How Helm works out the depth under your boat

A nautical chart shows you a sounding. It does not show you whether your boat clears it today. Helm closes that gap by reading the depth from the chart, adding the predicted tide where it can, and reporting the result against your own draft.

Where the number comes from

Source	What it contributes
NOAA ENC	The electronic navigational chart for the coast and the Great Lakes: depth areas and soundings. Authoritative, but a sounding on it can be decades old.
USACE Inland ENC	The Army Corps of Engineers chart for the inland rivers, with its depth areas.
NOAA CO-OPS tide prediction	The predicted tide from a station within 25 nautical miles, added to the charted depth. A chart datum number is a floor, not the current depth.

What the card tells you about the number

Two soundings that read the same can mean very different things. The depth card says which kind of figure you are looking at: a charted sounding, a depth band from the chart, or water the chart has no soundings for. A depth band is judged on its shallow end.

Where the tide number comes from, and why it never makes a route passable

The tide figure is the published NOAA CO-OPS prediction for the nearest tide station, and Helm names that station and how far away it is. With no signal, Helm keeps using the last prediction curve it downloaded for the station nearest the boat. That curve runs about a week ahead, and Helm marks it as an old reading once it is more than twelve hours old.

Routing is computed at chart datum, MLLW, so the tide figure never decides whether you can go. A tide that would carry you over a bar is reported to you as information for that hour, and the route plans only across water your boat clears at datum. The reason is the failure mode: a route that was only passable because the tide cooperated is a route that traps you when you are an hour late leaving.

What counts as enough water

Under your keel, Helm treats three feet as the floor for calling water clear. Anything below that reads as tight, because a foot or two of margin is smaller than the things that move the real number around on any given afternoon: the error band on a tide prediction, the trough of a passing wave, and the uncertainty CATZOC itself declares, the quality field an electronic navigational chart carries to record how a survey was done and how much its depths can be trusted.

If you set your own safety margin larger than three feet, yours wins. A skipper who enters a metre of margin sees tight water all the way up to a metre, which is the point of entering it.

Bridges use the same idea, upward

Tap a bridge or an overhead cable and Helm shows the charted vertical clearance, measured above mean high water. Fixed bridges and the down position of an opening bridge are both read, so a bascule you would have to wait for does not look like open sky. The check against the air draft you entered happens when Helm plans the route for you, in the coastal and Great Lakes packs that carry routing data.

When a bridge carries no clearance figure in the chart, Helm reports it as unknown rather than printing zero. A zero on a chart field is the absence of data, and rendering that as a hard stop would send you the long way around a bridge you could have cleared.

Your draft is part of the picture

Enter your draft once and Helm stops showing you abstract numbers. Wherever the chart carries depth areas, the water you cannot cross is shaded. In the coastal and Great Lakes packs that carry routing data, routes plan around it too, and the measure tool reports the shallowest point along any line you draw rather than the average.

The measure tool

Draw a line anywhere on the chart and Helm gives you distance and bearing, and in the coastal and Great Lakes packs that carry routing data, a depth profile along it — including where the shallowest point falls. Useful before you cut a corner or take an unfamiliar cut on a falling tide.

Common questions

Does this work on lakes?

On the Great Lakes, yes: they are on NOAA charts and have charted depth. The state lake packs show lakes by name and shoreline from the USGS National Hydrography Dataset, and Minnesota adds state contour lines drawn on its lakes.

Does it need a signal?

No. Once the chart pack for your area is downloaded, depth works offline like the rest of the app.

Does it replace a depth sounder?

No. A sounder tells you what is under the hull this second. Helm tells you what to expect ahead and how much to trust it.