Checking potential collision course?

[Lucky Duck]

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.

Tim,

I suspect you meant to say that if both are moving then it does not work otherwise if neither are moving they will never collide or have already done so

However Ken's observations do match my own;

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

 

[DJE]

I've used the shore method (when there's shore to be seen) for years and it works.

I'm with you Ken. Much quicker and more accurate to use the shore method. I only reach for the handbearing compass when there's no land background and then I find myself trying to average out a +/- 10 degree swing in the compass card.

 

[YELLOW BOAT]

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.
__________________
regards David - DSW Marine Engineering
www.dswmarineengineering.com


Of course principially youre right!
But I don't think anybody will start bworrying when a ship is not on a possible collision course!
May be I should have added "when a ships is on a possible collision course".
Ad

 

[Beadle]

I wonder if there was some confusion.

I've used the technique on the river Humber where you must keep close to a line between buoys and you have strong tides running in odd directions.

Set the buoy you are aiming for in line with some object on the shore - and keep there so you sail a straight(ish) line in the right direction even if you are pointing in a completely different direction to where you want to go. Aiming straight for the buoy and and not checking your line invariable involves a meting with the Humber mud.

 

[YELLOW BOAT]

off course that's true, but something completely different from avoiding a collision with a ship on a crossing course!
Ad

 

[snowleopard]

I'm with you Ken. Much quicker and more accurate to use the shore method. I only reach for the handbearing compass when there's no land background and then I find myself trying to average out a +/- 10 degree swing in the compass card.

If you work out the geometry of the situation the shore method does not give an accurate answer. To take a simple case imagine two vessels travelling at the same speed on a slightly converging course. A line through the other vessel will travel along the shore at the same speed as the vessels, i.e. the other vessel will appear to be drawing ahead relative to the land even though a it is on a collision course.

It is a useful approximation provided the distance to the shore is considerably greater than the distance between the vessels.

 

[ancientsailor]

I've used the shore method (when there's shore to be seen) for years and it works ... 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

back to the forums - been away a while ...

It works in the right circumstances

Try this (the animations further down the page in particular):

http://www.sailskills.co.uk/Webdemo_ColRegs/sailskills_sample_animations_compass_bearing.html

It was used in the earlier forum ... I try to set out the strengths and limits of this method -which I was first taught in tidal waters on my YM practical

 

[YELLOW BOAT]

Very nice example.
I wonder what to do far out at sea , crossing a traffic separation zone with no land in sight.
Mabe use the sun or moon?
The one example in the video ( slightly converging courses ) indeed is a problem which however can be handled without ataking a bearing!
You will have time enough because the courses are only slightly converging and a little change in course will be enough .
But in fact you are right, there are situation where things can go wrong. Collisions even happened because of this ( faintly remember something near New York a long time ago).
Ad

 

[ancientsailor]

I wonder what to do far out at sea , crossing a traffic separation zone with no land in sight.
Mabe use the sun or moon?

With care, yes. In suitable conditions I've even used a distant cloud to give an indication.

Be aware of the dangers and limitations of the method ...

The cloud, sun, star or moon could all be changing bearing in such a way as to give the impression that the approaching vessel's bearing is changing when it is not

There is only one absolutely reliable method - which is of course compass bearings - but the ColRegs do warn:

such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range​

 

[cgull]

I think ultimately its the warm feeling in the seat of your pants that confirms you are going to collide!

 

[webcraft]

.
I find a pair of decent bins with built in compass much easier to use than a HB compass, particularly at night - and the bins are always to hand.

- W

 

[Jamesuk]

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 expect the other instructor would say "when in open water I use clouds instead of landmarks."

Keep to the RYA methods. Do everything the RYA way and then when you have passed you can do hopefully the "safe way" when your on your own.

 

[timbartlett]

Tim,

I suspect you meant to say that if both are moving then it does not work otherwise if neither are moving they will never collide or have already done so

Sorry, yes, you're quite right. It works if one of you is stationary.

However Ken's observations do match my own;

Over very short periods of time, and with a suitably distant landmark, it is a reasonable approximation. But consider a yacht sailing past a landmark at six knots, six miles from a landmark. The bearing of the landmark is going to be changing by 1degree every minute.

If the landmark is only a couple of miles away, the effect will be three times worse.

 

[Twister_Ken]

Sorry, yes, you're quite right. It works if one of you is stationary.

Over very short periods of time, and with a suitably distant landmark, it is a reasonable approximation. But consider a yacht sailing past a landmark at six knots, six miles from a landmark. The bearing of the landmark is going to be changing by 1degree every minute.

If the landmark is only a couple of miles away, the effect will be three times worse.

FFS, you don't monitor it constantly and you're not taking a bearing on a landmark - of course that'll change over time. Just keep checking and every time you look (which only takes a couple of seconds) you can see whether the bearing is changing or not by observing the land behind the boat in relation to his forestay. Keep checking and you'll determine whether you're going to be clear ahead, clear astern or whether you'll need to do something. IT'S NOT HARD!

 

[peterb]

It's a very effective method when used at night. Compare the nav lights of the other vessel with the nearest star; much more sensitive to relative movement than using a hand bearing-compass.

 

[Marmalade]

I'm not sure whether I've ever consciously had to take a bearing on a vessel when the shore's in sight - you can just *tell* whether you're on a collision course. Maybe I'm subconsciously applying the techniques described here.

The HB compass compass comes out in the shipping lanes where there are no external reference points and instinct proves inadequate.

Actually the HB compass normally stays unused as the radar is our first port of call...

 

[Jamesuk]

Actually the HB compass normally stays unused as the radar is our first port of call...

I generally leave the Hand bearing compass where it is and use the ships compass for reference on vessels in the distance as I find it gives a more stable reading.

Radar has its limitations but we too will have Radar on at night.

As for collisions as long as your keeping a sound course - put on Auto pilot if you are concerned. Then you should be able to use the structures of the yacht to help you. I has one contact recently going through the straights and I said it is as the end of the boom now I can see Green So I am going to expect it to go from Left to right. As we were keeping a good course it slowly went along the boom to the point that it was on the bow and off it went to STBD (boom was on port tack)

The COLREGS love it!

 

[snowleopard]

There is only one absolutely reliable method - which is of course compass bearings - but the ColRegs do warn:

such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range​

Again using the slowly converging course example, as a bigger ship gets closer its bow will seem to draw ahead because the angle covered by the whole ship is wider. At the same time if you concentrate on the stern that will seem to be falling back. Most people look at the bows as that is the bit that does the damage!

 

[snowleopard]

A quick question

I'll calculate the rate of change of angle for a near-miss to clarify how much the bearing changes during the approach. I propose to work on the basis that passing less than 1/2 mile ahead is dangerous. Is that a reasonable minimum distance?

 

[ancientsailor]

I'll calculate the rate of change of angle for a near-miss to clarify how much the bearing changes during the approach. I propose to work on the basis that passing less than 1/2 mile ahead is dangerous. Is that a reasonable minimum distance?

I am going to jump firmly on both sides of the fence here and say, "It depends".

In the open sea with a large fast commercial vessel (I'm usually under sail these days at less than 8 kts) I would be very uncomfortable with so small a distance

Plymouth, from where I usually sail gives a different perspective. In places the channel's very narrow with plenty of water around it. A vessel half a mile away in the channel may even have a couple of bends to turn through. Small Frigates do this quickly, big amphibious landing ships do this slowly.

Sailing across where I'm only going to be in a few moments and my intentions are clear ... I'd probably be much happier - especially if the vessel has a "guard boat" of some sort which is yet to reach the area I intend to cross