Plotting Fixes - using reciprocal

[Chris_Robb]

SWHBO is doing her shore based Nav this winter - not because she wants to be able to pick me out of the water, but to get home without me. BUT her instructor has said that when plotting fixes, one should always convert the bearing to an point, into a reciprocal bearing and plot using the reciprocal and NOT the direct bearing to the point. The instructor is addement that you should do it this way

WHY -
I cannot add 180 to the bearing in my head - so I'll probably get it wrong!
Only point I know is expressing the bearing from a land based point in a MAYDAY call.

Anyone know why this is a better way?

 

[Twister_Ken]

Dunno. Not the way I was taught to do it when I did my Yachtmaster xxty years ago. Seems perverse.

 

[Chris_Robb]

I don't want to interfer with the instructors teaching - but in this case I cannot understand it. I also don't want to confuse her - she's great at cooking, sewing.... but Navigation - she is struggling!

If you are using Parallel rulers and always 'walk' from the compass rose, then I suppose there is a point. but who uses them anymore?

 

[bigmart]

I suppose it depends on the type of plotter you are using. I can imagine that it may be a good practise when using a Breton or Portland type. The conversion, for the reciprocal, can be made visually and it should be faster to plot your fix as you would not need to move the plotter around until the bearings aligned.

Martin

 

[Chris_Robb]

I was thinking it might be applicable to the old parallel rules, which you had to place on the compass rose.

 

[jimi]

Does it really matter? As long as the lines form a cocked hat surely it does'nt matter whether you are using the bearings or the reciprocal. Indeed I'm sure a lot of us do it both ways unknowingly ie If I use the hand bearing compass it gives the reciprocal, if I use the bins it does'nt, but I plot both in the same manner using the Portland plotter. What is important to get right is when giving position which should be bearing and distance from a known point.

Jim

 

[jamesjermain]

Seems odd but...

As you say, the instructor seems to be adding in the possibility of error. However, he is also, I speculate, trying to eliminate a potentially greater error. Thus:

The bearing you take is of a headland or similar from your BOAT. But when you plot it you will be drawing a line from the HEADLAND (or similar) through your position. I guest that, in order to avoid drawing the line in the wrong direction, he feels you need to work out the reciprochal for absolute clarity. I can think of very few circumstances when a navigator might be confused by this, but strange things happen when you are tired, and maybe a line which is a couple of degrees out is better than one which is drawn from the mark away from you rather than towards you.

I have never bothered to work out the reciprochal; I tend to note it menatally when I have the parallel rules on the compass rose.

JJ

 

[jamesjermain]

I do. nm

.

JJ

 

[halcyon]

In the back of my mind the YM always lists bearings from the object, and are writen in the log as such. Thus if you take a bearing you write as the reciprocal, and plot as such.

Back to my notes now to check memory.

Brian

 

[Chris_Robb]

Re: I do. nm

James - I used to - but was converted when I navigated on another boat without my (then) beloved parallels. Sorry - Did'nt mean to be patronising on this!

 

[bedouin]

Not sure about his justification. But of course in practice you have to use the reciprocal. After all what you plot on the chart is the line representing the bearing from the object, not the boat, so you have in effect used the reciprocal bearing.

Calculating the reciprocal in the head is rather prone to errors, but many of the devices we use for plotting do this for is implicitly (e.g. just going "the other way").

Maybe the Instructor is just doing this to ensure that the class know what is really going on.

 

[Chris_Robb]

In the purety of it - you are right. However in practice you point and squirt your plotter and get the same result using bearing too. But I now think I see why the Instructor is insisiting they do it this wasy.

 

[jamesjermain]

The Devil

No probs... I know there are better ways of plotting, particularly on a small yacht chart table but it is a question of the devil you know...

JJ

 

[oldsaltoz]

G'day Chris,

I suspect most have misinterpreted the post. Perhaps the Instructor is advocating the use of reciprocal bearings 'between fixes' so that she can steer to them and bring the boat home.

Not complicated, and better to note then when your not in a panic, Trying to get the boat home without any other help for the fist time.

Avagoodweekend Old Salt Oz /forums/images/icons/cool.gif

 

[Chris_Robb]

G'day mate - The bit about falling overboard was why SWMBO is doing the course. She has determined that this is the quickest way to get at my assets and be rid of the boat!

One posting I think explained why the instructor is insistant about plotting a reciprocal. This is because he wants his trainees to remeber that to plot a position in the literal sence one has to use the reciprocal - Today we use the breton plotter type which means that we actualy cheat in how we plot.

The point of the post was to try to understand why the instructor was so insistant they used this method. I understand - but don't agree!

Middle Daughter comes back from one year in Oz on Tuesday - wonder if I'll recognise the little sheila!

Have a good week end

 

[bedouin]

I think it is probably a good idea - makes sure the class understands what is going on.

I doubt if the Instructor advocates doing it that way in practice - but a lot of the theory is about learning to do it "correctly" - then learning how to do it in practice!

 

[Chris_Robb]

Yes - Like learning secondry ports and tidal streams! Actaully you can go very wrong on some secondry orts by cutting corners!

At least I'll be able to explain to here that I am not a complete idiot! and why the instructor is telling her this way.

 

[webcraft]

What about simple understanding?

A competent navigator should be able to use whatever suits at the time.

I woldn't like to go to sea on a serious passage with a skipper who was depending on a flowchart system of navigation, or who used acronyms to remember what they should be doing.

A little understanding of first principles is required.

<font color=blue>Nick</font color=blue>
<A target="_blank" HREF=http://www.bluemoment.com>

</A>

 

[chippie]

I agree with James Jermain, also the reciprocal will give a line to work from a known point (eg headland) to a third party should a distress message be sent.

 

[david_e]

I am doing year 2 YM, (jumped straight in without doing year 1) and so far we have not (from memory) been asked to do this. It also doesn't mention this in the RYA books and the question papers all state whether it is a bearing to or from.

Reason she finds it confusing is because she is not doing it for real or 'learning through involvement', it's the same with me.

I also believe the RYA books, whilst good aid memoires, are not up to the task.

BTW, just in case she asks, there is no mention in passage planning about having your will up to date)