A cable length (or simply “cable”) equals one-tenth of a nautical mile — approximately 185 meters, or about 608 feet.
Why a Shorter Unit Than the Nautical Mile
The nautical mile is the standard for navigational distance, but at 1,852 meters it’s too coarse a unit for many shorter, practical distances — describing how far a vessel is anchored from shore, the separation between two anchored vessels, or distances within a harbor. The cable length fills this gap, giving a genuinely useful shorter unit that still relates cleanly to the nautical mile (exactly 10 cables per nautical mile).
Common Uses
- Anchoring distances — describing how far a vessel lies from shore, a hazard, or another anchored vessel
- Coastal pilotage — short-range distance references in coastal or harbor navigation, where nautical miles would be imprecise
- Traditional chart notation — some older charts and traditional navigation references use cables for shorter distances
Cable Length vs Shackle: Don’t Confuse Them
A cable length (a distance unit, 0.1 nautical miles) is entirely different from a shackle (a unit of anchor chain length, 15 fathoms) — both are traditional maritime units, but measuring different things, and confusing them would be a genuine error.
Modern Usage
While GPS and modern electronic navigation have reduced day-to-day reliance on the cable length for precise position work, it remains genuinely useful shorthand in practical ship-handling communication — “anchored two cables off the breakwater” conveys a clear, useful distance without needing decimal nautical mile figures.
Exam Relevance
Cable length, its relationship to the nautical mile, and its practical anchoring/coastal use are covered within Navigation and Ship Knowledge topics in deck officer competency exams.