In many situations you can safely navigate along coastal areas using only a chart and your boat's depthfinder. But why in this era of GPS chartplotters and navigation apps for smartphones and tablets would you want to navigate without them? There are several reasons to be able to use this skill:
- An electrical failure can suddenly disable a chartplotter
- The battery of a smartphone or tablet may go dead, or the unit become wet and dysfunctional
- You may have just turned on the unit and it hasn't found the GPS location yet
- You may simply prefer to use paper charts
- You are alone and cannot do traditional chart navigation with parallel rules and dividers to determine your exact position
With an even depth contour along shore, such as shown in the chart image above, you can track your position using your depth readings without even having to take a compass bearing to some feature on shore. Imagine you are approximately at the position indicated on this chart segment by the red flag, and you are sailing WSW along this coast. You don't need to guess your distance from the island or to know the precise bearing to any shore feature as long as you stay in deep enough water. For this segment of the chart, for example, you will be safe as long as you stay in water over 10 feet deep or so.
The next page shows a trickier situation where the depthfinder alone cannot keep you safe and a visual bearing needs to be included.