Cell coverage ends before the water does. Helm is built so that the moment you lose signal is not the moment the app becomes a blank screen.
Coverage is divided into 161 downloadable packs. You take the water you actually run and leave the rest, so you are not waiting on a multi-gigabyte download for a country you will never cross.
| Feature | Offline |
|---|---|
| NOAA charts | Yes, from the downloaded pack |
| Depth against your draft | Yes |
| Route planning | Yes |
| Anchor alarm | Yes |
| Track recording and logbook | Yes |
| Tides | Yes, predictions are computed locally |
| Live weather and AIS | No — these need a connection by nature |
Downloads run in the background and resume if they are interrupted, so a pack that stalls at the dock picks up where it left off rather than starting over. Helm checks you have the disk space before it begins.
Chart packs are fetched through a system background transfer rather than by the app itself. That distinction matters at the ramp: you can put the phone in your pocket, switch to another app, or have iOS shut Helm down entirely to reclaim memory, and the transfer keeps going. When it finishes, the system wakes the app to file the pack away.
It is the difference between a download you have to babysit and one you can start and forget. Starting a large pack, locking the phone and driving to the water is a normal thing to do.
A pack is a single vector tile archive holding the chart for that area, together with the depth data behind it. Vector rather than pictures of a chart is why it stays sharp at any zoom and why the file is small enough to be worth keeping several. The tide predictions that pair with it are computed on the device from the station’s harmonic constants, which is why the water level still moves through the day with no signal at all.
Two of the rows above need a connection by their nature. Live weather is a forecast someone has to send you, and AIS is other boats reporting their position in real time. Neither can be packaged in advance. Everything that answers “where am I and what is under me” is already on the device.
Most of them are small. The middle of the catalogue sits around 6 MB, which is less than a couple of photographs, and the great majority are under 10 MB. The heaviest pack in the set is a little over 200 MB, and that is a dense coastal area carrying a lot of detail. All 161 together come to roughly 5 GB, so even a boater who downloaded everything would not fill a phone.
This is the practical reason to take more coverage than you think you need. A neighbouring pack usually costs you a few megabytes, and the trip where you wish you had it is the one where you took a detour you did not plan.
Of the 161 packs, 48 are inland lakes and another 16 are rivers and waterways. That split exists because lake and river boaters are the ones most often left with a blank screen: NOAA charts thin out fast away from the coast, and a lake with an unfamiliar bottom in a low water year is exactly where you want a depth number and a downloaded chart.
Download before you leave the dock. Once you are out of range there is nothing to fetch. Helm shows each pack’s size and checks the device has room before it starts.
Offshore is the obvious case. The less obvious one is inland: many lakes have no usable coverage a few hundred yards from the ramp, and that is exactly where an unfamiliar bottom and a low water year cause problems.
Most sit around 6 MB and the largest is a little over 200 MB. Helm shows the exact size before you download and checks that the device has room.
Yes, and Helm asks for confirmation first so you do not lose coverage by accident.
Yes. It runs on GPS, which does not need a connection.
Helm Boating