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index > nautical facts and information > nautical units and angles

NAUTICAL UNITS AND ANGLES


By: Jo Edkins

Nautical distance and depth
Points of the compass
Angles


I must confess that I know nothing of sailing, so I hope that I haven't made too many mistakes in the following!

Nautical distance and depth
6 feet = 1 fathom
15 fathom = 1 shackle
10 cables = 1 nautical mile
   
6080 feet = 1 nautical mile
Full fathom five thy father lies;
Of his bones are coral made:
Those are pearls that were his eyes:

Nothing of him that doth fade,
But does suffer a sea-change
Into something rich and strange.

Shakespeare - The Tempest

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.
The Observer newspaper had a lot of trouble with the definition of a ship's tonnage. It appears in their readers' corrections column 3 weeks running. So for everyone else, here is their final definition:

Tonnage is a measure of the internal volume of a ship, devised as a basis for charging habour dues. It is said to originate with 'tuns' of wine, one tun being equal to 252 gallons, roughly 42 cubic feet. The modern tonnage measurement, introduced by the Merchant Shipping Act 1854, is 100 cubic feet. Gross tonnage is the internal volume of the ship and Net (or Register) is the Gross tonnage less non-earning spaces, such as engine room and crew accommodation.
I had an email from 'an ex-Marconi seadog' who told me some nautical slang - nothing to do with units of measure but I can't resist it!

Tab Nabs = light treats (food)
Doby = clothes for washing - verb dobying - doby dust = washing powder.
To put someone ON THE SHAKE = to wake them up
Deck head survey = sleeping

Compass
Points of the Compass
Most people know the four main points of the compass, North, South, East and West (see left). If you can't remember which way round "West" and "East", they read "we"! However, there are 32 points of the compass. The best way to see this is to build it up a bit at the time. Half way between North and East is North East. This gives 8 points. The North or South always comes first, and the East or West comes second.
Now, imagine half way between East and North East. This is called East North East. (You could think of it as East North-East). The single point (North, South, East, West) comes first, and the double point (North East, North West, South East, South West) comes second. This makes sixteen points. Compass
Compass Now imagine half way between each of these points. This makes up 32 points.
To get the name of a point like this: take the nearest single or double name (such as North East). Then describe the movement to the new point by saying 'North East by North' to go anti-clockwise or 'North East by East' to go clockwise. So, starting from N and working clockwise, the points are: N, N by E, NNE, NE by N, NE, NE by E, ENE, E by N, E etc.
I'm afraid I'm NOT going to illustrate this!
 


As you can see, this is quite complicated. Now directions are given by degrees. North is zero, and the degrees go clockwise. So East is 90 degrees, and West is 270 degrees. This has the advantage that all bearings are equally simple, but it doesn't sound so romantic.
 


While angles are, of course, not confined to the sea, they follow on from the previous subject. There are 360 degrees in a circle (or 'full turn'). This is a useful figure, since it can be divided in many ways, so if you want to divide a round cake into 3, it's 120 degrees, 4 pieces are 90 degrees, 5 pieces are 72 degrees, 6 pieces are 60 degrees, and so on. One degree is a very small amount, but if you want something smaller, then there are 60 minutes to a degree, and 60 seconds to a minute. So there are 1,296,000 seconds to a circle!

However, in Mathematics, we meet another way of measuring angles - radians. There are 2 pi radians to a circle, where pi is the ratio of the diameter of a circle to its circumference (round its edge). It should have its own symbol, but this might not work on all browsers, so I'm not using it. It looks like a minature Stonehenge. It is a very strange number, approximately 3.1416, but however many places of decimals you write, you never reach the accurate figure of pi, and the number sequences never start to repeat themselves continually. This is called an irrational number, and pi is, perhaps, the most famous example. I am tempted to call mathematicians pretty irrational to use a unit where you not only don't get a whole number of them to a complete circle, you don't even get a rational number (or fraction)! They would defend themselvese by stating that if you cut a slice of cake with angle of one radian, then its rounded edge is the same length as its straight sides (or the radius of the original circle).

There are also formulae which are easier to deal with if you use radians rather than degrees.
Another angle unit is the mil. This is a military unit for defining angles. The name derives from milliradian, and they are used because an angle of X mils is X metres wide at a distance of X kilometres. This means that if you drop a shell 200 metres to one side of the target (according to your map) and it's 4 km away, 200 divided by 4 is 50, so you swing your aim by 50 mils. Note that current OS maps have the magnetic and grid deviations shown in mils as well as degrees.

There are 6283.1853 mils in a circle, but they seem to be rounded off to various values in use, such as 6283 and 6280. However, the US military made things 'simpler' by standardizing on 6400 mils in a circle. To make things even more interesting, the Russians, and perhaps others in Europe, use 6000 mils in a circle!


Anyone who seriously disapproves of metric systems may be pleased to hear that the French Revolutionary lot tried to define a 'metric' angle of measurement. This was gradians, and there were 400 to a circle (so 100 to a right angle). It never caught on. They also tried to define time as 20 hours to a day, and 100 minutes to an hour, with an equal lack of success. (I'm surprised that they didn't try to define 400 days to a year!)


Jo Edkins was born in 1953 in England, and she learnt her tables at school as did all children of that time. This included not only the multiplication tables, but Lengths and Measures as well, and these were usually to be found at the back of our exercise books. All people of around that age know vaguely about 'rods, poles and perches' even if they can't remember what they are!

However, when Jo's son was born, children were only taught metric, and Lengths and Measures became boring powers of ten. Jo thought it was a shame that this information should die out altogether, so she looked it up in various ancient and falling-to-pieces reference books, and put it in a website. This website has been greatly expanded by many people writing to her with additional information (and corrections where she's made mistakes!)

Jo has no experience of sailing, boats or ships at all (apart from punts), and has lived over a hundred miles from the sea for most of her life - quite hard to do in England! However, she loved the Swallows and Amazons books of Arthur Ransom as a child, and added a page on Nautical measurements and Angles, and another on Weather, to her website. This has been expanded by other people's contributions.


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