Whether we like it or not, for better or worse, for richer for poorer, till death us do part, etc., etc., the European Union rules much of our lives these days – including what we use in the way of weights and measures.  However, being (once) a seafaring nation we consider it our prerogative to decide what goes on at sea, and so have retained many of the traditional measures (if only to spite and confuse the ‘damned foreigners’).  This is a summary of some of the ‘good’ old ways in use at sea.

1 fathom =          6 feet
1 chain =          100 feet
6 chains =          1 cable
1 cable =          608 feet (say (600)     =          100 fathoms
10 cables =          (approx) 1 nautical mile (NM)
1 NM =          6080 feet
  =          1/60th of 1 minute of arc (1 degree of latitude)
  =          1/60th of 69.091 (i.e. 1.1515) statute miles
1 knot =          a speed of 1 NM per hour
US cable =          120 fathoms
1 log-line =          450 feet                       =          75 fathoms
Note: The US cable and the log-line above are approximate.

The fathom is the traditional imperial unit for measuring soundings, or depths, at sea, although Admiralty Charts now use metres.  The OED ascribes its etymology to the Old English word fæÞm, from Germanic roots, for two arms outstretched in a straight line to their full length, since standardised to six feet.  For taking soundings, a line was attached to a lead weight which was swung to windward, by a man (called the leadsman) standing in the main-chains, in the direction the ship was moving, so that by letting go the line at the right moment and then hauling up the slack, a skilled leadsman could get an up-and-down sounding when he passed over the lead; a task that, though skilled, was physically undemanding, often given to an unfit seaman who was envied by the rest of the crew – perhaps the origin of the expression ‘swinging the lead’ used for someone suspected of shamming inability in order to get an easy time.

Two weights, or plummets, were used: one for shallow waters up to about 20 fathoms, called the ‘hand-lead’ and weighing about 9 pounds and the other for deeper seas, unsurprisingly called the ‘deep-sea-lead’ and weighing up to 30 pounds.  Both were an elongated cone-shape and were used on approaching land after a sea voyage.  Not relevant to measures used, but for interest, the bottom of the lead would be liberally coated with tallow, which would pick up shells, mud, gravel, etc., that would stick to it, thus aiding the knowledgeable navigator, who would usually have such information marked on his charts.

The hand-lead-line was usually 20 fathoms in length and was marked at 2 fathoms by two strips of black leather; at 3 fathoms by three strips of black leather; at 5 fathoms by a piece of white bunting; at 7 fathoms by red bunting; at 10 fathoms by black leather; at 13 fathoms by black leather; at 15 fathoms by a white rag and at 17 fathoms by a red rag.  The corresponding depths were called ‘marks’ where one of these marks was seen to be close to the surface of the sea, whereas unmarked depths were estimated and called ‘deeps’ or ‘dips’.

The deep-sea-lead was marked at 20 fathoms with two knots; at 30 fathoms with three knots; at 40 fathoms with four knots, and so on to the end, with the intermediate lengths of 25, 35, 45 fathoms, etc., each marked with a single knot.  The deep-sea-lead was more efficiently used when the ship had been ‘brought to’, or stopped, due to the time taken by the lead plummet dropping to the deep sea-bed.

The calls of the leadsman were made in a chant resembling the cries of street-hawkers of the day; thus if when using the hand-line the red bunting was seen near the surface he would call ‘By the mark seven!’, but if it was seen to be about a fathom above the surface he calls ‘By the dip six!’ etc.; with more accurate estimates being given by such calls as ‘And a quarter six!’ or ‘And a half five!’ or ‘And a quarter less six!’, etc.  Samuel Clemens, whose early life included a spell as a steam-boat pilot on the Mississippi, adopted ‘Mark Twain’ as his nom de plume from the leadsman’s call for two fathoms.

Early ships would carry a stock of logs for the galley fire (some still do) and the speed of the ship would be estimated by tossing a log, attached to a long line for prudence sake, overboard at the bow and measuring the time it took to pass the stern, a known distance that would be used to calculate the speed, which would then be recorded in the ‘log-book’.  This rough and ready calculation method using what was known as the ‘common log’ was improved over time by the ‘chip log’ being made of a quadrant of light wood, weighted to float upright and thus increase resistance in the water, and by knotting the line at intervals of 47 feet 3½ inches and running it out for 28 seconds – measured by a 28 second sand-glass – from which it was calculated that a speed of ‘one knot’ was 6,080 feet per hour (28 × 6,080 ÷ 3,600 = 47.29ft).  Then the ‘patent log’ was invented in the 17c and developed quickly from unsatisfactory early types to a more streamlined form in the early 19c, similar to that in use today, which was a rotator with fins, towed astern by means of a non-twisting log-line, or fixed into the hull, and used to measure distance as well as speed.  The modern log uses an electromagnetic field sensed by electrodes under the hull.

A cable length differs from the cable attached to a ship’s anchor, which is measured in ‘shackles’ or ‘half-shackles’.  Until about 1950 a shackle equalled 12½ fathoms, or ⅛ of 1 cable, but it has since been revised to 15 fathoms.  The bower cable of the ill-fated HMS Hood was 41 shackles of 12½ fathoms, a total length of 3,075ft – over half a mile!

The Admiralty designated the nautical mile as 6,080ft, or 1.1515 statute miles, but it was eventually superseded by the international nautical mile which is about 3ft 10½in shorter, at 1,852 metres.  So now the conversion from nautical miles to statute (land) miles is a multiplier of 1.15 and to convert from statute miles to nautical miles, a multiplier of 0.87.  Consequently, 38 statute miles equals 33 international nautical miles.

In practice, the nautical mile is generally taken as 2,000 yards, or 10 cable-lengths.  The nautical mile is particularly handy, enabling the polar circumference of the Earth to be expressed as 21,600 miles (the number of minutes in a circle), corresponding to 24,860 statute miles.  But, beware of the international nautical mile, as stated above.

The geographical mile measures one minute of longitude at the equator, giving a length of 6,087ft 0¼in, but the nautical mile is measured at the meridian, which means its length varies with latitude.  The value of 6,080ft chosen by the Admiralty is about right for latitude 48º N.  The value of 1,852m is appropriate at latitude 45º N, in the region of Bordeaux and Nova Scotia.  Dividing the distance from the equator to the north pole by 5,400 (the number of minutes in the quadrant) the average nautical mile is nearly 6,076ft 10in, ranging between 6,046ft 3in at the equator to 6,107ft 6in near the pole.

Historically, a vessel’s tonnage was the number of ‘tuns’ or barrels she could carry, but ship tonnage today has distinct meanings in the merchant and Royal navies.  The mercantile marine all over the world uses ‘registered tonnage’, which is the internal capacity of the ship measured in hundreds of cubic feet: so 1 registered ton equals 100cu.ft.  On the other hand the Royal Navy uses ‘full-load displacement tonnage’, which is the weight of the ship with all her equipment, stores and armaments, representing the weight of water she displaces.

The size of a rope is its circumference in inches; easily measured by the length of a piece of twine needed to wind once round the rope.  The increasingly used alternative is the diameter of the rope measured in millimetres, but the diameter is much harder to measure exactly without instruments.  Fortunately, the two measures are easily related; for the diameter in millimetres is roughly (near enough) eight times the circumference in inches (2.54 × 3.1416 or 25.4 ÷ 3.1416 = c.8).  So a 2½in rope has a diameter of 20mm.  Some small ropes are supplied in lengths of 20 fathoms and their size measured by weight: e.g. if we refer to a 2½lb line we mean that 20 fathoms of it weighs 2½lbs.

This article would not have been possible without taking advantage of the mine of information in “The General Rule” (ISBN 1-978-906069-01-8) by Vivian Linacre, who did all the hard-slog investigative work, over many years.  Vivian Linacre has a passion for tradition and has been the lifeblood of, and is now the President of, the British Weights and Measures Association.

For general enquiries try the link up on the right.  BWMA is an organisation dedicated to the resistance to the criminalisation by the EU of traditional weights and measures.  My role was reduced to the easy bit, or ‘plagiarism’ as those of you of a cruel nature would call it; consequently, if any income accrues from this article it will be donated to the BWMA.  Please support them if you value tradition.