distance between two fix

[conachair]

Originally Posted by Jim Cate

Having a stout current helping you along gets you to the destination faster, but means little with respect to how well the boat (and the skipper) were doing.

Means a lot with respect to the skippers passage planning though
Ocean currents are on the pilot charts >
https://opencpn.org/OpenCPN/info/pilotcharts.html

Best I've ever had in a day was 70 free miles from the current.

[conachair]

Originally Posted by clownfishsydney

Record the track on chartplotter and then note this at the end of the day or later, review (or download) and see how far you travelled. This is by far the easiest and most accurate option.

A lot of cruising boats won't actually run the plotter offshore - uses too much power.

[captmikem]

Originally Posted by henris

Hello,

On ocean crossings, skippers try to know the distance sailed in 24h. With the intention of writing an article on the various techniques (and tricks) used by sailors to determine this distance, I would like to know from our fellows blue water sailors how they do it. (Or if they do this daily routine).

Henri

This is a very simple question with a very simple answer. Crossing an ocean, the prudent skipper has a paper chart covering the ocean on the chart table. At Noon a fix (celestial, electronic or DR) is marked on the chart (circle with a dot for a non electronic fix, triangle with a dot for an electronic fix semicircle with a dot for a DR position). The next day at noon another fix is marked on the chart. The navigator then takes his dividers and (using the latitude scale on the side of the chart) measures the distance between the two fixes.

M

[NevisDog]

Originally Posted by captmikem

... At Noon a fix (celestial, electronic or DR) is marked on the chart ...

And do professional navigators record 'noon' as (a) local as in celestial fix, (b) local as in current time zone, or (c) 24 hr/global as in GMT/UTC/etc?

[captmikem]

Originally Posted by NevisDog

And do professional navigators record 'noon' as (a) local as in celestial fix, (b) local as in current time zone, or (c) 24 hr/global as in GMT/UTC/etc?

Always up to the Master as to Ships Time but normally we flog the clocks every 15 degrees of Longitude except in very high latitudes, and log noon as ships time Noonsights are recorded on the chart as LAN giving us a latitude check.

In the old days of running down your latitude morning sights were advanced (using DR to the time of the noon site to get a days run from the past days LAN, which depending on the speed and direction of the ship was around 24 hours (very rarely exactly 24 hours).

Nowadays the days run is used in several calculations (eta for the pilot, propeller slip, fuel economy etc.) and is normally reported to the company as required.

Just as a note often the moon is visible during LAN and you can shoot it and cross it with your LAN to get an actual fix. Otherwise you just get Latitude and it is not a fix, it is latitude and a time reference.

M

[captmikem]

Originally Posted by fxykty

Zulu is the old name for UTC, so the same time zone.

Actually, and this is only for interest, not correcting you... Zulu is the military designate of GMT 0 zone (GMT has been replaced by UTC) World time zones were ALPHA (GMT +1) Bravo (GMT +2) November (GMT -1) Oscar (GMT _2 ) Etc.

Juliet was sometimes used as a reference to local time

M

[Jim Cate]

Just as a note often the moon is visible during LAN and you can shoot it and cross it with your LAN to get an actual fix. Otherwise you just get Latitude and it is not a fix, it is latitude and a time reference.

Mike, years ago when we were still navigating with celestial, I wrote a little program for my HP-41 that did a least squares fit to a parabola, and would input a few sequential altitudes spanning LAN. The resultant location of the peak of the parabola gave a pretty accurate time for LAN, and hence a decent look at longitude... a good enough fix for a grotty yottie. Maybe not good enough for official use or for the office wallahs.

Jim

[captmikem]

Originally Posted by Jim Cate

Mike, years ago when we were still navigating with celestial, I wrote a little program for my HP-41 that did a least squares fit to a parabola, and would input a few sequential altitudes spanning LAN. The resultant location of the peak of the parabola gave a pretty accurate time for LAN, and hence a decent look at longitude... a good enough fix for a grotty yottie. Maybe not good enough for official use or for the office wallahs.

Jim

That is pretty neat Jim! I had never thought of that, your timing between the spaces had to be pretty exact I would imagine, or...your program took that into account, Just thinking about it gives some good thought. I remember using an SR71 (calculator, not plane) to write corrections (average) for the moon, it was a pleasant task. I was second officer on a RO/RO working between Miami and Panama, all we had were my sights and DR...(Loran was useless after 100 miles or so) doing 23 kts thru the Mosquito Channel ... I was shooting everything I could whenever I could. Used to shoot everything 3 times, then average the sight and time to try and get as accurate as possible.

Between 10:00 and 14:00 not a lot to shoot and using that would have at least kept me busy playing with it.

Too bad, nowadays no one knows what least squares or log tables are for anymore.

Thanks for the thoughts and memories.

M

[Jim Cate]

Too bad, nowadays no one knows what least squares or log tables are for anymore

Log tables??? That's where you stack the firewood, right?

And re the timing of the sunsights... no, random times were fine. One just input time/altitude pairs whenever it was convenient and the program sorted it out. Pretty standard statistical analysis stuff, just applied to something useful!

Cheers,

Jim

PS I think I saw Turmoil offered for sale a while back. Still one of the prettiest yachts I've ever seen.

[cyan]

Originally Posted by captmikem

This is a very simple question with a very simple answer. Crossing an ocean, the prudent skipper has a paper chart covering the ocean on the chart table. At Noon a fix (celestial, electronic or DR) is marked on the chart (circle with a dot for a non electronic fix, triangle with a dot for an electronic fix semicircle with a dot for a DR position). The next day at noon another fix is marked on the chart. The navigator then takes his dividers and (using the latitude scale on the side of the chart) measures the distance between the two fixes.

M

Does anyone else go full 18th-century with a sextant and a spring-powered marine chronometer? Our original idea came over too many beers when someone asked that prepper question: What if all our electronics or all satellites went down? (local lightning strike, global war, invading aliens, etc)

I suppose this Armageddon talk and an old copy of "On the Beach" piqued my interest in celestial navigation for a crossing. The led to our proposed "end-of-the-world" contest for the next ocean crossing. The 18th century team (IE "the Virgins") plot the daily celestial positions (sans electronics) directly on a chart while the 21" century team (the wh-- umm "Non-Virgins") separately plot the GPS positions on a big vellum overlay. (hidden daily from the Virgins)

The wager (for all the remaining rum) would involve the Virgins' ability to make the intended landfall without a course-correction from the Non-Virgins.

This might be better in a thread entitled: "What to do on a 3-week crossing when the books run out?" Does anyone have experience in similar Luddite practices? (my plastic sextant might disqualify my 18th century claim) Other long-crossing contests? (we had one involving fish once)

[patprice]

Originally Posted by woc

I may be the one off but I don’t care how many miles I sailed in 24 hours calculating fix to fix. What I do care about is how many miles I made good towards my destination. Not much math needed. For those that like to fix to fix calcs.....there is an app for that.

See my earlier. You put it better.

[KiwiKen]

"Does anyone else go full 18th-century with a sextant and a spring-powered marine chronometer? Our original idea came over too many beers when someone asked that prepper question: What if all our electronics or all satellites went down? (local lightning strike, global war, invading aliens, etc)"

When I trained as an Air Navigator with the Air Force in the early 1960s,
we went "18th Century", using the Mk 9 Bubble Sextant, and a spring Chronometer wrist watch ( International Watch Company), and in the B8 Canberra, we used a periscopic Sextant (similar to Mk9), and a similar sytem on most military transport and Bomber aircraft - the USAF B52 had a very complex and expensive modern sextant called an astro tracker that worked on the same basis. I came off flying Duties in 75, but astro was used consistently in the Forces, until the advent of GPS.
We did have quite hi tec (for the time) Ground Position Indicators using a doppler system to give a constant lat and long, or distance down track and to side of track, but these were monitored once in each met zone ( because the doppler could unlock over the sea) by comparing the machine position with an Astro fix 3 star or Box at night or MPP Sunline during the day, and as long as the machine position was within the circle of error of the observed fix the machine position was accepted. Our techniques were pretty slick, but the workload was reasonably high.
I have also been professionally involved in marine navigation, and have instructed navigation for both air navigators and candidates for ALTP, plus Marine navigation for both recreational and professional sailors. However I have been well and truly retired for 12 years now.

Modern aircraft do not have just a single GPS, usually there is a triplex system, to ensure the aircraft does not go astray if one system developed a fault.

[NevisDog]

Originally Posted by cyan

Does anyone else go full 18th-century with a sextant and a spring-powered marine chronometer? ...

Wouldn't be without sextant (we use one each) but a wind-up watch or clock is maybe a step too far; solar powered digital watch is a huge step forward. Are you allowed time signals on this proposed 18th-century voyage, or must we check with an eclipse of Venus?
I just gave away our air nav tables (so at least one other person must be keen to learn) since a basic calculator makes life so much easier in working sites, so that's one more electronic gizzmo that could fail when armageddon strikes.

[cyan]

Originally Posted by KiwiKen

When I trained as an Air Navigator with the Air Force in the early 1960s,
we went "18th Century", using the Mk 9 Bubble Sextant, and a spring Chronometer wrist watch ( International Watch Company), and in the B8 Canberra, we used a periscopic Sextant (similar to Mk9), and a similar sytem on most military transport and Bomber aircraft - the USAF B52 had a very complex and expensive modern sextant called an astro tracker that worked on the same basis. I came off flying Duties in 75, but astro was used consistently in the Forces, until the advent of GPS.
We did have quite hi tec (for the time) Ground Position Indicators using a doppler system to give a constant lat and long, or distance down track and to side of track, but these were monitored once in each met zone ( because the doppler could unlock over the sea) by comparing the machine position with an Astro fix 3 star or Box at night or MPP Sunline during the day, and as long as the machine position was within the circle of error of the observed fix the machine position was accepted. Our techniques were pretty slick, but the workload was reasonably high.
I have also been professionally involved in marine navigation, and have instructed navigation for both air navigators and candidates for ALTP, plus Marine navigation for both recreational and professional sailors. However I have been well and truly retired for 12 years now.

Modern aircraft do not have just a single GPS, usually there is a triplex system, to ensure the aircraft does not go astray if one system developed a fault.

Wow, Ken- much respect! I wish I could have experienced your navigation classes.

My first navigation "instructor" was in one of those official-sounding 3-number classes that leads to a fancy 3-letter certificate, for which I paid real money. The "teacher" handed out a single page Pseudo-cylindrical map of North America. (Robinson projection?) Think squished pumpkin view, but North America tilts to the right because this was a cropped cut-out of the full world-view that is centered on Greenwich, England.

He had us each place 2 pencil dots on the Latitude 35 N line where it intersects the East and West coasts of the US. Then we had to draw a straight line from each dot to the North Pole. Here's where it gets fun. We then measured the LENGTH of each line, noting that the West coast 35 N to the pole is a bit longer than the East Coast 35 N to the pole. (ONLY BECAUSE OF THIS PROJECTION'S DISTORTION, DUH)

Fun turns to magic: The "instructor" then stated that THIS difference (map projection perspective) is the reason why there is a magnetic variation, the difference between true North and magnetic North. (!) Oh, to have a globe handy.

I was the only one that laughed. Silence. The other 7 aspiring sailors looked confused. The "instructor" doubled-down and hammered his warped [SIC] point home, detailing the mysterious math of compass reading addition and subtraction.

The right thing to do was to stop the absurdity and speak up, pointing out that this "teacher" was not only wrong but possibly SO wrong that these newbie sailors would be more likely to end up on the rocks because of him. I did NOT do the right thing that day. Two students told me to shut up in a dicky way because my laughing was rude. I complied.

I eventually received my fancy certificate. However I wonder if any of my former classmates are now lost at sea because I was too polite. Probably not, but this I regret.

[cyan]

Originally Posted by NevisDog

Wouldn't be without sextant (we use one each) but a wind-up watch or clock is maybe a step too far; solar powered digital watch is a huge step forward. Are you allowed time signals on this proposed 18th-century voyage, or must we check with an eclipse of Venus?
I just gave away our air nav tables (so at least one other person must be keen to learn) since a basic calculator makes life so much easier in working sites, so that's one more electronic gizzmo that could fail when armageddon strikes.

I like the solar-powered watch idea! (+1 point to you for Venus eclipse)

My fellow sailor Todd would point out that an EMP or lightning strike might take it out, thus a mechanical spring is the best bet. Todd also pointed out that using the oven as a Faraday cage to avoid lightning damage could result in a melted watch when you forget about it whilst getting ready to bake bread. He claims to have lost a VHF radio this way, filling the galley with melting plastic smoke.

We are not taking Todd to Tahiti.