When we started looking at the electronics specifications for Wind Horse, we considered adding a depth finder with a bottom echo display, if it would also help us tell the difference in bottom conditions. Our reasoning was that it was worth having this gear in areas where it was hard to find a nice spot in which the anchor could dig in – if it really worked. So we checked with several friends that had powerful depth sounders and they all told us the same thing, that they could not consistently tell one type of bottom from another.

As a result we stayed with a simple digital read out – no moving display. This is the same approach as we’ve used for the last 20+ years.

One of the benefits of the Furuno CH 270 sonar we’ve just installed is that when the transponder is aimed down (vertical) it works as a very high-end depth sounder. During recent testing, with Furuno sonar guru Steve Bradburn aboard, we asked about interpreting bottom condition from the visual echo trace.

Steve explained to us about second and third echoes. Essentially, when the depth sounder emits its pulse, depending on bottom condition, more or less of that energy will be bounced from the bottom back to the sea surface, and then off the sea surface back to the sea bottom, and then back up again. The harder the sea bottom, the more of these bounces occur and the stronger the second and third echoes show up on the screen. Conversely, softer bottoms, or those covered in weed or grass, reflect less energy and have less total and/or weaker traces.

The image above is in roughly 50 feet (15m) of water, off Ventura Harbor (Southern California). Because each successive bounce adds additional time to signals out and return, they show up as deeper. In the image above you can see three distinct lines. The upper, thickest, is the true bottom. The next, at roughly 100′ (30m) is the second echo. The lowest, at 150′ (45m) is the third echo. To see these, echo rejection must be turned off and sufficient depth range must be showing on the screen. The three of these are an indicator of a hard bottom.

Now let’s look a little further along.

In the image above, notice how the lowest (third) echo is thinner, and in some areas not present, compared to the first image we just showed you. This is an indication of a softer bottom.

Here is another area of even weaker third echo. So, this is probably a wider area of soft bottom, or possibly an area covered with grass.

Notice how all three of the traces on each of the images show a waviness? This could be an indication of the bottom contour – in other words, it is not flat. Or, it could be the rise and fall of the boat in the waves, which were running about 4 feet (1.2m) when we snapped these screen shots.

Water temperature, salinity, and debris (dirt and various forms of sea life) all affect how these echoes play out. Steve Bradburn suggested in each cruising region going from an area of known bottom – perhaps where there is a hard coral pan or granite – to another with good anchoring conditions, perhaps mud or sand – and then noting how they show up on the depth finder image.

Bottom slope also affects second and third echo returns. Flat bottoms are best as they bounce the image straight back to your transducer. The more slope to the bottom, the less energy will be reflected. So, in a rapidly shoaling sea bed, you would expect to see a lot less of these additional echoes.

Using these techniques doesn’t work 100% of the time, Bradburn told us, but they will give you a pretty good idea 80% of the time. We’ll find out a lot more in the near future.

Based on what we’ve learned here, if we were doing this over again, we’d probably fit a higher end depth finder. If you are thinking along these lines, and have one of the standard sized through hulls, it is often possible to replace an existing low-end unit with something that has a bit more sophistication and power, without adding a hole in the boat. Simply find a transducer which will fit into your throughhull fitting. In our case, this is a standard 2″ (50mm) B and G fitting, made for them by Airmar. As Airmar makes the majority of the transducers for the depth finder industry, the odds are they will have something which will fit your throughhull.