Life at sea is practical, it has to be, therefore measurements had to be practical as “life at sea” was tough “back in the day”. Units had to be easy to “fathom out” and practical in use.
A fathom was the distance between a “sailor’s” out stretched hands, when hauling a “sounding lead” when measuring the depth of the sea.
The “Cable” causes much confusion, partly due to the method of cable construction and by its use as a measure of distance.
The heaviest Royal Navy cable-laid anchor cables were constructed by twisting together 3 hawser-laid ropes (clockwise); each hawser-laid rope was constructed by twisting together 3 ropes (anti-clockwise). This repeated twisting produced very strong water-laid cables which absorbed little water. Each stage of twisting reduced the length of the cable.
A rope is a group of yarns, plies, fibres or strands that are twisted or braided together into a larger and stronger form. Ropes have tensile strength and so can be used for dragging and lifting. Rope is thicker and stronger than similarly constructed cord, string, and twine.
Rope may be constructed of any long, stringy, fibrous material, but generally is constructed of certain natural or synthetic fibres.
Common natural fibres for rope are manila hemp, hemp, linen, cotton, coir, jute, straw, and sisal. Synthetic fibres in use for rope-making include polypropylene, nylon, polyesters , polyethylene (e.g. Dyneema and Spectra), Aramids (e.g. Twaron, Technora and Kevlar) and acrylics (e.g. Dralon).
Synthetic fibre ropes are significantly stronger than their natural fibre counterparts, they have a higher tensile strength, they are more resistant to rotting than ropes created from natural fibers, and can be made to float on water. However synthetic rope also possess certain disadvantages, including slipperiness, and some can be damaged more easily by UV light.
The twist of the strands in a twisted or braided rope serves not only to keep a rope together, but enables the rope to more evenly distribute tension among the individual strands. Without any twist in the rope, the shortest strand(s) would always be supporting a much higher proportion of the total load.
Some ropes are constructed of mixtures of several fibres or use co-polymer fibres. Wire rope is made of steel or other metal alloys. Ropes have been constructed of other fibrous materials such as silk, wool, and hair, but such ropes are not generally available. Rayon is a regenerated fibre used to make decorative rope.
Marine Rope measurement
The long history of rope means that many systems have been used to state the size of a rope. In systems that use the “inch” (British Imperial and United States Customary Measure), large ropes over 1 inch (2.5 cm) diameter such as are used on ships are measured by their circumference in inches; smaller ropes have a nominal diameter based on the circumference divided by three (rounded-down value for pi). In metric systems of measurement, nominal diameter is given in millimetres. The current preferred international standard for rope sizes is to give the mass per unit length, in kilograms per metre. However, even sources otherwise using metric units may still give a “rope number” for large ropes, which is the circumference in inches.
Manufacture of a 100 fathom cable would require:
- 3 x 120 fathom hawsers
- each hawser would require 3 x 150 fathom ropes.
In 1830, the UK Admiralty defined the following:
- Cable’s Length (distance): Tenth of a nautical mile (approx 101 fathoms).
- Cable-laid cable: 100 to 115 fathoms.
- Cablet: 120 fathoms.
- Hawser-laid cable: 130 fathoms.
- 6 feet.
- Shackle of cable (UK RN, old):
- 12½ fathoms. Length of a section of (anchor) chain. (Used until 1949).
- Shot or shackle of cable:
- 15 fathoms. Length of a section of (anchor) chain between joining shackles or swivels.
- Scope of cable:
- The length of (anchor) cable paid-out. Measured by counting shackles. Approx 5 times depth of water, depending on conditions.
- Cable (UK RN and Germany):
- 0.1 nautical mile. Approx 101 fathoms.
- Metric Cable (France and Spain):
- 200 metres. Approx. 109 fathoms.
- Cable (USA):
- 120 fathoms.
- Other Cables:
- Russia: 100 fathoms.
Holland: 123 fathoms.
Portugal: 141 fathoms.
- Nautical mile (international):
- 1852 metres (approx 6076 feet).
- Nautical mile (UK RN, old):
- 6080 feet. 10 cables. After 1970, Admiralty charts were changed to use the 1852 metre international nautical mile.
- Nautical league:
- 3 nautical miles.