Checking potential collision course?

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scotty123

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On a recent RYA Theory course, a question was asked about a Skipper checking if another vessel was on a collision course. The answer given, was he should take bearings with a hand held compass or check the bearing against a stanchion. Should the bearing remain the same, a possible collision situation exists.

One of the other students, had been told by another instructor, that another method, was to take a suitable landmark & check the bearing of the other vessel against that. If it moved ahead, it was going faster & if it dropped back, it was going slower. This then confirmed that no collision is possible.

Is this method, not mentioned in any RYA books, correct ie will it check if there is a likely collision potential?
 
I'm afraid it's not true. You can't use a landmark to give the relative positions of you and another boat.

Think of it this way. Suppose the two of you were abreast, but you don't know whether you're converging, diverging or running on parallel courses. Somewhere in the background will be an object in transit. Five minutes later, the object will have dropped back, but the other boat may still be abreast, and you still have no idea whether it's closer or not.
 
scotty123 said:
One of the other students, had been told by another instructor, that another method, was to take a suitable landmark & check the bearing of the other vessel against that. If it moved ahead, it was going faster & if it dropped back, it was going slower. This then confirmed that no collision is possible.
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There was a recent long conversation about this.
The upshot was that it is not a totally sound idea, but handy if used with discretion.
 
Constant Bearing Is The Only Sure Way

The way to check if you are on a collision course is with a bearing. Other visual methods using your eye may give you an indication that two vessels are not on a collision course, but it could be wrong.

Measuring the bearing is the only sure way when using your eyes. I have made the mistake of assuming that moving land behind the other vessel indicates a non collision situation but it was demonstrated graphically that it can be wrong.

I believe that this is why the RYA dont endorse a transit or a relative motion across the land as definitive indicators of a whether a collision situation exists or not.
 
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doug748 said:
There was a recent long conversation about this.
The upshot was that it is not a totally sound idea, but handy if used with discretion.
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Exactly.

It's a technique that is often used for port tackers to shave the sterns of starboard tackers during racing, where you don't want to miss by any more then you have to. But it has to be used sensibly.

For the OP:
The hand bearing compass is the proper way, but do be aware that you might have to wait a while for the bearing to start changing noticeably as the ship gets closer.

Be very cautious of lining up a stanchions to determine if you're on a collision course. There are strong reasons why this can give you the wrong answer, from erratic helmsmen to waves throwing the boat aroud.
 
While as mentioned it's not a fool proof method of foretelling a potential collision I think in most cases it is accurate enough to tell you whether you need to get the compass out for some accurate bearings.

Obviously the stanchion method also only works if you stay in the same position, but again is good enough to tell you whether you should be worrying or not.
 
BlowingOldBoots said:
The way to check if you are on a collision course is with a bearing. Other visual methods using your eye may give you an indication that two vessels are not on a collision course, but it could be wrong.

Measuring the bearing is the only sure way when using your eyes. I have made the mistake of assuming that moving land behind the other vessel indicates a non collision situation but it was demonstrated graphically that it can be wrong.

I believe that this is why the RYA dont endorse a transit or a relative motion across the land as definitive indicators of a whether a collision situation exists or not.
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How was it "demonstrated graphically"?
 
That is an interesting idea - but I think to be effective the land must be much further away than the other vessel, so that the bearing of the landmark can be considered constant during the time you are assessing collision risk.

Probably at least as reliable as lining up on a stancheon
 
I think that the landmark behind the other vessel has to be a good distance behind the vessel for this to give an reasonable estimate, presumably at least a mile or two.

Even if the conditions just happen to fall right, it's much easier to check against a stanchion for an initial distant estimate as the conditions are always right that I can't really see why anyone would use the "landmark" method

Richard

beaten to it by bedouin!
 
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I absolutely don't understand the discussion!
There's only one right way.
Take a bearing from the ship on the possible collision course .
Supposing the ship is on your starboard side :
A bearing thet doesn't change means there will be a collision.
A bearing that gets less e.g. from 150degrees to 140 degrees means the sip will pass in front of you.
The other way around it will pass behind you.
A ship on the port side its the other way around.
Ther less change , the closer the CPA ( closest point of approach).
The closer the ship is the more often the bearing must be taken.
Off course a landmark cannot be of use because : what to do without any land sight?
As for currents, these only affect the correctness if there is a difference in curent for both ships which can be near a cape.
Ad
 
simon14b said:
... it does have its limitations.
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The limitation is that one of the vessels (either you or the other) has to be stationary! If either of you are moving it does not work.

The explanation is simple:
If you are on a collision course with another vessel, then his bearing from you is constant (there are a few very special cases where this is not necessarily true, but the principle is sound, and well-established)

If you are moving then the bearing of a landmark from you is bound to be changing unless your track is directly towards or way from the landmark.

So if you are moving, and another vessel remains lined up with a landmark, then either:-
The bearing of the landmark is changing, ergo the bearing of the other vessel is changing
or:-
Your track is straight towards the other vessel, in which case a collision can only happen if he is stationary.
 
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Even more confused.

BlowingOldBoots said:
This is the post where the discussion happened. The graphic is in the post.

http://www.ybw.com/forums/showthread.php?t=227481&highlight=collision
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I've also looked at that post, but now probably even more confused.

Could you expand on those diagrams please.

The one showing constant bearings using a hand bearing compass I can understand, since it shows an unchanging bearing, with the distances for both boats at the intervals bearings were taken along their course lines, also unchanging, which presumably means that the approach speed (& direction moving) of the target is unchanging.

The one showing bearings taken at similar intervals on the spire, seem to show increasing intervals along the targets course line. Does this mean that it would have to be accelerating to achieve crossing these lines? Not sure what I am looking at.

I can also understand that if the target moves backward, it will pass astern (going slower) & if it moves forward it will pass ahead (going faster), but how does that solve 'will it collide (or how close will it get?).
 
RichardS said:
I think that the landmark behind the other vessel has to be a good distance behind the vessel for this to give an reasonable estimate, presumably at least a mile or two.
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That's exactly right. Imagine that the 'landmark' is the moon, low in the sky. For practical purposes its distance is as near infinite as makes no difference. So effectively, you're taking a bearing, albeit that there's no need to put a number to it. The nearer the reference object, the less this effect applies.
 
YELLOW BOAT said:
. . . . There's only one right way.
Take a bearing from the ship on the possible collision course .
Supposing the ship is on your starboard side :
A bearing thet doesn't change means there will be a collision. . . . . .
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No it doesn't!

If a ship to starboard at say 7 cables is making 5 knots on a course 270°T

You are also on a course of 270°T and also sailing at 5 knots.

Within 10 minutes, half an hour and one hour you take other sightings and the ship remains in exactly the same position relative to you. You will not collide. ;)
 
lenseman said:
No it doesn't!

If a ship to starboard at say 7 cables is making 5 knots on a course 270°T

You are also on a course of 270°T and also sailing at 5 knots.

Within 10 minutes, half an hour and one hour you take other sightings and the ship remains in exactly the same position relative to you. You will not collide. ;)
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He could also be on a constant bearing of 270 degrees yet moving away from you. A collision is unlikely in that situation as well unless you get hit by the other ship you didn't see because you were fussing over the first one so much.
 
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.

All it takes a second or two to determine what's what. Several looks over the course of a few minutes will keep a check running - remember that you or he might be changing course because of being headed or lifted, or just rank bad helming (on his boat, of course!).

As you get closer and a collision looks feasible you have to decide what to do - call starboard or overtaking or windward boat on him, or do something yourself to get out of his way if you have no rights.
 
scotty123 said:
.... Could you expand on those diagrams please. ...
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Ask the person who posted them. Sufficient to say I am convinced now on the limitations of the transit and relative motion methods. Prior to that post I had a flawed understanding.
 
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