Checking potential collision course?

Well I'll start with 1/2 mile and adjust later. The situation here is a ship travelling West 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

So the question is - how accurately can you take bearings at sea? You might manage 1° with an azimuth ring on a gyro compass on a stable platform but how about a hand-held compass on a small boat? Until the ship is within 1 mile, the change in bearing is just 5°
 
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snowleopard said:
...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
1.5__81
1.3__81
1.1__82
0.9__84

So the question is - how accurately can you take bearings at sea? ...
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As I mentioned earlier, if you are doing 6 knots and you are six miles from the landmark, the bearing of the landmark changes by 1 degree per minute. (As it would if you were five miles from the landmark, doing 5kts. That is more than the rate of change of bearing of the ship in your example until it is within a mile and a quarter. In other words, the landmark method would conceal the risk of collision until you are within just over a mile of the ship.

This is one of the great beauties of AIS: AIS bearings are accurate to a degree, so you can detect a change of bearing from 077 to 078.
 
YELLOW BOAT said:
I absolutely don't understand the discussion!
There's only one right way.
Take a bearing from the ship on the possible collision course .
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What was being discussed was methods of 'taking a bearing' ....

And there isn't only one right way of doing this - there are lots of right ways ...
 
Please explain.

snowleopard said:
Well I'll start with 1/2 mile and adjust later. 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

So the question is - how accurately can you take bearings at sea? You might manage 1° with an azimuth ring on a gyro compass on a stable platform but how about a hand-held compass on a small boat? Until the ship is within 1 mile, the change in bearing is just 5°
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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.

The Yacht is steering 360, so any compass bearings on a hand held compass reference anything directly in front of the yacht, would be around 360 & anything to the West (the Ship "travelling" East) must be between 270 - 360.
 
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.
 
jimi said:
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.
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This is fine when the current is slow relative to your speed - but fails when it is a significant element.
 
timbartlett said:
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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"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."

Could you explain please.

Trying to get my head around how a collision occurs if you or the other vessel is/are stationary.
 
jimi said:
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.
Click to expand...

At last, someone else who gets it!
 
Twister_Ken said:
Rowlocks - works perfectly fine it the Solent with a fair bit of tide.
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How can it? - with current as a significant proportion of your boat speed you're both passing the scenery - therefore any scenery is constantly changing behind your target vessel. I know this through racing dinghies in little wind where you travel at around 2-3 knots with a current of 1-2 knots ...

When you've got a small proportion of current then the affect is less noticable and the practice can be used to aid collision or not mental calculations - as long as you're crossing at around 90° or so ...
 
scotty123 said:
Surely, if there is any current, it affects both vessels, regardless of how fast, so can be eliminated as a factor.
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Yes - but you're effectively taking a transit with the other vessel and land (or object fastened to the land in the case of a buoy or post) - land isn't affect by the current (or shouldn't be anyway!) and therefore shouldn't be used to judge the collision potential! As I said previously - if the current is an insignificant element compared to your speed then the errors are marginal and it then becomes a good rough indicator.
 
fireball said:
Yes - but you're effectively taking a transit with the other vessel and land (or object fastened to the land in the case of a buoy or post) - land isn't affect by the current (or shouldn't be anyway!) and therefore shouldn't be used to judge the collision potential! As I said previously - if the current is an insignificant element compared to your speed then the errors are marginal and it then becomes a good rough indicator.
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I think you're wrong. The relevant vectors are relative to the ground if comparing with land, and the ground vector of the vessel is combination of vessel course and direction ples current vector .. so theoritically it still works. Practically it always worked for me when I was based in the Solent.
 
jimi said:
I think you're wrong. The relevant vectors are relative to the ground if comparing with land, and the ground vector of the vessel is combination of vessel course and direction ples current vector .. so theoritically it still works. Practically it always worked for me when I was based in the Solent.
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I agree with jim, the whole point is that it does take into account the tide and to be honest, I'm not sure, or really care, about the geometry of this, all I know is that it works. I use it every day on both my yot (snailing along at 6 knots) and on my fishing boat (creaming along at 15).

I agree that the coast has to be a fair way off but from a practical point of view the land as seen from roughly the middle of the Solent is easily far enough off for the method to work.
 
fireball said:
Yes - but you're effectively taking a transit with the other vessel and land (or object fastened to the land in the case of a buoy or post) - land isn't affect by the current (or shouldn't be anyway!) and therefore shouldn't be used to judge the collision potential! As I said previously - if the current is an insignificant element compared to your speed then the errors are marginal and it then becomes a good rough indicator.
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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?

Also, are you saying, that as your speed over the ground, is comprised of current + speed thru the water, then if both are enjoying the same current, should this be the dominant factor for this combined speed, it will be impossible to gauge what the effect of the boat speed thru the water?
 
scotty123 said:
"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."

Could you explain please.

Trying to get my head around how a collision occurs if you or the other vessel is/are stationary.
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In that instance the other vessel could be a channel marker or a moored boat.
 
I think you're wrong in your view - if you have a 2 knot current and you're being swept along with it as is the target vessel then the land you can see will be different as you progress - this will be noticable if your speed through the water is not high.

It is the equivalent of two boats on a converging course (ie not quite parallel) - you can see the other boat to your beam - but there is no point trying to transit against the shore as it will be changing as you travel along.

I think you can use it if the current is weak and you're travelling quickly and on a crossing situation .
 
ChrisE said:
I agree with jim, the whole point is that it does take into account the tide and to be honest, I'm not sure, or really care, about the geometry of this, all I know is that it works. I use it every day on both my yot (snailing along at 6 knots) and on my fishing boat (creaming along at 15).

I agree that the coast has to be a fair way off but from a practical point of view the land as seen from roughly the middle of the Solent is easily far enough off for the method to work.
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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.
 
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