Checking potential collision course?

We use this all the time when we're racing the dinghy, it really dos work a treat to determine if boats on oppposite tacks are on a collision course. Just line up the other chaps mast against a suitable landmark and then see if his mast moves in front of or behind the mark on the shore, no compass required, if it drops back you'll pass in front and vice versa... have used it racing 34 footers in the ionian regatta, but not sure if I'd like to chance something really big....
 
scotty123 said:
I've been trying to follow some of the more 'technical/geometric' offerings on this thread, a few I understand, many I admit defeat in understanding.
Maybe a tutorial is required.

In the above, could you please explain that diagram, because it seems to me, that the ship is on a constant bearing from the yacht and is seen moving along, but not "eating up" the coastline which I always understood (particularly from some posts on this thread) meant that the 'target' was moving ahead (or behind) a steady bearing against a moving land background.
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As you say, the ship is on a steady bearing from the yacht. That is because they are heading towards a collision. A "steady bearing" is the classic test of a risk of collision.

Here's a modified version of the same diagram. The only amendment is that I have added letters on the coastline.
At T-18, the skipper of the yacht sees the ship directly in line with the land at A, on a bearing of (roughly) 210. The land at B and C is ahead of it.
At T-12, both vessels have moved. The bearing of the ship from the yacht is the same. The range of the ship from the yacht has reduced. The skipper of the yacht now sees the ship in line with the land at B -- i.e. The ship has "eaten up" the stretch of coastline between A and B.
At T-6, both vessels have moved. The bearing of the ship from the yacht is the same. The range of the ship from the yacht has reduced. The skipper of the yacht now sees the ship in line with the land at C. i.e. the ship has "eaten up" the stretch of coastline between B and C.
 
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timbartlett said:
As you say, the ship is on a steady bearing from the yacht. That is because they are heading towards a collision. A "steady bearing" is the classic test of a risk of collision.

Here's a modified version of the same diagram. The only amendment is that I have added letters on the coastline.
At T-18, the skipper of the yacht sees the ship directly in line with the land at A, on a bearing of (roughly) 210. The land at B and C is ahead of it.
At T-12, both vessels have moved. The bearing of the ship from the yacht is the same. The range of the ship from the yacht has reduced. The skipper of the yacht now sees the ship in line with the land at B -- i.e. The ship has "eaten up" the stretch of coastline between A and B.
At T-6, both vessels have moved. The bearing of the ship from the yacht is the same. The range of the ship from the yacht has reduced. The skipper of the yacht now sees the ship in line with the land at C. i.e. the ship has "eaten up" the stretch of coastline between B and C.
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I accept that the ship is moving from A-B & B-C, thats not in question.

Perhaps an earlier post by TwisterKen might explain it better than I can.

"I've used the shore method (when there's shore to be seen) for years and it works. BUT it's not exactly a transit on the shore. What you're looking for is the movement of the shore relative to the other boat. If he's moving up the shore, he's beating you. If shoreline appears to be coming out from in front of him, you're beating him. When he stays pretty well constant against the shoreline then there's a collision risk."
 
scotty123 said:
I accept that the ship is moving from A-B & B-C, thats not in question.

Perhaps an earlier post by TwisterKen might explain it better than I can.

"I've used the shore method (when there's shore to be seen) for years and it works. BUT it's not exactly a transit on the shore. What you're looking for is the movement of the shore relative to the other boat. If he's moving up the shore, he's beating you. If shoreline appears to be coming out from in front of him, you're beating him. When he stays pretty well constant against the shoreline then there's a collision risk."
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Well - I don't doubt both TK and Jimi do use this method and avoid collisions - but I think there is more too it than they are explaining - as TB's diagram neatly explained - the Ship looks like it is moving up the shore - therefore beating you ... but your bearing to the ship is constant and there is a collision risk - so that method doesn't work at the range and in the timescale that TB demonstrated.
Where I think it may work is when you take short looks at the target (5-10 seconds?) - with a shoreline a reasonable distance off when no significant change in background would be viewable within the much reduced timescale. What you're effectively doing is using the land to establish a bearing at a point in time and assessing - at that point in time - if the target vessel is on that steady bearing. A minute later you do the same thing - but with a different background.
It fails to work consistently with continuous observation, and I still contest that it will fail to work when current is a significant proportion of your speed - but at that point it is not difficult to avoid collisions anyway.

Half the problem is that we do many things automatically through instinct or experience and it is difficult to explain the process behind them. I'm still going to try this method when I next go out - but I can still see the huge limitations of it and at the moment I certainly wouldn't want to teach it as a collision detection system to a novice.
 
scotty123 said:
I accept that the ship is moving from A-B & B-C, thats not in question...If he's moving up the shore, he's beating you. ... When he stays pretty well constant against the shoreline then there's a collision risk."
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I'm afraid the distinction between "moving from A-B & B-C" and "moving up the shore" is too subtle for me.

What we can see is that in my example the ship is destined to collide with the yacht unless one of them takes avoiding action. There is clearly a collision risk, in spite of the fact that she is "moving from A-B & B-C", and is not staying "pretty well constant against the shoreline".
 
OK, guys, what software can I use to create these diagrams?

FWIW TB's diagram is downright misleading as the scale is inappropriate to see the rate of change (and the increasing rate of change) as the vessels approach each other
 
Joker said:
Sorry, folks, Tim is right.

You cannot use the shore as a reference for potential collision with another boat.
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Sorry folds the method is right. You can't trust anybody else to know what they're talking about :)

No one has yet put up a convincing argument as to why a method that patently works for so many of us, er doesn't.
 
Yes, it really works. Look:
bang.gif

'We're perfectly safe, darling - look, it's moving against the shoreline.'

BANG!
 
Whatever the diagrams say, if the land horizon moves against the other boat, you can tell if you will clear ahead or behind it. It has never failed for me, on the Thames or the Solent. It's the classic case of "it works in practice, but will it work in theory?".

You have to use it to prove it.
 
Joker said:
<headbang>One day, the headbang will be a real 'BANG!@</headbang>.

You don't need theory. Look at the pretty picture.
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The method deniers are beginning to sound like the scientists who used to claim that according to the laws of physics bees can't fly.

It seems to me that the geometries and models that have put up to date are missing something. I wouldn't claim to know what it is but the fact that the method works so well for many of us suggests to me that the modelers are missing something. A bit like global warming? I'll get my coat as I leave:)
 
ChrisE said:
The method deniers are beginning to sound like the scientists who used to claim that according to the laws of physics bees can't fly.

It seems to me that the geometries and models that have put up to date are missing something. I wouldn't claim to know what it is but the fact that the method works so well for many of us suggests to me that the modelers are missing something. A bit like global warming? I'll get my coat as I leave:)
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It is simple - the method works fine over short timescales where the land is comparatively far away - several times the distance between the boats.

All the antis post pretty pictures where the shore is close and talk about situations evolving over a comparatively long period of time.

The method seems excellent meeting a boat of similar size - and who ever uses a compass in those situations?
 
bedouin said:
It is simple - the method works fine over short timescales where the land is comparatively far away - several times the distance between the boats. <snip>
The method seems excellent meeting a boat of similar size - and who ever uses a compass in those situations?
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which is why it isn't taught as a method because it is a rough reconner that works for particular scenarios but doesn't work across the board.
 
At least 4 of the people responding in support are qualified yachtmasters(practical) .... now what was that what you said about certification?
 
Joker said:
That makes it all the worse (I hope none of them are instructors!).
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"It was a fine sunny day with the wind about F3. I set off into the unknown. Everything was fine for the first few hours, then came the first big puzzle.

On the horizon were sails – not just one or two, but perhaps forty or fifty, all bunched together. I looked at the chart – Worthing? Littlehampton? Forty yachts? Unlikely. Then ... maybe they were coming out of the Solent. Perhaps I was making better time than I thought I was. I tacked inshore, and saw the sails turn round and head in. Closer to, I saw dozens of Laser dinghies on the beach*. Chichester, I thought. Well, press on, and I can make Portsmouth.

But the wind died, and I found myself drifting about a mile from the shore. Then, to my surprise and alarm, I felt the boat bump. And bump again. And stop. I was aground. A couple of hundred metres away I could see a post**, obviously marking something – though I didn’t know what."



Any further comment?
 
or shall I continue quoting?

"I suppose I was stuck for about an hour, until the tide started rising and I was able to bump my way out. I could see the Isle of Wight, but the wind had now filled in again, and the island was dead to windward. Thus I was trying to beat against the tide to get there – a fairly fruitless pastime.

It was also getting quite late now – sunset couldn’t be that far off. I decided I was getting nowhere – why not turn back and head to what I thought was Chichester? But by the time I got there the sun was below the horizon, and what I could see didn’t look at all like Chichester. I hove to and scanned the chart. Nothing I could see bore any resemblance to anything on the chart.

But it was getting dark enough for buoys to start flashing, and I saw something – six short, one long. I scanned the Solent chart. Nothing there. And if I was near Chichester, then I should be seeing other lights. But there was just this one. In desperation, I pulled out the chart for Selsey Bill to Beachy Head, and started scanning that. Yes! Off Littlehampton. And everything fell into place."
 
Well done, sir, you've read my blog. Here you are, you can all read it.

Gosh, I got things wrong. Even you get might things wrong occasionally. Heavens, I even write about them.

But I don't go around saying that 2 + 2 can often make 5, depending on the situation.
 
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