AIS, alarms and man overboard

1 Where AIS comes from

Race Control reads AIS off the same NMEA 0183 wire your instruments are on. If your AIS receiver or transponder puts its output on the boat’s bus — which nearly every one does — and your gateway forwards that bus to the tablet, the targets appear. There is nothing to switch on and nothing to configure.

If the tablet is on a FaRo feed rather than a 0183 gateway, there is no AIS: the FaRo’s protocol does not carry it. That is not a setting you have got wrong, and the app will not pretend otherwise — see “No AIS on this feed” below.

Your own transponder’s reports are read and never drawn as a target. They are useful all the same: they are the cheapest proof that your transponder is actually transmitting, and they show up in the sentence count on the diagnostics card.

2 The target list

Open it from SETTINGS → ALARMS → ALARMS → AIS TARGETS…, or by tapping the red band when a ship raises one. Every row reads: the name (or the MMSI, if the ship has not sent its name yet), the type of vessel, range and bearing from you, CPA and TCPA, what the ship is doing, and how long since its last report.

The order is by threat, not by distance. Anything inside your guard zone is at the top, then the smallest CPA, then the soonest. Distress beacons come above everything, whatever their geometry — a liferaft is not moving, so a list sorted by closing rate would bury the one row that means somebody is in the water.

CPA and TCPA, and what they assume

CPA is how close the two of you will pass; TCPA is how long until that happens. Both are computed from your position and course over ground and the ship’s, on the assumption that neither of you alters. That is the standard meaning of the numbers and every bridge in the world reads them that way — but it is worth saying out loud: a ship that is turning has a CPA that is about to change, which is what the rate-of-turn column is for.

A negative TCPA means the closest point has already happened and you are opening. It is printed as past rather than hidden, because “0.2 nm four minutes ago” and “0.2 nm in four minutes” are different sentences and only one of them is a reason to alter course.

Every CPA on the list is measured from your boat’s own position, taken from the instruments and never from the tablet’s GPS. If the boat’s position goes stale, every CPA becomes an em dash and the screen says why — a number measured from where you were a minute ago is worse than no number.

How long a target stays

A target leaves the list when it has stopped reporting for longer than its own kind of transmitter would ever go quiet:

On the chart

Targets are drawn on Navigation and Path ahead as small outlined triangles pointing the way the hull is pointing — heading when the station sends one, course over ground when it does not, and a ring when it sends neither, because a triangle would claim a direction nobody gave. Anything inside your guard zone fills in red. Names appear when you are zoomed in far enough for them to be readable; a guard-zone target and a beacon are labelled at any zoom, because the point of a label is to name the thing you are about to talk about on the radio.

A virtual aid to navigation — a wreck mark or a cardinal that exists only as a transmission, with nothing to see out of the window — is drawn as a diamond rather than a triangle, so it can never be mistaken for a hull.

Distress beacons

AIS-SART (in a liferaft), AIS-MOB (in a lifejacket) and AIS-EPIRB transmit on reserved identities that are never issued to a vessel, so Race Control knows what they are from the very first burst — before any name arrives, and often that first burst is all there is.

The row says which kind it is and whether it is ACTIVE or under TEST, in words rather than in a colour you would have to remember the meaning of. A device transmits both signals — a status field and a name — and where the two disagree we read it as ACTIVE. A test escalated to a live alarm costs you a look at a screen; a live man overboard demoted to a test costs the person.

A beacon under TEST is a red row and not an alarm. Monthly self-tests are routine within VHF range of a marina, and an app that sounded a critical alarm on every one of them would teach a crew that the man-overboard banner is usually nothing — which is the most expensive thing this feature could possibly do.

3 The seven alarms

SETTINGS → ALARMS → ALARMS lists every rule, armed or not, with its threshold and one line saying what it does. The set is closed on purpose: every rule is a fact the tablet can establish from your own instruments and your own plan, against a number you set. An alarm that could not fire on a tablet with no signal in the dark would not be an alarm.

The depth alarm, and the one thing to get right

The number you set is a sounder reading, not clearance under your keel. Race Control publishes depth exactly as your instrument measures it — from the transducer down — and deliberately does not apply the offset your gateway may be configured with, because that offset’s sign decides whether it means “below the waterline” or “below the keel”, and a delivery skipper can retype it between two regattas.

So the value the app proposes — your boat’s draught plus a metre and a half — is a starting point to check against what your own instrument reads alongside, not a calibration we can perform for you. Set it once, at the dock, against the number on the display. Nothing on this tablet navigates on it: the job of this rule is to wake somebody up.

Why XTE ships wide

A racing yacht is routinely half a mile off the rhumb line by choice. This rule is not about tactics; it is about a boat that is no longer steering the leg it is meant to steer, and about a channel narrow enough for 100 m to matter. Tighten it where the water is tight.

Arrival is once per waypoint

Sailing past a mark, being set back onto it by the tide and passing it again is one arrival, not three. The alarm clears when you advance the route, and coming back onto a waypoint you have already been told about does not raise it again.

The anchor watch

It ships disarmed, and not out of timidity: with no anchor point there is no circle to measure from, so an armed rule would be one that could never fire. DROP ANCHOR HERE takes your position now, sets the circle, and arms the rule — the button describes the setting, so it makes it. WEIGH ANCHOR takes the point away and leaves the rule armed with nothing to measure.

The circle defaults to 50 m, which covers the scope, the boat’s own length and a GPS aerial that is not at the bow. Tighter is available and will cry wolf on the first wind shift of the night.

The collision alarm

Two numbers: how close is too close, and how far ahead you want to know. 0.5 nm and 12 minutes is what it ships as. Half a mile is the standard limit on commercial installations for coastal work. Twelve minutes is four miles of run at a twenty-knot closing speed — which is what it takes to get a kite down and alter course under sail. Six minutes is the commercial default and it assumes an engine and a bow thruster.

Inside three minutes the alarm goes from a warning to critical: twelve minutes is time to plan a manoeuvre, and three is time to make one.

Feed lost

Thirty seconds of silence while the boat is live. The slowest thing a healthy instrument bus sends is once a second, so thirty seconds is thirty missed packets — wide enough to ride out a wifi roam, narrow enough that you are told before you have sailed a quarter of a mile on a frozen picture. It cannot fire at the dock, and it cannot fire before anything has ever arrived.

Acknowledging, and what it does not do

ACK silences; it never clears. A depth alarm you have acknowledged stays on the banner and in the settings room for as long as the water is shallow, because the water is still shallow. The only thing that takes an alarm off the screen is the condition ending.

Acknowledging is per occurrence. Silence the depth alarm in one gully and it will sound again in the next — a new occurrence is a new thing to know about. Two ships inside your guard zone are two alarms, and silencing one does not silence the other.

SOUND OFF stops the tablet making a noise and changes nothing else: every banner is exactly where it was. That is the night-watch setting — below decks off watch, a chime wakes four people who cannot help.

Sound, vibration and the notification shade

Alarms use their own Android notification channel, called Safety alarms, separate from the quiet one the live-boat notice uses. That means you can turn the safety channel up or down in the tablet’s own settings without silencing — or being pestered by — the ongoing “live on…” line. If you never granted Race Control permission to post notifications, everything still works: the banner is the primary surface and the shade is the second one.

4 Man overboard

The MOB button is red, and it is on the strip along the top of the screen. It is there and not on a tab because the strip is the only thing that is on screen everywhere — every tab, and over the settings room too. A button whose whole value is being findable without looking has to live in the one place that never goes away.

One press. There is no “are you sure”. Race Control asks before anything expensive, and this is the exception that rule exists for: a confirmation dialog costs the seconds between somebody going over the side and their position being recorded, spent by a person who is not looking at the screen.

What the press does, in order:

• records the boat’s position and the boat’s time — from the instruments, like a line ping, and written to disk before it returns;
• raises the alarm, whatever your settings say. There is no state of the world in which you want that press ignored;
• puts a full-width red banner across the top of every tab with the bearing back to the mark, the distance, the time since counting up, and the position in degrees and decimal minutes — the form you read onto a VHF;
• draws the mark on both charts, over everything else.

If there is no position at that moment the mark is still dropped, the clock still runs, and the banner says No position recorded rather than inventing one. A second press does not move the mark: the position somebody went in at is the first one.

The mark stays on the tablet. Nothing about it is sent to the platform: the event vocabulary the app is allowed to push has no kind for a man overboard, and we would rather record nothing than record it as something else and have it re-derive your races around it. The mark survives the app being killed, the tablet restarting, and the rest of the day.

5 Replaying a race

Alarms and AIS are computed while you replay a recorded race, and drawn just as they would have been — seeing where the depth alarm would have fired is a good reason to open a replay in the first place. Nothing rings. A race sailed across a bar in April replays with April’s soundings, and a tablet on the flight home is not going to sound a shallow-water alarm over an aeroplane. The banner says REPLAY so what is on screen can never be mistaken for now.

A replay carries no AIS at all: a recording holds the boat’s own telemetry and nothing about the ships that were around it.

6 When something looks wrong

SETTINGS → SYSTEM → DIAGNOSTICS has an AIS group once a single AIS sentence has ever been read, and an ALARMS group always. Between them they answer the questions support will ask:

• sentences and per second — is anything arriving at all? A sea with nothing on it and a gateway that has stopped forwarding AIS are the same empty list, and this is what tells them apart. The rate is a mean over the whole run rather than a one-second window, because AIS arrives in bursts and a live figure would read zero four times out of five on a perfectly healthy receiver.
• targets tracked — how many are in the table right now.
• refused — sentences that were AIS and could not be read. A few on a noisy wire is normal; a large and growing number is a wiring problem.
• types not read — AIS messages we do not model (base stations, binary broadcasts, safety texts). Counted rather than hidden, so “nothing useful on this wire” and “plenty on this wire and we want none of it” are distinguishable.
• alarms active and unacknowledged — what is going off, and what has not been dealt with.

The empty target list says which of the three silences you are in: No AIS on this feed when nothing on the wire has ever been an AIS sentence, No targets in range when they have, and a separate line when the boat has no position and there is therefore nowhere to measure a range from.

7 Where to go next

• Race Control on the boat — the tablet from the licence to the first gun, including the telemetry setup flow that has to work before any of this does.
• Race plans — the route the cross-track and arrival alarms are about.
• Weather — the other thing that has to be aboard before the dock lines come off.