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.

Hold it for one second. The button fills from the bottom while it is held, and the alarm is raised when the second is up. Lift your finger before that and nothing is raised: the button opens the MAN OVERBOARD panel instead. There is no confirmation dialog — the one-second hold is the confirmation. A sleeve, a knee or a sheet across the screen touches the button for a fraction of a second, which is not long enough to raise anything. The position is taken at the moment the hold completes.

What the hold 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 alarm settings say;
• 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;
• sends it to the boat’s instruments on the outputs that are switched on: Expedition’s MOB mark, and the Exocet’s MOB start — see Man overboard to the instruments below;
• records it for the debrief: a record the tablet sends to the platform with its other race records.

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. While a mark is down, a second hold does not move it: the position somebody went in at is the first one, and the button only opens the panel.

The MAN OVERBOARD panel

A tap on the MOB button opens it. From the top: the alarm that is raised, or No alarm raised; the MOB, SART and EPIRB beacons heard on AIS, or No beacons heard; an INSTRUMENTS list while the boat’s instruments report a man overboard; a Man overboard on this boat control marked HOLD 1 s; and CLEAR while this tablet holds a mark. The control in the panel raises the alarm on the same one-second hold as the button on the strip, and a tap on it does nothing. Once a mark is down it reads MARK DOWN. Closing the panel changes nothing: it does not clear a mark, silence an alert or acknowledge an alarm.

Each INSTRUMENTS row names the instrument — EXOCET, EMITTER 000FF, BUTTON or INSTRUMENTS — with the distance and bearing when a position was reported, and a note where there is one: what triggered the Exocet’s MOB, RAISED FROM THIS TABLET, or TEST.

Every man overboard is recorded for the debrief. The tablet sends one record when an alarm starts and one when it ends, each with the time, what raised it — the MOB button on this tablet, the boat’s instruments, or an AIS beacon with its MMSI — and the position when there was one. A test transmission is not recorded, and neither is the tablet’s own man overboard coming back from the instruments (below). The records go to the platform the same way as the start gun and the line pings, so a tablet with no connection sends them when it reconnects, and the debrief player shows them on the timeline and the track map. The platform notifies nobody: it keeps a record for the debrief and is not a rescue or alerting service. The mark itself survives the app being killed, the tablet restarting, and the rest of the day.

Man overboard from the boat’s instruments

Race Control also raises the MOB alarm when the boat’s own instruments report a man overboard. It reads two sources: a $--MOB sentence on an NMEA 0183 feed, and the MOB box of an Exocet the tablet reads. The alarm is raised whether or not the MOB rule is armed: the armed setting applies to AIS beacons, which can belong to another boat, and not to the boat’s own MOB system.

A $--MOB sentence from the boat’s MOB system, a plotter, or a gateway that converts NMEA 2000 to NMEA 0183 is read by its status letter:

Each emitter id is a separate alarm. A sentence that cannot be read is counted and ignored.

On an Exocet, route the MOB box’s MOBStatus, EventIsTriggered, MOBLatitude, MOBLongitude and MOBTriggerSource outputs to the Exocet_out box that sends to the tablet. The MOB box’s own name is the group, and any name works.

• MOBStatus 7, or EventIsTriggered true, raises the MOB alarm with the reported position. The alarm ends when the Exocet reports that its MOB is no longer triggered. EventIsTriggered is read only beside one of the other four outputs, or from a box named MOB.
• The position is read in the unit the Exocet declares for it: degrees, or radians converted to degrees. Any other unit, or a position of exactly 0° 0°, is no position; the alarm still sounds.
• A graph that routes none of these outputs raises nothing and reports no error. The field map is not involved: mapping these outputs to channels does not change the alarm.

An alarm from the instruments turns the MOB button solid. When it is the man overboard the screen describes (the order is below), the banner names its source — for example MOB · EXOCET · 320 m · 245°T, or MOB · EMITTER 000FF when there is no position to measure. The SOURCE line names the instrument: for an Exocet, what triggered its MOB — the Exocet, navigation software, NMEA 2000, an AIS beacon or the external button; for a $--MOB, the emitter id and, where the sentence says so, a manual button, a low battery, or a position estimated by the vessel rather than reported by the emitter. TIME and TIME SINCE count from when the tablet raised the alarm. With no reported position, POSITION reads No position reported and there is no bearing or distance.

• CLEAR clears only the tablet’s own mark. An alarm from the instruments ends when the instrument reports that it has ended.
• If the instrument stops reporting, the alarm stays for one hour after its last report. A dropped feed is not a sign that the person is back aboard.
• Selecting a different telemetry source clears every man overboard the instruments on the previous source reported.

More than one man overboard

The banner, the Man overboard alarm window — which opens over the screen when the man overboard it describes starts — and the top of the panel describe one man overboard at a time, in this order: the mark held on this tablet, then a man overboard from the boat’s instruments, then an AIS beacon. The panel lists every one of them.

Each one has its own cross on both charts. The tablet’s own mark is a red ring with an upright cross, labelled with the time since it was raised: MOB 1:35. A man overboard from the instruments is a red ring with a diagonal cross, labelled with its source and the time since: MOB EXOCET 1:35. It is drawn only when the instrument reported a position, and under the tablet’s own mark where the two overlap. AIS beacons are drawn as targets and labelled at any zoom.

Man overboard to the instruments

The hold sends the tablet’s own man overboard on two outputs, once, at the end of the hold. A second hold on a mark already down sends nothing, and nothing an instrument reports is ever sent back out.

• Expedition’s protocol. With the EXOCET card’s OUTPUT for slots over Expedition’s protocol switched on, the hold sends Expedition’s MOB mark, #M,MOB,MOB,lat,lon,X, with the mark’s position. It is sent whether or not DRIVE THE FAR END’S TIMER AND LINE is on. A hold with no position sends nothing on this link.
• The Exocet’s own protocol. The EXOCET card’s OUTPUT for this app’s own numbers carries two boolean variables beside them, NordicStars_StartMob and NordicStars_StopMob, both low at rest and both repeated in every value frame. In the Exocet’s graph, wire NordicStars_StartMob to the MOB box’s StartMOB input and NordicStars_StopMob to its StopMOB input.

The hold sends NordicStars_StartMob: high for three seconds, then low. It is not sent when the Exocet’s MOB box already reports a man overboard. A command that does not reach a streaming Exocet link within its three seconds is not sent later. Two commands never overlap: the second waits for the first to end, and then for one second low.

CONFIRM CLEAR sends NordicStars_StopMob, and only for a man overboard this tablet started on the Exocet. The tablet counts it as started from here when:

• it receives the MOB box’s outputs, the Exocet still reports its man overboard, and that man overboard is the one recognised as the tablet’s own coming back (below); or
• it does not receive those outputs, and NordicStars_StartMob for this mark reached the Exocet link while the link was streaming.

A man overboard started on the Exocet by anything else — its own control, navigation software, NMEA 2000, an AIS beacon or another button — is never stopped from the tablet. After the tablet restarts it no longer knows what it sent, so it sends no NordicStars_StopMob: stop that man overboard on the Exocet. A NordicStars_StartMob still waiting to be sent when you clear is withdrawn.

The tablet’s own man overboard coming back

While the tablet’s mark is down, a report that first appears within ten seconds of the tablet sending on a path that can produce it is the same man overboard, not a second one:

• an Exocet MOB triggered by its external start input, or stating no trigger, after NordicStars_StartMob reached the Exocet;
• an Exocet MOB triggered by navigation software, or stating no trigger, after #M,MOB was sent;
• a $--MOB with no emitter id after #M,MOB was sent.

That report raises no second alarm, opens no alarm window, draws no second cross and sends nothing back. The panel lists it as RAISED FROM THIS TABLET, for as long as the instrument reports it and also after CONFIRM CLEAR. A report from any other trigger, or one that arrives after the tablet restarts, is an alarm of its own.

What the tablet does not read

• A FaRo feed carries no man-overboard report.
• Race Control reads no NMEA 2000 MOB message itself. An NMEA 2000 man overboard reaches the tablet only as a $--MOB from a gateway that converts it to NMEA 0183, or through the Exocet’s MOB box.

This page describes what Race Control reads and sends. It does not describe what Expedition, the Exocet or a plotter does with what it receives.

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. A man overboard reported by the live instruments is not used in a replay either.

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.