it’s going to pass astern

Both methods indicate the ship’s going to be close – I for one would not like to stand-on at five knots across the bows of a ship doing 20 – without some other information such as a change in aspect of the approaching vessel.

IMO the results illustrate the points made in the posts, the method can be dangerous. If you relied solely on this method, ignoring clues such as the ship’s aspect (including at night) you could be in trouble. It does work well especially when the vessels are close, and the land is distant.

I’ll nail my colours to the mast and say I use it frequently. I do monitor the situation and check by other means if I’m concerned – usually by hand bearing compass. If I’m going slowly (usually the case!) and the bearing of the approaching vessel against the land, sun, moon, star or cloud is changing very rapidly I am happy. I guess it comes down to experience.

As stated, at six miles and six knots the bearing of the land will change by a degree a minute - on or close to the beam. This is the scenario that has been set – the land’s not much forward of the beam at 6 miles. If the land’s ahead the bearing won’t change ... in absence of leeway and set.

I feel I’ve entered an argument which goes a little like this, “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.


If anyone wants to check the maths ...

The figures in order are:

- Time from Crossing Point (mins)
- Separation of Vessels (nm)
- Angle on bow of ship from Yacht (deg)
- Angle on Bow of Object from yacht (deg)
15 5.6 77 78
14 5.3 77 79
13 5.0 77 80
12 4.6 77 80
11 4.3 78 81
10 3.9 78 82
9 3.6 78 83
8 3.2 78 83
7 2.9 78 84
6 2.5 79 85
5 2.2 79 86
4 1.9 80 87
3 1.5 81 88
2 1.2 82 88
1 0.8 84 89
0 0.5 90 90

 

[snowleopard]

Can you go over that again please.

I may be missing something, but in your example, if a ship is going East & a Yacht is going North, then surely the Ship will be to Port of the Yacht.

Yup, senior moment. Corrected the post.

 

[jimi]

If I was a scientist and I came up with a theory that predicted different results from what actually happened , then I'd revisit my theoretical calculations and try and work out what was wrong with them ..

 

[ancientsailor]

If I was a scientist and I came up with a theory that predicted different results from what actually happened , then I'd revisit my theoretical calculations and try and work out what was wrong with them ..

Fair point. Theory and practice do accord - provided it's remembered that the bearing of the land does change with own vessel's movements.

The effects of this are more significant in faster vessels, with the land close and close to the beam.

This method is really, really effective in the way it's been described in the posts - usually two small slow vessels relatively close with the land relatively distant.

It literally only takes a second or two to make the intitial assessment - while continuing to monitor of course!

 

[Touchwood]

Collision avoidance - the "Father Ted rule"

If the other boat is getting smaller - no worries
If it stays the same size - no worries
If it gets BIGGER - Look Out!

Seemples.............

 

[scotty123]

Yup, senior moment. Corrected the post.

(1) so I can understand, are bearings changing from 77 up to 90, or am I getting confused?

(2) the same figures seem to appear in Ancientsailor's version,
"I’ve just got side-tracked by this thread into putting all the figures into a spreadsheet for the example above (I really should have been doing something else – I must get out more)."
Is he also having a senior moment, or a copy/paste situation?

 

[ancientsailor]

(1) so I can understand, are bearings changing from 77 up to 90, or am I getting confused?

(2) the same figures seem to appear in Ancientsailor's version,
"I’ve just got side-tracked by this thread into putting all the figures into a spreadsheet for the example above (I really should have been doing something else – I must get out more)."
Is he also having a senior moment, or a copy/paste situation?

No I think the figures are correct.

The figures quote from the yacht's head, rather than compass bearings. I just followed the same convention to keep life simple.

At the start of the list the approaching vessel is 77 degrees on the yacht's port bow - measured from right ahead.

Gradually to this figure increases to 90 degrees when the vessel is on the yacht's port beam.

As the yacht is heading due north, all these figures should be subtracted from 360 degrees to get a "real" compass bearing (I'll re-do the spreadsheet if requested!)

++++ addendum - in the orginal scenario, which has been cut and paste a couple of times, the ship was heading east (so she was on the yacht's port side). Now the scenario's been edited she's heading west, so the bearings are compass bearings and the ship's on the yacht's starboard side. Takes so long to post a reply, the forum's moved on ++++

 

[scotty123]

No I think the figures are correct.

The figures quote from the yacht's head, rather than compass bearings. I just followed the same convention to keep life simple.

At the start of the list the approaching vessel is 77 degrees on the yacht's port bow - measured from right ahead.

Gradually to this figure increases to 90 degrees when the vessel is on the yacht's port beam.

As the yacht is heading due north, all these figures should be subtracted from 360 degrees to get a "real" compass bearing (I'll re-do the spreadsheet if requested!)

Thanks, understood, I think, now.

 

[snowleopard]

The figures quote from the yacht's head, rather than compass bearings.

I deliberately chose a north heading so that the angle is both a compass bearing and an angle on the bow.

When the yacht crosses the ship's track, the ship is exactly abeam and due East, i.e. 090°