Admiralty Nautical Mile

[MainlySteam]

While the International Naurtical Mile has now long overtaken the Admiralty Nautical Mile (aka Admiralty Mile, Imperial Nautical Mile, Admiralty Measured Mile, etc) I was wondering about some of the history behind the Admiralty one.

Know all the obvious stuff like how long it is and relationships to the Admiralty Knot, latitude, etc but can anyone fill in the bit as to who in English history determined it and when.

Thanks for any responses.

John

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[powerskipper]

try
here
or

<A target="_blank" HREF=http://powerskipper.mysite.freeserve.com/>http://powerskipper.mysite.freeserve.com/
Julie

 

[MainlySteam]

Thanks Julie, but none of them actually say who in English history determined the Admiralty Nautical Mile (as opposed to the international one) and when.

Like the Admiralty NM is approx 1,853.181 meters whereas the international one now universally used is 1852 meters (for whom I am indebted to Tome for the calculator face aid d'memoir - I always just rounded it to 1850 in my mind before that for convenience). Somewhere back in English history at some time, someone decided upon the 1,853.181 meter bit - I have not found who and when.

Obviously it would have been related to some assumptions about the dia of the earth (obviously pretty accurate ones so I suspect the Admiralty NM came about in relatively recent times, perhaps 19th Century might be earliest?) but beyond that I have drawn a blank.

An Admiralty knot would also have been slightly faster than the current knot too (though not by very much).

Not a life or death thing /forums/images/icons/smile.gif but any help is appreciated.

John

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[halcyon]

One date seen is a figuer of 6080 feet set around 1600 for the Admiralty Nautical mile, or the Admiralty measured mile.


Brian

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[Guest]

Its ok quoting metres - but only recently have these measures been UK used ...... reference to the older Imperial style measurements will remove a load of decimals and revert the measurements to true units and more understandable in numerical form.
Remember that Admiralty used Chains, Feet, Fathoms etc. etc. not the imported 'Frenchifile' bits.

There are many things tio do with admiralty and Brit at sea from the past that lead to interesting reading ... signals is another - things such as Nelsons famous signal .... leave that one for you lot, but you'll be surprised about HIS wording and the actual FLOWN wording and why they were so different etc.
Steering commands in RN being opposite to nowadays .... port for starboard turn etc. etc.

Distance measurement and terms is only the start of the interesting saga ......


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[Guest]

6080 ft is true for Nautical Mile when used in a calculation .... but in reality the nautical mile is one minute of arc on earths surface = 1 nautical mile.

The statute mile was designated by ... I believe .... Elizabeth 1st and has basis in Hampton Court ....... can't remember the exact bits ....but know that it had no physical relationship ....

<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[MainlySteam]

Yes the mile was designated by Q Elizabeth I and as you say 6080 is the imperial of the Admiralty NM.

But do you know when and who by the Admiralty NM came about from. I have since confirmed that, rather obviously, it was fixed by the Admiralty but do not know when and if any particular person was involved.

Regards

John

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[Guest]

OK - now I stick my leg out .... the figure of 6080 was derived from a particaulr latitude reference .....

I can't answer completely - because some b.....d stole my Navigation College notes years ago and its in there.

As the NM is variable depending on the latitude you are measuring at .... it was necessary to have a standard NM that could be used for calculations. It was decided that based on large number of vessels of the RN being in certain latitude range - the mean would provide the basis of the NM for calc purposes .... true or not I don't know. But it is true that a certain latitude did give us the 6080 ft. As to when and who deicded ???????


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[trev]

Can't say I've ever heard of the 'Admiralty' or 'International' nautical mile in over forty years of seafaring - its always just been the nautical mile of one minute of arc of latitude or 6080 feet as a fairly good mean.

<hr width=100% size=1>Trev

 

[bob_tyler]

Nigel, that's how I have always understood it although I must admit that I always considered 2000yards good enough against 1760yards for the statute mile.

Not perhaps a close enough approximation in these days of GPS.

Bring back Feet & Fathoms. I can't get my mind round metres for depth.

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[Piers]

I believe you will find the average nautical mile is approx 6076 feet.

As has been said, the absolute is one min of arc of latitude.

But since the world is not a perfect sphere (it's an oblate spheroid - rather like me as my children say....) the actual length of the nm varies.

Hence the average of around 6076 feet. Loosely taken to be 6080 feet.

<hr width=100% size=1>Piers du Pré
http://www.dupre.co.uk

 

[MainlySteam]

Seems quite a few are stuck on the belief that 6080 ft is a nautical mile, however that is incorrect as the internationally accepted international nautical mile is 1852 meters. Even the UK National Maritime Museum has it wrong on their internet site (in the FAQ section) even though the UK (including UKHO) changed to the international NM over 30 years ago (1970)!

All fairly trivial really but at least the meters bit is something I use all the time. The anchor watch alarm radius on our GPS is set in decimals of a NM and, of course, charted depths are in meters so a conversion is needed if one wants to set it greater than the chain veered. I generally just round it to 1850m for that if doing it mentally but as Tome pointed out to me once, on a calculator keyboard the layout makes it easier to both remember and enter the exact 1852.

Some still using the old Admiralty NM but no one seems to know when and who in the Admiralty was responsible for it /forums/images/icons/shocked.gif. The history of English seafaring is obviously going to the dogs or maybe it was borrowed from the Portuguese and no one wants to admit it /forums/images/icons/smile.gif.

John

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[peterb]

CA Library

The Cruising Association Library may have the info that you're looking for. I'll try to remember to look next time I'm there.

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[Guest]

I finished MN training in late '75. The college instructors / lecturers were all using 6080 ft and that is what we used in examinations.

I am not arguing that 1852 mtrs may have been accepted by UK in 70 .... but it seems strange that one of the foremost MN colleges in UK should make such an error ?

And anyway - NM is defined and stated in a definite measurement for calculation purposes. The true NM is one minute of arc on the earths surface. If not - then the whole worlds charts would have to be re-drawn ....


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[MainlySteam]

<<<The true NM is one minute of arc on the earths surface>>>

But where on the earths surface (the earth is an obloid)? Or are you saying that a nautical mile varies according to where one is (which is contrary to the definition, whether the old Admiralty - 6080 feet - or the more recent international NM - 1852 meters).

I accept that we all in practical chart work just take a nautical mile as being 1 degree of latitude of the chart in the latitude that we are working.

On your training in 1975, I don't know but assume that all the UK (Admiralty) charts were also still in fathoms then too - I understand that some still are.

Regards

John


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[terryd]

I briefly remember from my old Navy days that 1 Nautical Mile was given as the distance of 1 minute of Arc at 48 degrees North or South. As rightly pointed out with the earth not being a true sphere it has to be somewhere as a nauitcal mile cannot change as you move around the world. However I cannot remember why or when it was choosen as this point on the Earth's surface.

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[trev]

I think this is a load of B*ll*cks. The metric global measurement is Universal Transverse Mercaror (UTMs) which is a surface measurement in metres labelled North/South and East/West. - you probably have it in your GPS.
I can't think of any purpose for a 'metric' nautical mile.
I took my Master Mariner's cert in 1973 and have attended several up-dates since - I also work with full time professional mariners - never heard of this before. Where did you get this?

<hr width=100% size=1>Trev

 

[MainlySteam]

<<<I think this is a load of B*ll*cks>>>

You may think that but I am afraid that you are wrong. Any simple internet search for nautical mile or international nautical mile will demonstrate that. The international nautical mile is 1852 m and it is now universally used.

If you want an authorative reference try at least Bowditch under "Nautical Mile" in its "Glossary of Marine Navigation" where it is explained. Is starting to look like you need a refresher course.

Sorry.

John

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[Guest]

The nautical mile is 1 minute of arc on the earths surface and yes it does vary with location - that is why an abitary figure had to be devised to be able to use it in calculations ..... and it was based on a particualr Latitude..... not on any physical measurement.

As regards charts - I started with Fathom AND metric .... as they were still finishing off the change-over - and yes I believe there still are some fathom charts around, but they will be obscure and little used areas.
I used to like the old fathom charts ... with all sorts of interesting remarks printed on .... mermaids, dolphins, whales etc. etc. Metric charts are too uninteresting now.

As I do not sail commercially anymore - only for fun and have links to ships via my work - I don't really have to keep up with this sort of topic - but find it interesting to read / comment.

I shall continue with my dividers on side of charts ....


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[Guest]

Extract from Web ....

<<Fathoms measure depth. They have been in use since before 1600, and may be derived from "faethm", the Anglo Saxon word for "to embrace" (because it is roughly the distance from one hand to the other if your arms are out-stretched). Tim Chafer from Germany says "The English term is related to the German term 'Faden', which is also the general term for 'thread'. A 'Faden' or 'fathom' would thus originally have been the length of rope or thread you could stretch between the hands with the arms outstretched."

Nautical miles measure distance. 1 nautical mile is the angular distance of 1 minute of arc on the earth's surface. As these differ slightly (6108' at pole c.f. 6046' at equator) 6080 was adopted (this being it's approximate value in the English Channel). The International nautical mile is 1852 metres, so is very slightly different from the UK nautical mile.


1 UK nautical mile = 1.00064 international nautical miles
Now the International nautical mile is used throughout the UK, except in the Statutory Instrument 1995 No. 1804 which defines it as 1853 metres! If you want an irrelevant fact, one minute of arc on Mars is close to a kilometre (.987 km to be more accurate). Perhaps the French who defined the kilometre were really Martians!
A knot is a nautical measure of speed, one nautical mile per hour (or about 1.15 mph). The name comes from the knots tied in the log line used with the sand glass. The log line was thrown onto the sea and the knots in the line were counted as they ran out during the sand glass interval. The knot has been used since the 17c. It is sometimes called the sea mile. Again, the international knot is very slightly less than the UK knot.

The following information is given by Dick Grindley. "A cable was originally the length of (believe it or not) a hemp cable defined by the RN (although further complicated by whether it was a rope or a hawser laid cable), the shackle came later when anchor chains started being made of lengths of a number of un-dividable links joined up at a 'joining shackle'. So an anchor cable will always be an integral number of shackles long. In chart work the cable is still in use for the simple reason that charts are usually large & when you use them you tend to fold them to get at the bit you want. Having folded them the metric linear scale is never visible (must be someone's law ?) but at either side of the chart is the latitude scale in units of mins, sub-divided into 10 (a.k.a. the cable). So, when I did my chart work training at Dartmouth (a few years ago I must admit) we quite happily mixed metric depth & height with nm & cables for distance."

The shackle has given me some trouble. I've been given the values of 12 fathoms, 12.5 fathoms and 15 fathoms (which is the correct figure) to the shackle. I am indebted to Richard Sheppard for the following information.

Apparently, according to a quote from a 19th Century Seamanship Manual, ships were usually equipped with 12 shackles of *bower cable* where each shackle was a 12.5 fathom length. (Perhaps this is where the figure of 12 comes from?) These 12.5 fathom lengths were joined by shackles (hence the name) and by swivel links to overcome twisting. When paying out the anchor cable, counting the number of shackles passing gave a measure of the length used.
In 1949, the Royal Navy switched from 12.5 fathom shackles to 15 fathom shackles.
In 1970, the RN Hydrographic Charts Dept. switched their Nautical Mile from 6080 feet to 1852 metres (the International NM). At the same time, their Cable became a tenth of the International Nautical Mile.
Modern heavy mooring chain is usually sold in 15 fathom lengths or *shots*. The specification sheets quote the number of links per shot. Much of it seems to come from the Far East!
Light chain can be bought in arbitrary lengths. >>

So it seems there is a point to all this ..... technical and not affecting daily life on boat .... but seems true .....

I cannot of course say the source copied here is correct !!


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/