Seamanship

BYE BYE BIRDIE # 2 Bathymetric Navigation

- Robert Reeder

Bathymetry, as per Merriam-Webster, is defined as, “The measurement of water depth at various places in a body of water; also: the information derived from such measurements.”

BATYMETRIC NAVIGATION: LINE OF SOUNDING METHOD

Even vessels with the most austere complement of navigation electronics will usually have a depth sounder. Rarely, however, do we recognize the full utility of this unassuming tool, especially for navigation in the absence of GPS.

I first learned the art of navigation aboard U.S. Navy submarines, years before GPS was available to military or civilian mariners (no, really, it wasn’t that long ago). The technique we’ll be working with here was part of our routine navigation, just one of the ways in which submariners use depth sounders. It’s an awfully handy tool and works equally well for small cruising powerboats or sailboats. This technique demands a committed navigator, at least for as long as it takes to derive a fix. It is simply not possible to maintain a proper helm and lookout while navigating with a depth sounder. Later in this series we will explore satellite-free navigation techniques that can be used safely while single-handing a small vessel, but for now, I am assuming that navigation is the primary job of one of your crew.

Most commercially available depth sounders provide a speed through the water reference that is independent of GPS speed over ground. If your depth sounder does not provide this data, you must still know your speed through the water by some means (various methods for obtaining this will be the subject of a future article).

In addition to a depth sounder and standard plotting tools, such as a paper chart with printed depth contours, you will need something akin to tracing paper. For this exercise I used a piece of white tissue wrapping paper, just to demonstrate that really any semi-transparent paper will suffice.