Slocum's Navigation

[sneuman]

We know (according to Sailing Around the World Alone and a bit of historical research) that Spray's "chronometer" was probably this "Little Lord Fauntleroy" model nightstand clock - of dubious accuracy and without even a second hand!

Considering that one second could mean being a half-nautical mile off even if his sun, moon, or star shots were perfectly accurate (and, let's face it, they probably were damn good), Slocum still would have had to visually interpolate the seconds on that clock. Then who knows how much did that piece of junk gain/lose?

How the hell did he do it?

[callmecrazy]

mark the site, start counting until the minute hand clicks. subtract.

[sailorF54]

He seems to have taken few shoots, and used the old lunar distance method that did not require accurate time when he had to. He was probably one of the last to use it...

[boatman61]

I guess folks were more intuitive back then. also they were not as obsessed with pinpoint accuracy... one made as accurate a general direction as calculably possible then made adjustments as/when visble marks appeared to confirm ones position on the chart... working within errors was a way of life
Pretty much how I sail today... when GPS dies...

[Blue Stocking]

Originally Posted by boatman61

I guess folks were more intuitive back then. also they were not as obsessed with pinpoint accuracy... one made as accurate a general direction as calculably possible then made adjustments as/when visble marks appeared to confirm ones position on the chart... working within errors was a way of life
Pretty much how I sail today... when GPS dies...

Sounds like a plan to me.
So I sail South, note on my chart where I turn left, circumcise the globe until I get back to my noted position--and tun left again.

[PamlicoTraveler]

He used dead reckoning for longitude and apparently only took one lunar observation during the entire circumnavigation. He used an approximate time, and Noon Sun sights for latitude.

I think the guy was just winging it and got lucky.

[sneuman]

Originally Posted by sailorF54

He seems to have taken few shoots, and used the old lunar distance method that did not require accurate time when he had to. He was probably one of the last to use it...

Thanks, you made me look this up. Interesting. The semi-diameter for the moon and parallax correction (needed for lunar sights) are both listed in modern Nautical Almanacs, so, in theory you could still do this.

[boatman61]

Originally Posted by Blue Stocking

Sounds like a plan to me.
So I sail South, note on my chart where I turn left, circumcise the globe until I get back to my noted position--and tun left again.

No.... left at the 3rd whale or you'll run aground.... these days one use's Air France from St Martin to judge left turns on Trans Atlantics...(shortage of whales)..... remebering its only a 45degree turn or one ends up in Iceland... a 45 takes you str8 up the Western Approaches...


Thanks, you made me look this up. Interesting. The semi-diameter for the moon and parallax correction (needed for lunar sights) are both listed in modern Nautical Almanacs, so, in theory you could still do this.

Nope... in FACT you can still do this

[skipmac]

Originally Posted by sailorF54

He seems to have taken few shoots, and used the old lunar distance method that did not require accurate time when he had to. He was probably one of the last to use it...

This was my recollection from the book, that he found his longitude by lunar observations which do not require accurate time. In fact, didn't it say that his clock didn't even had a minute hand, much less second hand?

Did not recall any mention that he took only one lunar on the voyage but as I started rereading the book this weekend I'll keep an eye out for mentions of the navigation.

[maxingout]

Slocum was an experienced mariner with a knowledge of the movement of celestial bodies. Knowing where you are isn't that big of a deal if you know your sky.

Modern mariners are addicted to GPS, and they feel that they must know their position within a few meters. That level of accuracy is nice when your EPIRB transmits the position of your life raft to rescuers. But for mariners who stand watch, such accuracy isn't necessary.

For thousands of years people have done latitude sailing which is very easy. Simply by looking at the height of the north star above the horizon, you instantly know your latitude in the northern hemisphere. With latitude sailing, you simply sail on the latitude of your destination until you arrive there.

Every old time navigator knew the latitude of their destination, and they simply sailed up to or down to that latitude, and then stayed on that approximate latitude until they arrived at the desired port.

Arab navigators used a Kamal for their latitude navigation. A Kamal was a stick that they held in their hand, and they tied a string to the stick. At every destination, they would hold the stick in front of them so that one end of the stick touched the horizon and the other end of the stick touched the north star. They would then make a knot in the string to designate the length of the string necessary for the Kamal to subtend the latitude of that destination. When they wanted to sail to Bongo Congo, they would place the Bongo Congo knot in their teeth, stretch the string out in front of them, and then sail north or south until the Kamal stick was touching the horizon on one end and the north star on the other end of the stick. Every different destination had a different knot in the string. It was a basic method of latitude navigation that worked.

Latitude navigation still works today. When I sailed across the Atlantic from Cape Verde to Barbados, it really did not matter my exact position during the trip. But at the end of my trip, I did need to finish at the approximate latitude of Barbados. A Kamal would have worked fine for that trip.

[sneuman]

Originally Posted by maxingout

Slocum was an experienced mariner with a knowledge of the movement of celestial bodies. Knowing where you are isn't that big of a deal if you know your sky.

Modern mariners are addicted to GPS, and they feel that they must know their position within a few meters. That level of accuracy is nice when your EPIRB transmits the position of your life raft to rescuers. But for mariners who stand watch, such accuracy isn't necessary.

true, but "running down [your] latitude" could also mean "running on the rocks" if you don't have a good idea of longitude from your DR position, too.

[zopi]

Originally Posted by Blue Stocking

Sounds like a plan to me.
So I sail South, note on my chart where I turn left, circumcise the globe until I get back to my noted position--and tun left again.

Didn't know you were a Mohel....

Slocum was a lunarian...He calculated time based on lunar position...the math is a bit thick...and very few people bother now...and corrected his time and position when meeting other ships which answered hail...

Slocum was a very accomplished navigator (gambler) for those days, those men were largely fearless, they knew how to play the odds, take an area of uncertainty around one's assumed position created by taking sights and reducing them and proceed along a DR course in the direction of your destination, correcting that course utilizing what you know, suppose, or observe about current, leeway and speed made good...they usually didn't know or much care exactly where they were, except in proximity of land, which was avoided in poor visibility or foul weather...if one has a 10-20 mile AOU in the middle of the pacific, and your DR plot is good, and one confirms the other...big deal. GPS, Radio, RADAR, RDF, LORAN, Satellite phones...let us plot our positions to within small fractions of arc seconds...if it ain't foggy, who cares? as long as one can make landfall safely from ocean side, such precision isn't really necessary...but it is nice.

Whatever else Slocum may or may not have been...he was one hell of a Seaman...a term which IMO has lost some of it's meaning...guy had Muy Cojones...

[sailorF54]

Originally Posted by maxingout

...

Latitude navigation still works today.

There was a time it didn't...
Look up for example
Scilly naval disaster of 1707 - Wikipedia, the free encyclopedia

[roverhi]

Slocum did indeed use lunar sights to get a fix, both lat and long. It is my understanding that the computations are quite complex to reduce a shot this way and the tables for reducing the shots were removed from Bowditch in the very early 1900s. Chronometers had become so accruate and reasonably cheap that mariners no longer used the moon shots by the the beginning of the last century. IIRC, The moon data in the current almanac and Bowditch believe is meant to be used with a chronometer to find your way. It's not the sight form that Slocum used.

Slocum couldn't have made it more than a few hundred miles relying on DR Navigation. Even a skilled navigator would have to use the sun and stars to maintain a reasonably accurate DR position as the effects of wind, current, etc. would multiply throwing your navigation way off.

He could have relied on latitude sailing for navigation without a clock using the various methods described above. He was too good a navigator to have had to rely on latitude sailing, however. He obtained his longitude from the moon using the old tables.

Slocums clock not only had no second hand but had lost its minute hand also. Slocum was on a tight budget, to say the least, and recycled a clock that had been discarded.

[MichaelC]

Slocum didn't have jet contrails.
When I was preparing for my first ocean crossing, I took a class in celestial navigation. The instructor was incoherent. I was talking to an old fisherman at Seattle's Fisherman Terminal [this was in the 70s], and he scooped the snoose out of his mouth, and told me how to find Honolulu. Go out the Strait of Juan de Fuca, hang a left. When your AM radio picks up San Diego, hang a right. Follow the jet contrails. They are all going to Honolulu. I took some books and my radio and my watch and a Davis plastic sextant. He was right about the contrails, and I taught myself to navigate on the way over.