Safe water: a navigational identity, not water quality

A safe water mark indicates navigable water around it and does not mark a danger. It can provide a landfall reference, an approach reference or a mid-channel mark.

The Portuguese name águas limpas is the nautical family name, rather than an environmental assessment: it says nothing about pollution, transparency or drinking water. The English term safe water mark describes the navigation function, with the same international characteristics.

Stripe orientation matters. Red and white vertical stripes differ from the horizontal bands of cardinals or isolated danger.

The upper sphere is single and red, whereas isolated danger uses two black spheres. A spherical buoy body and a narrow body carrying a spherical topmark are two presentations of the same family; the diagram separates body and topmark to make that relationship visible.

Iso means equal light and dark durations in the cycle. Oc means occulting, with total light duration longer than darkness.

LFl is a long flash lasting at least two seconds; in this family’s alternative it repeats every ten seconds. Mo(A) represents Morse A: dot followed by dash.

These are alternatives, not four signals every buoy emits in sequence. Neither Iso/Oc period nor all Morse phase timings can be inferred from the family name.

R1001 includes uses such as landfall, port or estuary approach, channel entrance and mid-channel, including indication of the best passage under bridges. The particular function belongs to its published context: the same identity does not put every mark on a channel’s geometric centreline.

The family announces neither a depth in metres nor keel clearance for every boat. Navigable water is information related to the place, rather than an individual certificate for all draughts.

Compare two descriptions using the same white colour: Mo(A)W has one short and one long element, whereas Fl(2)W has a group of two flashes. The first is a safe-water alternative; the second belongs to isolated danger.

Counting light appearances is insufficient because duration and organisation also matter. The purpose is to relate notation to the described sequence, rather than memorise unexplained letters.

This comparison connects the family lesson with the more general lesson on rhythm and period.