Anchoring Electronics

Anchoring is the one activity where a cruising boat uses its navigation system at maximum precision and minimum alertness: you need metre-scale awareness of position, and you need it while asleep.

That combination is why anchoring deserves deliberate electronics setup rather than habit.

#Choosing where to drop

The chart tells you what was surveyed. The sounder tells you what is there now.

Before committing:

• Read the chart critically. Anchorage symbols, charted depths, the nature of the seabed where shown, and the Zone of Confidence for the area — see marine charts Australia. A popular anchorage in a poorly surveyed bay is common in remote Australia.
• Compute the tide. You need depth at low water for grounding risk and depth at high water for scope. The tides guide covers the arithmetic.
• Survey the spot with the sounder. Motor slowly across the intended swing circle watching depth. This takes five minutes and reveals slope, isolated shallows and the actual depth range you will swing through.
• Consider forward sonar where fitted, for isolated coral heads or rock in unfamiliar water. It is an extra input, not a guarantee — see marine sonar.
• Check swinging room against other boats, moorings, the shore and the depth contour, for a 360° wind shift
• Check the forecast direction for the whole time you intend to stay, not just now. An anchorage tenable in a southerly can be untenable when the wind backs at 0300.

#Setting the anchor alarm properly

This is the single most misconfigured feature on recreational boats.

The geometry you actually need:

          anchor
            o
            |\
            | \   swing radius = scope laid + bow-to-stern
            |  \
            |   \
            o----o  boat at full scope

The alarm radius must be measured from the anchor, and must cover:

• the horizontal distance from anchor to bow at full scope
• plus the boat's own length, because the stern swings further
• plus a margin for GNSS position scatter
• plus any allowance for the anchor's position being imperfectly known

The common failure is setting the alarm from the boat's position at the moment of setting, which puts the circle centre roughly where the boat is rather than where the anchor is. The boat then swings out of a circle it was always going to leave, and the crew concludes anchor alarms are useless and stops using them.

Better technique, available on most modern systems:

1. mark the anchor position at the moment it goes to the bottom
2. drop back and set
3. set the alarm centred on the marked anchor position
4. set the radius to scope plus boat length plus margin
5. check the alarm actually sounds — many are configured silently

Where the system does not allow an offset centre, drop the anchor, note the position, then set the alarm after you have finished backing down, accepting a slightly larger radius.

Also set a depth alarm — a shallow alarm at your low-water margin is the alarm that matters if the anchor drags into shallower water or the tide falls further than predicted. A deep alarm catches dragging into deeper water where the scope ratio collapses.

#Heading at anchor

At near-zero speed, GNSS-derived course over ground becomes unstable, because tiny position changes produce wildly varying direction calculations. That is not a fault; it is arithmetic.

A real heading source — a solid-state AHRS or a GNSS compass — gives stable orientation at zero speed, which matters at anchor for:

• knowing which way the boat is actually lying
• interpreting the anchor-watch track
• radar orientation if you are keeping a radar watch in a crowded anchorage

See heading sensors.

#Windlass and chain data

A chain counter integrated into the network can display deployed chain length on the chartplotter, which makes scope calculation quick and repeatable.

Two cautions:

• It must be calibrated, and it must be re-zeroed when the chain is fully retrieved. A counter that has drifted is worse than none, because it is trusted.
• Marked chain is the authoritative cross-check. Paint or cable ties at fixed intervals cost nothing and cannot fail electrically.

Windlass current draw also belongs in the electrical power picture: anchor retrieval is often the largest short-duration DC load on a cruising boat, and it frequently happens with the engine running for exactly that reason.

#Cameras

A bow camera showing the anchor and chain is more useful than it sounds, particularly on:

• large catamarans, where the helm-to-bow sightline is poor
• motor yachts with a high helm and a long foredeck
• shorthanded boats where the skipper is at the helm and nobody is on the bow

It shows chain direction and tension, whether the anchor is fouled on retrieval, and whether the snubber is loaded. See catamaran navigation for the wider camera argument on wide-beam boats.

#Keeping an actual anchor watch

Electronics support an anchor watch; they do not constitute one.

A workable arrangement in settled conditions:

• chartplotter anchor alarm set correctly, audible from the sleeping cabin
• depth shallow-alarm set at the low-water margin
• a phone anchor alarm as an independent second layer, on charge
• the anchor-watch track left displayed, so the morning check shows the actual swing pattern

In deteriorating conditions, add:

• a physical watch rotation
• transit bearings on shore lights or features, checked visually — an independent, non-electronic confirmation
• the engine ready to start
• a plan for where you will go if you have to leave, decided before you need it
• the handheld VHF charged and to hand

The anchor-watch track is worth reviewing even on a quiet night. A track that shows a clean arc is reassuring. A track that shows the arc's centre migrating downwind is an anchor that has been slowly dragging, which is exactly the failure a single alarm radius can miss.

#Common setup errors

• Alarm radius set from the boat rather than the anchor
• Scope computed on arrival depth instead of high-water depth
• Depth offset misconfigured, so the shallow alarm is meaningless — see depth sounders and transducers.
• Alarm volume set too low to wake anyone
• Chain counter never calibrated or never re-zeroed
• Anchor alarm left running on the previous anchorage's position
• Phone app relied on alone, then the phone slept

#Bottom line

Mark the anchor, set the radius from the anchor, compute scope on high-water depth, set a shallow depth alarm, run a second independent alarm, and look at the track in the morning. None of that costs money and all of it works.