Manual
How to sail with it
What every tab does, and what each number is worth. It also says where each number stops being trustworthy. Read it once ashore, not on the water.
Contents
Before you leave the dock
- Mount the phone upright, screen facing the helm. The heel calculation assumes the phone stands on end. Flat on its back it has nothing sensible to measure. Portrait or landscape both work, and heel stays positive to starboard either way.
- Grant the location permission. The app asks the first time you open a tab that needs it. Choose precise location, because a coarse fix is worthless for speed and course.
- Allow notifications if you want to record. The recorder runs as a foreground service, and Android makes it show a notification while it runs.
- Give the GPS a minute in the open. Speed, course and the track all wait for a real fix. Under a boom cover or below deck there is nothing to be had.
- Swing the compass once. Draw a figure of eight in the air with the phone before the start.
The screen stays on while the app is in front. A race instrument that blanks is no use. The battery drains faster than you expect, so bring a power bank for a long day.
DATA. Speed, COG, heading, heel
Three readouts, nothing else. Landscape puts them side by side, portrait stacks them. Each number keeps a fixed position, so a new digit or a minus sign never shifts the block or its neighbours.
SOG
Speed over ground in knots. It comes from the receiver's own speed measurement, never from the change in position. The last few fixes are smoothed and weighted by the accuracy the receiver reports for each, and very uncertain fixes are dropped.
Under it sits AVG, the running average over the last minute. That is the number to
compare between two beats. The instant value moves with every wave.
HEADING / COG
Tap the block to switch it. HEADING is where the bow points, from
the phone's compass. COG is your course over ground from the GPS, so it includes
leeway and current.
A short haptic tick confirms the switch. The choice survives a tab change.
COG shows -- when there is no course. Usually you are too slow for
the GPS to derive one. It never falls back to the magnetic heading, because two different
measurements under one label are worse than an honest gap.
HEEL
Heel angle from the accelerometer, positive to starboard. Fore-and-aft pitch does not affect it, so bow-down in a wave stays out of the number. Below it runs a ten-second trace on lighter smoothing, so you can see whether the boat is settling or still swinging.
TIMER. The countdown
A start countdown of 1 to 10 minutes, five unless you change it. START runs it,
PAUSE holds it, RESET puts it back to the length you set. It needs no
GPS fix and no setup of any kind, so it works from the moment you open the app.
Setting the length
Use − and + above the clock, in whole minutes. The app remembers the
length for next time. Once the countdown runs the two keys disappear and the length is fixed, so
a stray tap cannot change it in the last minute.
SYNC
If you started the countdown late, tap SYNC at the next official signal. It snaps
the clock down to the whole minute it is heading for, so 4:52 becomes 4:00. You realign with the
committee boat instead of carrying the error to the gun.
It only ever moves the clock forward.
The countdown is recorded
While a recording is running, the logbook file carries the countdown alongside every position, so the analysis page can separate the pre-start from what happened after the gun. It costs you nothing: start the countdown as you always would.
A sequence you pause leaves those seconds blank in the file, and one you reset and run again — a general recall — is recorded as a second sequence rather than overwriting the first.
What you hear
| At | Signal |
|---|---|
| 5:00, 4:00, 1:00 | Single beep on the mark |
| Last 20 seconds | A short pip every second |
| 0:10 | Double pip, so ten is recognisable without looking |
| 0:00 | A longer, clearly different tone. The gun |
The signals stay at 5, 4 and 1 minute whatever length you set, so a shorter countdown simply
starts below some of them: a 3-minute countdown gives its first beep at 1:00. Every press of
START answers with a beep and a flash, so you know it took.
Tones go out on the alarm stream, so they stay audible when the phone is silenced. Two seconds after the gun the app switches itself to the DATA tab. At that moment you want speed on screen.
The countdown keeps running when you leave the tab. Look at the chart mid-sequence and come back, and the clock is where it should be. It stops only when you stop it.
TRACK. Chart and recording
Your position and your sailed track, drawn live over a topographic chart, with a scale bar at the top left.
CHART ON / CHART OFF at the top right switches the chart itself. Off,
the track and your position are drawn in white on the app's own dark background, and the tab
loads nothing from the network. Everything on this screen works either way: the chart is there
for orientation, not for the recording.
One button starts both the track on the chart and the logbook file. The label follows what the
recorder is doing, so a recording that fails to start does not leave the button claiming
otherwise. While recording it is red and reads STOP.
The chart needs a data connection. Tiles are fetched from a map server and cached, so water you have already looked at will usually still draw when you lose signal. That is a side effect of caching. Do not plan a day around it.
Everything else works with no connection at all: the readouts, the timer and the recording.
The © OpenStreetMap · Thunderforest credit at the top of the chart is required by
the map data licences.
Sessions and the logbook file
SESSIONS lists every recording on the phone with its start time, duration,
distance, average and maximum speed. Share one through Android's normal share sheet, by mail,
cloud or cable. Or delete it there.
SHOW puts an earlier session back on the chart, framed as a whole, with its figures
in a panel above the buttons. LIVE returns to the live track, and so does starting
a new recording.
ANALYZE opens the session in the analysis page, inside
the app: tacks and gybes, the timeline, heel, the wind estimate and the polar, exactly as on
this site. It needs no network, and nothing is sent anywhere. CLOSE or the back
gesture returns to the list.
- The figures are computed exactly like the tiles on the analysis page, so the same file shows the same numbers in both places. Average and maximum speed are smoothed over five seconds, so a single GPS spike doesn't set the maximum.
SHOWandANALYZEare unavailable while a recording is running. During a race the phone shows the race.- The recording currently being written has no delete button, because the recorder still holds the file open.
- Deleting always asks first. Until a session has been shared it exists in exactly one place, and there is no undo.
What is in the file
Plain CSV, one line per second, with a header row. Nothing proprietary, so open it in a
spreadsheet or a script. The analysis page reads it directly and
turns it into tacks, legs and a polar, in the app with ANALYZE or here in the
browser.
timestamp,lat,lon,sog_kn,cog_deg,gps_accuracy_m,time_to_start_s
2026-08-23T10:00:00Z,54.420000,10.160000,5.31,183.4,4.1,120
2026-08-23T10:00:01Z,54.420021,10.160013,5.28,183.9,4.1,119| Column | Unit | Notes |
|---|---|---|
timestamp | ISO-8601 UTC | One row per second, truncated to whole seconds |
lat / lon | Decimal degrees | GPS receiver only, never a network position |
sog_kn | Knots | From the receiver; left empty rather than zero if unavailable |
cog_deg | Degrees true, 0–360 | From the receiver; empty when it carries no course |
heel_deg | Degrees, signed | Positive means starboard side down; empty whenever the phone is not upright in its mount |
gps_accuracy_m | Metres | So you can judge a leg afterwards instead of guessing |
time_to_start_s | Seconds, signed | Positive before the gun, negative after it, empty when no countdown is armed |
Two further columns are already fixed in the format and will be filled later:
heading_deg (bow direction from the compass) and compass_quality.
A reader that expects them gets empty values rather than an error. Older recordings stay
readable once the columns arrive.
Heel is the one column the recorder decides about itself. The angle only means anything if the phone is upright in its mount with the screen facing you; lying flat on a chart table it would read a tidy 0°, and a recording full of zeroes looks like a boat that sailed dead level. So the cell is left empty whenever the phone is not upright, and empty again if the sensor stops reporting — which it may do while the screen is off. Position, speed and course keep being recorded throughout.
Gaps are left as gaps. If the GPS drops out, no rows are written for that stretch. Interpolated points would look like real measurements.
Decimal points are always dots, never commas, whatever language the phone is set to.
What it does not do
Worth knowing before you rely on it:
- It is not a navigation system. The chart is there to show where you sailed. It has no depth data, no routing, no AIS and no collision warning, and it is not a substitute for an official chart or a proper lookout.
- It measures no wind. There is no anemometer in a phone, so there is no wind speed and no wind direction anywhere in the app.
- Speed is over ground, not through water. In a current those are different numbers, and the difference is the current.
- No polars, no target boat speed. The app knows nothing about how fast your boat should be going, only how fast it is going.
- The chart is online. Offline chart files are on the list, but they are not in the app today.
If something looks wrong
| Symptom | Likely cause |
|---|---|
| Speed and course stay blank | No GPS fix yet, or location permission was granted as “approximate”. Check the app's permission setting and give it open sky. |
COG shows -- |
Too slow for the receiver to derive a course. It comes back as you make way. |
| Heading is off by tens of degrees | Something magnetic near the phone: a speaker, a case with a magnet, a steel fitting. Move the mount and calibrate. |
| Heel reads high with the boat upright | The phone is not upright in its mount. It measures the phone's roll, and it can only be as level as the mount is. |
| The countdown drifted off the committee boat | Tap SYNC at the next signal. It snaps the clock to the whole minute it is heading for. |
| Chart is grey | No data connection, and these tiles have not been loaded before. The track still draws; CHART OFF gives a clean view of it. |
| Recording stopped by itself | Android stopped the service, usually through a battery optimisation setting. Exclude True Helm from battery optimisation for a long day. |
If something is not covered here, or behaves differently on your phone, write in. During testing that is exactly what is wanted.