lw395
Well-Known Member
0.5nm behind is cool, if it gives you maximum cpa on the next one!
MARPA - CPA values?
- Thread starter stiknstring
- Start date
T
timbartlett
Guest
OK, well let's assume for the moment that for some reason unexplained the ship is the stand on vessel. And the OP wants to know how to make the CPA bigger. And to do that, he needs to know whether the CPA is ahead or astern.
If the ship is crossing ahead, he can make it bigger by slowing down. If the ship is crossing astern and he slows down, he will make the CPA smaller, not bigger.
The kind of knowledge the OP is asking for is exactly the kind of knowledge that would have saved Wakhuna -- the Moody 40 that slowed down when she would otherwise have crossed safely ahead. Safely ahead, incidentally, of a large container ship whose master glibly accepted a CPA of a couple of cables when he was doing 25 knots through thick fog, and whose proper, high-quality commercial ship's ARPA was giving duff information because it was ground stabilised instead of sea stabilised.
And it "is it crossing ahead or astern" is exactly the piece of knowledge that was added to the RYA radar syllabus at the MCA's specific request after the Wakhuna incident.
So the OP may be a "radar baby" but at least he's asking intelligent questions and trying to find out intelligent answers.
Might is Right and sod the colregs is not an intelligent answer.
Angele
Well-Known Member
StiknString
Not sure how your MARPA is displayed, but together with CPA and TTCPA (time to ...), does it not also give current bearing and distance to target?
In the theoretical perfect world, where everything is stable and the output of your MARPA is giving a constant value for CPA then (assuming it is more than a few hundred yards) you would see a change in the bearing to target and that would tell you if it was going to pass in front or behind. If big boat is off to the left and the bearing is increasing it is going to pass in front (and vice-versa). If it isn't changing that is because you are on a collision course and the CPA ought to be a small number and I'd start worrying if I were you.
If the display doesn't show bearing on a list of MARPA targets, then I assume you can still find the info using the EBL.
In the real world, where the CPA is leaping about (for the reasons other have already mentioned) and it is a small number, then the machine probably doesn't know for definite which side of you it is going to pass. So, probably good job it isn't telling you as there is a strong possibility that it would give you the wrong answer.
In summary, if CPA is big and consistent then you are going to miss one another and it should be apparent visually or from observations on the radar which side of you it will pass. If CPA is small then take avoiding action, but I wouldn't rely on a small boat MARPA to tell me whether I needed to turn left or right or speed up or slow down. I'd use my own observations which (other than in fog and, admittedly, harder at night) would include taking into account the aspect of the boat relative to mine (and whether it appears to be changing) - something that MARPA cannot determine.
stiknstring
Well-Known Member
From the Radar Baby !
Let me answer some of the comments made. First a general one - I see nothing wrong in using every aid I have on board as a backup to a personal lookout when crossing busy traffic lanes. No point having a dog and doing the barking oneself. Refueller - thanks your comment. I have used radar for a number of years (just not one with MARPA before) and in condirions of fog, darkness and in very busy shipping lanes, not only across the channel but off Ushant and NW Spain. I believe also in the value of familiarisation with the kit even if that means using it in good viz rather than puzzling about it when it really is needed.Not so much a baby...
So, Refueller - You are quite right in principle that a CPA of 0.5m is too close if that is how close you are getting to the target. However, this value was given at several miles distance from the target (3-4 miles) in this crossing. There is nothing to prevent MARPA giving this reading when the target is 24 miles away - is that too close still for you? Clearly, if you get such a reading from a ship several miles away, you need to take notice and plan ahead. Some of the values given were from ships not visible to the eye and no more than 6-8 miles away. Deciding what the "picture" looks like at a range of 3-8 miles seems logical to me. Last thing I want to do is turn onto more of a reciprocal course in the hope that I can outrun the target by not understanding fully what the picture is. On the day in question, not unusually, there was a solid line of ships coming down the channel - in those circumstances, you do need to get close to them for crossing if you are not to conflict with the following ships. How do you teach that in the classroom? Waiting for a gap of 2-3 miles is simply impractical. Lastly, the MARPA values given change from one minute to the next due to the inevitable course changes when in a sailing boat.
Tim, Thanks your understanding of what I am trying to get at. It pains me that a simple question on this forum invokes unhelpful responses (not you or refueller as I know where you are coming from) from some posters. No wonder some people are discouraged from asking anything on here, which is a shame given the wealth of knowledge and generally good intent from the majority.
Let me answer some of the comments made. First a general one - I see nothing wrong in using every aid I have on board as a backup to a personal lookout when crossing busy traffic lanes. No point having a dog and doing the barking oneself. Refueller - thanks your comment. I have used radar for a number of years (just not one with MARPA before) and in condirions of fog, darkness and in very busy shipping lanes, not only across the channel but off Ushant and NW Spain. I believe also in the value of familiarisation with the kit even if that means using it in good viz rather than puzzling about it when it really is needed.Not so much a baby...
So, Refueller - You are quite right in principle that a CPA of 0.5m is too close if that is how close you are getting to the target. However, this value was given at several miles distance from the target (3-4 miles) in this crossing. There is nothing to prevent MARPA giving this reading when the target is 24 miles away - is that too close still for you? Clearly, if you get such a reading from a ship several miles away, you need to take notice and plan ahead. Some of the values given were from ships not visible to the eye and no more than 6-8 miles away. Deciding what the "picture" looks like at a range of 3-8 miles seems logical to me. Last thing I want to do is turn onto more of a reciprocal course in the hope that I can outrun the target by not understanding fully what the picture is. On the day in question, not unusually, there was a solid line of ships coming down the channel - in those circumstances, you do need to get close to them for crossing if you are not to conflict with the following ships. How do you teach that in the classroom? Waiting for a gap of 2-3 miles is simply impractical. Lastly, the MARPA values given change from one minute to the next due to the inevitable course changes when in a sailing boat.
Tim, Thanks your understanding of what I am trying to get at. It pains me that a simple question on this forum invokes unhelpful responses (not you or refueller as I know where you are coming from) from some posters. No wonder some people are discouraged from asking anything on here, which is a shame given the wealth of knowledge and generally good intent from the majority.
Blue5
Well-Known Member
Very true, sadly it is becoming more common, I am all for a bit of light hearted banter but some of the comments seem quite personal and dismissive at times to the extent that if you need to ask you shouldnt be sailing, and as you say a lot of folk will be discouraged from participating.
Refueler
Well-Known Member
Thank you ... at last a proper answer. To Tim etc. who think I say Might is Right etc. - that is not fair and not what I posted. Yes I do believe in steer clear of danger - in questionable cases as taught in Merch - bold alteration away ... make it clear what you do.
Wakhuna is actually not a good example for this - the report clearly indicated lack of overall knowledge on the part of the user. Having read and studied the incident - it is full of mistakes on part of the skipper.
Radar / Marpa / CAS - all aids to navigation and safe conning of a vessel. Limited in ability and also inputs. To be used allied to other means to determine probable risk.
As I posted ... 0.5nm in Solent is fine as you are pretty sure where and when ship etc. is going to turn / be etc. Open water ? No thank you sir. I want maximum reasonable distance from another vessel so I don't have to start 'drastic questionable evasive manouevres'.
Yes the OP question is a valid and sensible one ... it's just that he should accept that the answer is not what he likely expected. If I was skipper and he was helm, and suggested a questionable 0.5nm CPA in open waters ... I'd ask him to think again ! In fact even if he was Skipper - I'd question it !
Something I keep in mind and keeps me safe : Empty Water is Safe Water.
Refueler
Well-Known Member
Thanks for understanding that I wasn't trying to be rude.
Ok lets think about this ... at extended range and 'blind'. You have time to watch and gauge probability, in Channel unless it's a small coaster or fisherman - ships will be in lanes and reasonably predictable routes. It may be that 0.5nm is necessary at some stage due to a following vessel etc., but I am one who will do all to stay away from that short a distance - I don't like being thrown around by ships wake - especially if it's a Container or Reefer Ship at 20+kts !
Observation by all means will generally indicate what is going to happen in cases of long range as you example. Making a decision when ship is good distance away is OK as long as you are aware it's based on scanty information and suspect. Prepared to make an about turn or circle is always on my card. In fact I can think of an example when that's exactly what I did in Baltic when a container ship thought 2 cables was fine ... I had no idea if he would pass ahead or astern ... he was crossing from port side. I took the cowards way out - couldn't be sure he'd miss me, having watched his approach. Did an about turn and let him pass ahead ... called him up after and asked him if he saw me .... ( British Ship ) ... he said yes he'd tracked me for some distance .. 2 cables CPA. My reply is unprintable.... apart from my advising him that a yacht as mine is not able to determine such CPA and it visually appears to be a collision sit, plus 2 cables is too b***y close ! He apologised and reckoned he'd take note next time.
My problem I have to admit is having seen RAC at sea ... radar assisted collision, I fear it happening with yachts as it becomes more widespread. Radar is a marvelous tool when used correctly. But if operator is not careful - it can be deadly.
So simple question ...
You have your Radar on and you see a lot of targets all apparently heading your way. What do you do to avoid them ?
Simple answer actually .. and this was drilled into me by instructors at College and Masters at sea ... don't try and work out all sits ... take it one at a time by a) switching down range on the radar to get the closest threats only, b) ally that with Mk1 eyeball ... compass bearings etc. Once first sorted .. move on to next.
It may mean you manouevre more, but you are safe ...
stiknstring
Well-Known Member
Answer the question please
Refueller,
Please don't be patronising Refueller, first I am a radar baby and then you presume to tell me that I don't like the answers you give. It is not that I don't like your answers just that you haven't given any useful answers. Read the posts before replying please. My subsequent question was how do you get across a separation zone leaving, by implication, several miles clearance when there is a steady stream of traffic making that impractical? Secondly, the original question was if you see a CPA of 0.5m how do you know if this is ahead or astern. The question was further clarified by my telling you that this CPA value comes up even on targets at 6-8 miles (some of these targets being out of sight at that range). I still want to know what the picture is that far out . The question was answered sensibly by several of the respondents, for which I thank them. You unfortunately seem to want to make the tired point that 0.5m in open sea is too close. It might be if we were talking about clear open sea with no other ships or land around - but if you were helming and me as crew or vice versa, when crossing a busy shipping lane, I would not rubbish your judgement in crossing close to a ship, indeed crossing close to the stern of a crossing ship could give me, in theory, the maximum clearance from the next ship in the line.
I guess we will both have to agree that it is a good job we are not on the same boat or that I paid to be instructed by you. An inability to properly answer a question and your patronising attitudes would soon have me asking for my money back!
Refueller,
Please don't be patronising Refueller, first I am a radar baby and then you presume to tell me that I don't like the answers you give. It is not that I don't like your answers just that you haven't given any useful answers. Read the posts before replying please. My subsequent question was how do you get across a separation zone leaving, by implication, several miles clearance when there is a steady stream of traffic making that impractical? Secondly, the original question was if you see a CPA of 0.5m how do you know if this is ahead or astern. The question was further clarified by my telling you that this CPA value comes up even on targets at 6-8 miles (some of these targets being out of sight at that range). I still want to know what the picture is that far out . The question was answered sensibly by several of the respondents, for which I thank them. You unfortunately seem to want to make the tired point that 0.5m in open sea is too close. It might be if we were talking about clear open sea with no other ships or land around - but if you were helming and me as crew or vice versa, when crossing a busy shipping lane, I would not rubbish your judgement in crossing close to a ship, indeed crossing close to the stern of a crossing ship could give me, in theory, the maximum clearance from the next ship in the line.
I guess we will both have to agree that it is a good job we are not on the same boat or that I paid to be instructed by you. An inability to properly answer a question and your patronising attitudes would soon have me asking for my money back!
stiknstring
Well-Known Member
Look slike our posts crossed, Refueller. I now see your response to my previous. Apologies for calling you out on not answering the questions
T
timbartlett
Guest
I agree that the report indicated "lack of knowledge on the part of the user".
We could, also (perhaps) discuss whether it demonstrated a lack of knowledge on the part of the MAIB, who did not seem to appreciate that the accuracy of data available from a small boat radar -- even to a skilled radar operator -- might not have been sufficient for Wakhuna to make an informed decision. But that is a separate issue.
Clearly in Wakhuna's case the CPA (based on the Nedloyd Vespucci's version of events) was uncomfortably small. Wakhuna might have been able to make it bigger by speeding up. But she did not: she slowed down -- and by doing so reduced the CPA to nil and simultaneously lost steerage way. It is almost exactly the kind of situation the OP is trying to understand and avoid: how can he determine whether a vessel with a small CPA is crossing ahead or astern, in order to decide how to make a small CPA bigger.
I'm sorry, I accept that you did not say that. But the assumption that it is up to the small/recreational vessel to take avoiding action is often/usually in direct contravention of the colregs. And I'm afraid the tendency of yacht skippers to adopt a "might has right" rule is forcing ship's watchkeepers to defer taking avoiding action later than is really desirable because they don't know what any particular yacht is going to do.
Whether any of us like it or not, the rules do still say that power gives way to sail (except in certain defined circumstances) and they do still require the stand on vessel to stand on. So in most open water ship/yacht meetings it is up to the ship to take avoiding action. Tweaking the autopilot by five or ten degrees at a range of three or four miles is surely not a lot to ask, especially as at that stage, the yacht is not allowed to take avoiding action. She is only relieved of her obligation to stand on when it becomes clear that the ship is failing to conform to the colregs... and of course that never happens!!
If or when the colregs change, I would encourage people to support the new ones, but until then I'm afraid the idea that we should pick and choose which to obey or ignore is as alien to me as suggesting that I could go out and ride my motorcycle on the right, today, and expect others to respect my freedom of choice.
lw395
Well-Known Member
Does a ship comply with rule 8 b in 'tweaking' his course by 5 degrees? Is that 'large enough to be readily apparent'?
If not then the yacht is free to alter course IAW Rule 17.a ii.
And remember the requirement to hold your course and speed only applies in sight of another vessel which must keep clear of you. That means he can see you in your Ouzo, not just you can see his supertanker.
If not then the yacht is free to alter course IAW Rule 17.a ii.
And remember the requirement to hold your course and speed only applies in sight of another vessel which must keep clear of you. That means he can see you in your Ouzo, not just you can see his supertanker.
T
timbartlett
Guest
Assuming you are moving and he is heading for a point somewhere ahead of you (as he must be, in order to cause a collision) then altering course to aim straight at you is enough to be "readily apparent" because you will be looking at his bow instead of seeing one side. Depending on the geometry of the situation, five or ten degrees could well be plenty. In other situations, it might need thirty degrees or more.
Sorry but the yacht is not. 17a(ii) does not say that you are relieved of your obligation to stand on as soon as you do not see the other vessel taking action. It says you are relieved of your obligation when you see that she is not taking action.
I accept that it is a relatively subtle difference in wording, but the implications are significant:-
According to your interpretation, as soon as you see a ship that is not conspicuously altering course for you, you are relieved of your obligation -- even if the ship is so far away that there is no way that either of you could realistically have assessed the risk of collision. The fact that he is not taking action to avoid a hypothetical collision that might never happen is (apparently) irrelevant.
According to my interpretation, it has to be clear to you that the vessel is not taking action, despite the fact that it should do so.
It is impossible to assess whether someone else can see you, other than by their actions. It is quite possible that there is no-one on the bridge of the ship, or that they are wearing sunglasses at night, or doing paperwork, or are distracted by boiling a kettle (all genuine reasons given for actual collisions). But the fact that they are possible does not give any of us the right to assume that we have not been seen, and that we should therefore rip up all the rules apart from Number 19.
I appreciate that many yotties believe they are acting "safely" by running like startled rabbits whenever a ship comes within several miles of them. But that is not what the rules say. And until IMO changes the rules we are stuck with what we've got.
ancientsailor
Well-Known Member
Entirely agree.
The "in sight rules" apply when vessels are in sight of one another (not actually sighted!)
Now for a bit of a confession(?)
A sobering thought for the "startled rabbits". In a small boat at sea, approaching vessels are seen in my experience as "above the horizon" - but the horizon is generally pretty close. They are usually seen against the sky (or land)
From a bridge say 70+ ft above the sea (21 m or so) I personally found it often difficult to spot small boats with the naked eye until they were typically under about 4-5 miles (I didn't have brilliant eye sight - but passed the watchkeepers' tests). Others would do much better.
Small boats are often seen between the big vessel and the horizon - ie against the background of the sea not the sky. They could be difficult to see - particulary if there were many white caps and they were white-hulled. It was however pretty easy to see the big uns lumbering over the horizon
lw395
Well-Known Member
Taking an example of ship doing 20kts, yacht doing 5kts, courses at right angles, altering to aim straight at the yacht would be 14 degrees, if original course was a collision.
Ariadne
Well-Known Member
Not being sarcastic but:
1 how about going on a radar course and learn how to use thing properly.
2 if you use AIS then it gives the position, course, speed, etc what else do you need?
3 use your eyes and a hand bearing compass now and again (much more reliable than electronics when vessels are in sight), don't rely on electronics they give just when you need them most in my experience!
If you ask an ambiguous question your going to get ambiguous answers, of course you could also read the manual(s) as well.
Apart from that enjoy yourself sailing and keep at it.
1 how about going on a radar course and learn how to use thing properly.
2 if you use AIS then it gives the position, course, speed, etc what else do you need?
3 use your eyes and a hand bearing compass now and again (much more reliable than electronics when vessels are in sight), don't rely on electronics they give just when you need them most in my experience!
If you ask an ambiguous question your going to get ambiguous answers, of course you could also read the manual(s) as well.
Apart from that enjoy yourself sailing and keep at it.
D
DogWatch
Guest
I have completed 2 radar courses (1 RNLI and 1 computer training) and neither particularly dwelled on collision avoidance... I guess that is such a wide component it should be covered by YM training etc. The training I have seen is more about learning to use your radar and recognising what you are seeing. I tended to glaze over a little during this as I am/was a radar engineer and have a pretty good understanding of how they work.
YMonthly or was it PBO did a monthly training section on RADAR, again iirc they were a bit light on real world collision avoidance. Maybe the team it IPC Towers might like to run with this a bit.
T
timbartlett
Guest
Thank you for your confirmation of what I wrote:-
If you are heading N at five knots and he is heading SW at 25 knots, an alteration of 3 degrees would be sufficient.
T
timbartlett
Guest
FWIW, all RYA radar instructors are encouraged to allow at least two hours (25% of the course) for collision avoidance, with the suggestion that they should arrange their lesson plan so that they have plenty of slack to give it an hour or so more if necessary.
The "learning to use your radar and recognising what you are seeing" bit is important because if you can switch it on and get a picture, there's not much point knowing how to plot the blobs that you can't see.
It's not perfect. AFAIK no-one (least of all the RYA) pretends that it is. But if they extended the courses to two or three days, I seriously doubt whether many ordinary amateur sailors would attend.
I'm not suggesting that this is true of you, but there are a vociferous minority whose idea of "real world collision avoidance" is that someone should pat them on the head and tell them that it's all right, you don't need to bother with all those nasty rules, just do whatever seems best at the time." Anything that doesn't meet that specification is dismissed as "ivory towers" or "all right in theory"
st599
Well-Known Member
What's the margin of error on ARPA, mARPA and AIS?
I reckon 0.5 NM is probably, when taken with the maximum system error, a lot less!
I reckon 0.5 NM is probably, when taken with the maximum system error, a lot less!
LadyInBed
Well-Known Member
I have radar but not MARPA, I use AIS as the radar track tends to bounce around a bit, but to suggest an answer to your question:
I would slow down, the CPA will change.
If the CPA gets larger, he will pass in front
If the CPA gets smaller, he is passing behind.
Though the AIS target on the chart plotter screen normally makes it obvious.