YM Ocean questions

[Conachair]

This one and its companion are well worth a few quid for a kindle
http://www.amazon.co.uk/Astro-Navig...?s=digital-text&ie=UTF8&qid=1352358146&sr=1-4

 

[oldbilbo]

Can you explain how, prior to gps, navigators were successful despite sticking a pin in a chart?

Of course this is unlikely so we would just argue about where we are.

I do believe Captain Thomas Hubbard Sumner expounded the technique nearly 170 years ago in his booklet of 1843.

Being a Bostonian ( Mass.) his language and syntax was even more entangled than we are now accustomed to, but knowledgeable people persevered and his pamphlet was swiftly distributed to all the ships in the US Navy. His legacy as a 'thinking navigator' still trickles down to us today, despite the best efforts of the RYA Training crowd and the likes of Raymarine to reduce us all to passive screen-gawpers.

 

[laika]

turning up with star sights having used a davis done in mid ocean from a small you are a bull****er.

I've used a mk15 for stars. The normal criticism I've heard of more affordable sextants is inferior optics (compared with high end models) not transmitting as much light, but clear night, bright stars, the mk15 is perfectly usable.

I wonder if Uricanejack has had a negative experience of one particular Davis sextant or has extensive experience of the whole range

 

[Roberto]

Can someone please clarify

The RYA site description of the necessary sights for the exam states:
"The records must include as a minimum: planning, reduction and plotting of a sun run meridian altitude sight and a compass check carried out using the bearing of the sun, moon, a star or planet."

What is exactly in practical terms a "sun run meridian altitude sight" ?


Am I right in thinking one has to first compute an estimated sun meridian height from the estimated position, then make two sets of sights: a first one to determine latitude (measuring heights until the sun "stops"), and a second set of sights when the azimut has changed enough to give a decent crossing angle between the two lines of position, then shift (course and distance) the first LOP to the second to get a fix.
Is this correct ?



Re: Compass bearing check
IS it determining azimut at sunrise/sunset and checking the compass deviation on a single heading, say pointing the bow towards the sun ? Or must one take several differences at several headings, sort of rough deviation curve ? How can this be done during the few minutes a sunset lasts, the last deviation curve I made with a land transit took me a considerably longer time...

thanks

 

[oldbilbo]

The RYA site description of the necessary sights for the exam states:
"The records must include as a minimum...... a compass check carried out using the bearing of the sun, moon, a star or planet."

Re: Compass bearing check
IS it determining azimut at sunrise/sunset and checking the compass deviation on a single heading, say pointing the bow towards the sun ? Or must one take several differences at several headings, sort of rough deviation curve ? How can this be done during the few minutes a sunset lasts, the last deviation curve I made with a land transit took me a considerably longer time...

Periodic compass checks are required where a significant unsuspected fault/error in the steering compass could have serious consequences. In air nav, RAF-style, a formal Compasses Comparison check was required and logged at the start of each cruise leg, after every major change of heading, and every 15 minutes elapsed. Getting lost when jet fuel is scarce is not a good option. Getting lost in mid-ocean when water/food is scarce is also not a good option.

So, the RYA/Yachtmaster Standards Panel want us to understand the need and be able to carry out an effective 'compass check for gross error' so that problems with the steering compass ( they do occur ) can be picked up and addressed before one ends up heading for the South Atlantic instead of the Caribbean....

The technique involves use of a known True Bearing of a celestial object ( usually and conveniently the Sun at Rising and Setting, although other objects and times may be used, if one is showing off ) compared with the Compass.

If your Compass is sat on a binnacle/pedestal and is equipped with a 'shadow pin', the procedure is quite easy. Find a quiet, unlumpy patch of ocean and, using the 'reciprocal' of where the pin's shadow falls onto the bearing ring, read off the Magnetic Compass Bearing of the Sun at Sunset or Sunrise. Then compare this with the Tabulated True Bearing of the Sun at Sunrise or Sunset ( this is called a 'Sun's Amplitude' in the jargon ). Make an adjustment for the local Magnetic Variation, and the difference - the 'deviation' - is your Compass Error.

If it's big, you've discovered you have a problem. You can now consider what to do about this before the consequences impact on your peace of mind and reputation. The solution may be as simple as finding and moving that bag of tools that some eedjit ( you? ) stowed too close to the compass, or moving your hand-bearing compass which is stoopidly stowed alongside the main steering one.

If your compass is bulkhead-mounted without any means of providing a shadow from a shadow pin, the process is basically the same, but you have to use a 'workaround' to determine what is the Magnetic Bearing of the Sun when you want it. One way is to use a so-called 'Dumb Compass' or 'Pelorus' to measure the Sun's bearing relative to the boat's centreline and add this to the boat's Magnetic Heading at the required instant. This can be tricky to do with sufficient accuracy, if your bulkhead compass is small and the card dances around a lot.

For example, Boat's Magnetic Heading 240°M plus Sun's Relative Bearing 040°R gives Magnetic Bearing of the Sun 280°M.

If the Local Magnetic Variation from the chart is 5°W then the Sun's Corrected Bearing is 275°T ( 280 - 5 )

If the Tabulated Sun's Amplitude ( Setting ) for your date and latitude is 285°T, then your compass is under-reading by 10°.

That's about 20nm off course for each day of your passage, and I would suggest that could present you with a problem when you arrive at Cuba instead of Barbados.....​

Here's a pic of a 'Dumb Compass' cobbled together from a recycled/free bit of frigate, a slice of plywood, and a clamp. It permits accurate Relative Bearings from the boat's centreline to be added to whatever the bulkhead compass is reading, giving a Magnetic Bearing.

It's actually a whole lot easier to do than to read/write about, once you have your head around what's wanted. If anyone wants to know more, I can recommend 'Dutton', 'Bowditch' or here.

 

[Roberto]

So, the RYA/Yachtmaster Standards Panel want us to understand the need and be able to carry out an effective 'compass check for gross error' so that problems with the steering compass ( they do occur ) can be picked up and addressed before one ends up heading for the South Atlantic instead of the Caribbean....

thank you, it's interesting to know the logic behind it, I admit I stopped at "why should anyone want to check the compass with the sun when there are so many objects on land"

 

[Conachair]

thank you, it's interesting to know the logic behind it, I admit I stopped at "why should anyone want to check the compass with the sun when there are so many objects on land"

Slightly embarrassed to admit it, but I've sat in a bar in brazil with a bunch of cruisers and not one of us had any idea what the mag variation was. Probably no-one had more than briefly glanced at the compass for several thousand miles

 

[oldbilbo]

Slightly embarrassed to admit it, but I've sat in a bar in brazil with a bunch of cruisers and not one of us had any idea what the mag variation was. Probably no-one had more than briefly glanced at the compass for several thousand miles

Aye. I understand that. It may be rare that a serious compass failure/error creeps into one's gentle bumbling o'er the oceans, but this guy would want to be right on it if/when it occurs. That's me....

By the same token, a PRO navigator doesn't have the luxury of not bothering to check. His job's on the line and, quite possibly, his pink and chubby little ass. See the pics here.

I mean, how difficult is it?

If you're a week out from Gran Canaria, checking whether the butter has melted yet, and have read every pulp novel on board twice, surely to goodness you're casting around for something useful to do! It's not as if there's no time left in the day, the way it was before you left the office....

 

[AntarcticPilot]

Slightly embarrassed to admit it, but I've sat in a bar in brazil with a bunch of cruisers and not one of us had any idea what the mag variation was. Probably no-one had more than briefly glanced at the compass for several thousand miles

All this about compass variation reminds me that prior to the chronometer and the astronomical (lunar) method of getting longitude, using magnetic variation as a means of determining longitude was investigated by no less a luminary than Edmund Halley (of the comet). He carried out a voyage in the Paramour to the South Atlantic to chart the variation of the compass.

 

[Uricanejack]

I've used a mk15 for stars. The normal criticism I've heard of more affordable sextants is inferior optics (compared with high end models) not transmitting as much light, but clear night, bright stars, the mk15 is perfectly usable.

I wonder if Uricanejack has had a negative experience of one particular Davis sextant or has extensive experience of the whole range

My expierience is limited to 3. the cheep black triangle. which I found ok for horizontal sextant angle but not much use for anything else. I have used one new Mk 15 for sun found ok but optics not good got an acceptable position.
aquired a used MK 15. it was junk lose micrometer. index error 32 min. mirrors loose. and telescope very poor. A good new one ok for back up. i supose I am spoilt by having used a Ziess

 

[Uricanejack]

The RYA site description of the necessary sights for the exam states:
"The records must include as a minimum: planning, reduction and plotting of a sun run meridian altitude sight and a compass check carried out using the bearing of the sun, moon, a star or planet."

What is exactly in practical terms a "sun run meridian altitude sight" ?


Am I right in thinking one has to first compute an estimated sun meridian height from the estimated position, then make two sets of sights: a first one to determine latitude (measuring heights until the sun "stops"), and a second set of sights when the azimut has changed enough to give a decent crossing angle between the two lines of position, then shift (course and distance) the first LOP to the second to get a fix.
Is this correct ?



Re: Compass bearing check
IS it determining azimut at sunrise/sunset and checking the compass deviation on a single heading, say pointing the bow towards the sun ? Or must one take several differences at several headings, sort of rough deviation curve ? How can this be done during the few minutes a sunset lasts, the last deviation curve I made with a land transit took me a considerably longer time...

thanks

The sun run mer alt.

Is a Noon position from at least two observations of the sun.
The first a sun sight taken some time in the AM.
The second a Meridian Altitude. Taken at apparent noon which is when the sun is on your meridian or longitude and at its highest in the sky.

Essentially it is a running fix using the sun. much the same as a running fix of a single light house. you can do it in the afternoon but then you start with the meridian alt.

In a running fix of a light house. You take the 1st bearing. Which gives you one line of position. Which you plot on your chart. You are some where on the line but you don’t know how far along. If starting from a point anywhere on this line. You plot your course and speed for a known time to your next bearing of the same light house. You can “run” the 1st line of position down the course for your distance traveled and draw it again. Now your second bearing can be plotted and you will be on this 2nd line of position but have a good idea how far along as your first bearing which was “run” will cross the 2nd.

You do the same with the sun hence the term “sun run“. or “run to noon”

The reason the meridian alt is used is it is simple. The Vikings. Used probably used by Phoenicians you can figure out you latitude quite easily. If you can measure the suns height at noon.

If you accept the world is round ball in the center of the universe. And the sky is a perfect sphere around the world with the sun, moon, planets and stars moving on this sphere. (Galileo would not agree but he wasn’t a sailor)

The arithmetic works. The Sun moves north and south of the equator in a regular pattern through the year it changes constantly. The furthest north is the tropic of cancer. The Sun’s latitude is know as declination North or South.
If you know the day date and time you can look up the suns declination in the almanac.
If you are on the Equator the sun will be right above your head on the equinox at noon.
If you are on the North Pole the sun will be right on you horizon at noon

If you are anywhere else on this day the sun will be somewhere in between the horizon and right over your head or your zenith
.
The angle of the horizon to zenith is 90 degrees.
The further north you go the lower the sun gets lower.
Or your latitude =the opposite of the suns height or altitude..

On any other day this does not work.

But if you know the declination of the sun or the suns latitude you can do a bit of simple arithmetic and figure out your latitude.

Today is November you wont see the sun at the north pole.

If you are in the northern hemisphere and can see the sun. Measure how high it is at noon.. The opposite is the zenith distance. But if you add the suns south latitude to the zenith distance. This is your latitude. .

You have to think about it every time some times you add some times you subtract depending on which hemisphere you are in or the sun is in.

Now you know what a merr alt is.

If you know the time at Greenwich. Or GMT. You can figure out you longitude 1 hour = 15 degrees of longitude.

It is much more difficult to measure the exact time of the sun at its highest point than the highest angle so you latitude is good your longitude not so much. But if you have a sextant and take a sight earlier to run up and cross your latitude you know where you are.

There is a method of calculating a sight known as longitude by chronometer. This is not as popular as the Marc St Hillarie or intercept terminal position method. They are just different sums

Another chap gave a very good post on your compass error question. I agree with him on Dutton’s and Bowditch but like a little book I think is was called navigation by calculator.

Of course if you calculator has flat batteries? By a solar one at least it works when its sunny. Not so much for stars.

 

[oldbilbo]

I still can't programme the blurry VCR.....

 

[Uricanejack]

I still can't programme the blurry VCR.....

Programing a VCR still a mystery to me. along with sending a text on a cellphone

 

[Conachair]

Aye. I understand that. It may be rare that a serious compass failure/error creeps into one's gentle bumbling o'er the oceans, but this guy would want to be right on it if/when it occurs. That's me....

By the same token, a PRO navigator doesn't have the luxury of not bothering to check. His job's on the line and, quite possibly, his pink and chubby little ass. See the pics here.

I mean, how difficult is it?

If you're a week out from Gran Canaria, checking whether the butter has melted yet, and have read every pulp novel on board twice, surely to goodness you're casting around for something useful to do! It's not as if there's no time left in the day, the way it was before you left the office....

Can't remember if the conversation ever got as far as alternative methods of nav. Cruisers don't really talk about sailing, only maintenance But I doubt if the extremely unlikely (it seems) event of loosing gps onboard would have caused mass panic onboard any of the boats. DR/EP and ask anyone you come across where you are. Not that hard, Azores follow the contrails.
But after all that I do have a sextant onboard. Lovely it is too GPS loss might be on the ocean crossing list of bad things, but pretty far down the list, other bad things can and do happen to ruin your day.
Liking running out of pulp fiction..

 

[Roberto]

Essentially it is a running fix using the sun. much the same as a running fix of a single light house. you can do it in the afternoon but then you start with the meridian alt.

thank you


as for longitude, I once tried to measure a number of sun heights just before the meridian passage, then another series just after the meridian passage, put them in a graph and find the two times corresponding to the same height, the mid point between the two would give the (sort of) meridian passage time.
Not theoretically correct as the boats in the meantime moves, but if one thinks of sextant navigation as emergency positioning, then it's a bit of potentially useful information. By memory (I have only done it once) the longitude wasn't even too far from the real one.

 

[oldbilbo]

as for longitude, I once tried to measure a number of sun heights just before the meridian passage, then another series just after the meridian passage, put them in a graph and find the two times corresponding to the same height, the mid point between the two would give the (sort of) meridian passage time.
Not theoretically correct as the boats in the meantime moves, but if one thinks of sextant navigation as emergency positioning, then it's a bit of potentially useful information. By memory (I have only done it once) the longitude wasn't even too far from the real one.

Yep, Roberto, that's 'Latitude By Equal Altitudes of the Sun' as described in 'Dutton' and 'Bowditch', as I recall. Useful if a wandering cloud gets in the way, or a big spinnaker, or lunch intervenes......

More effective in lower latitudes than higher, as the 'rate of change of elevation' better defines the required moment. And can be helpful in giving an approximation of Local Noon, if both chronometers are thought to be wonky.

 

[mcframe]

The sun run mer alt.

Great explanation.

During the theory course I "got" the point about APs as being "If I was *there* at *that* time, then (the almanac & tables say that) $BODIES would be $ALTITUDE and $AZMUTH - I'm not there, but the time is the same, so I must be *here* (calculation based on the difference)"

I guess I might replace my (present from SWMBO a while back) Mk 15 if I ever get a chance to use it anger and find it lacking. Alas, on family sailing around the Solent, I don't think I've ever maintained the same course for >2 hours before and after local noon...

 

[mcframe]

Sorry - nothing do with YMO

I still can't programme the blurry VCR.....

Old Hat, Old Bilbo.

The yoof (and the enlightened) just use the Sky+ "app" on our iPads to tell our magic boxes to "series link" interesting things for us

Like:
http://www.channel4.com/programmes/how-britain-worked/episode-guide/series-1/episode-5 on Sunday 16th.

"Guy helps restore the oldest surviving Brixham sailing trawler, the boat that launched the modern fishing industry and transformed the way a nation ate.
Guy learns how the trawler's radical design saw off the competition, tries his hand at the precision joinery that made that design possible, tests his own version of an Industrial Revolution life jacket by jumping into the sea, and makes rope using the original machines that wove the ropes for Nelson's HMS Victory."

 

[oldbilbo]

Old Hat, Old Bilbo.

The yoof (and the enlightened) just use the Sky+ "app" on our iPads to tell our magic boxes to "series link" interesting things for us

Yup. That's as may be....

I used to earn my crust shoehorned into a tiny and uncomfortable 'office', watching intently a whole mob of dials, gauges and displays, alert for every little quiver and tell-tale variance that could presage a sudden departure from comfort and security, as we charged about the skies on Her Maj's business. Living on adrenalin....

A great part of the pleasure of sailing at weekends was freedom from all that and the simplicity of tugging on the odd rope here and there, waggling the stick-thingy now and then, and musing over whether to have the bitter or the lager with my lunchtime 'oggy' at the Pandora or Chain Locker. It still is.

And part of the pleasure now is watching the consternation and unease on the faces of 'the yoof and enlightened' when England drops over the horizon and, with it, the WiFi and 3G signals on which so much of their narrow lives now depend.

I watched this with interest just a couple of weeks ago in the company of a couple of 'very highly qualified' RYA Yotmeisters as we transited Biscay and down along the Spanish/Portuguese coast. They were forever playing with the apps on their smartphones and got decidedly uncomfortable when we went 'out of signal'. I had to mention at one point that the baro had fallen 7 millibars in 4 hours and explain the possible significance to the comfort of our passage. Blank incomprehension. We actually altered course for a while to get back into signal, and the relief on their faces when their mobos came back on was almost comic.

They were so focussed on the 'virtuality' of their clever kit that they couldn't see the 'actuality' going on around them. They had persuaded themselves that the bright and shiny info provided by Nokia, Samsung and Apple somehow had more validity. They couldn't even take pleasure from the coming of the dolphins...

I found myself wondering what on earth they were doing there.

 

[nortada]

Yup. That's as may be....

I used to earn my crust shoehorned into a tiny and uncomfortable 'office', watching intently a whole mob of dials, gauges and displays, alert for every little quiver and tell-tale variance that could presage a sudden departure from comfort and security, as we charged about the skies on Her Maj's business. Living on adrenalin....

A great part of the pleasure of sailing at weekends was freedom from all that and the simplicity of tugging on the odd rope here and there, waggling the stick-thingy now and then, and musing over whether to have the bitter or the lager with my lunchtime 'oggy' at the Pandora or Chain Locker. It still is.

And part of the pleasure now is watching the consternation and unease on the faces of 'the yoof and enlightened' when England drops over the horizon and, with it, the WiFi and 3G signals on which so much of their narrow lives now depend.

I watched this with interest just a couple of weeks ago in the company of a couple of 'very highly qualified' RYA Yotmeisters as we transited Biscay and down along the Spanish/Portuguese coast. They were forever playing with the apps on their smartphones and got decidedly uncomfortable when we went 'out of signal'. I had to mention at one point that the baro had fallen 7 millibars in 4 hours and explain the possible significance to the comfort of our passage. Blank incomprehension. We actually altered course for a while to get back into signal, and the relief on their faces when their mobos came back on was almost comic.

They were so focussed on the 'virtuality' of their clever kit that they couldn't see the 'actuality' going on around them. They had persuaded themselves that the bright and shiny info provided by Nokia, Samsung and Apple somehow had more validity. They couldn't even take pleasure from the coming of the dolphins...

I found myself wondering what on earth they were doing there.

JSASTC has a lot to answer for