With FPB

Depth Finders for Navigation

In the olden days, pre-SatNav and GPS, the depth finder was one of the most important navigation tools. By comparing depths to the chart you could often figure out where you were. They were also handy for watching depth trends, giving an early warning of a rise in the sea bottom.

On our own boats we’ve stayed with a simple digital depth finder – just a numeric read out. When we were thinking about the electronics for Wind Horse we considered going to one of the fancier “fish finder” style units, with a historic graphic display. But in the end, we opted to stay with a simple numeric output of the depth.

Fast forward two years and we’ve been learning to use our SONAR for navigation. One of its modes is looking straight down, the same way you would with a fish finder. In this mode the transducer works if it is retracted – which we like, as it is not vulnerable then to debris or kelp.

We’ve had the SONAR working in depth finding mode now all the way down the coast, and we’ve grown very comfortable with the extra safety factor this provides.

depthfinder for navigation

Here is a night time image in deep water. One of the cool things about this is the shape of the bottom. We’ve seen some amazingly steep profiles. These are typically on the charts, of course. But seeing depth contours on a chart and seeing them on the depth finder in real time are two different things. In the case above, there is an abrupt change from roughly 1700 feet to 1100 feet, and then back, in a very short distance.

Aside from the fun factor of watching the bottom go by, there is a big safety benefit from all of this. Even today, with GPS and electronic charting, people make expensive navigational mistakes. Maybe it is some detail on the chart that is missed. Or perhaps the computer freezes, the boat doesn’t move on the chart, and the person on watch is unaware of this.

What this graphical depth does is give you a navigation safety check. Take the left side of the image above. That’s an abrupt change in the sea bottom. It is enough to get us to take a quick look at the chart to confirm what is going on. The whole process takes maybe 30 seconds.

depth finder as navigation aid

As with any nav tool, there is some learning curve to interpreting this data – and we’re just starting to learn what it can do. Compare this image to the previous one. The settings are the same, and the depths are not that different. Yet there is a substantial difference in the bottom trace. This is the result of variations in sea water temperature, salinity, microorganism content and bottom condition. It will be a long time before we have a handle on these issues. What is important is that we can see the bottom trends now, without a big learning curve.

using SONAR depth finder to navigate

Here is another example of a depth transition that got our attention in a hurry. However, a quick perusal of the chart showed that this was as charted.

navigation with depthfinder

And by comparison, here is an image that has little cause for concern. That slight rise in the sea floor holds no risk factors.

This data represents what is under the transducer as you are passing over it. That is what is being shown to the right of all of these displays. The data then slides across the screen the the left, presenting the historic trend we’ve been discussing. What it does not do is give you a look ahead to see what is coming, forcing you to project into the future from current and historic data. This is where forward-looking SONAR comes into play.

In tighter navigational situations we deploy the transducer below the hull, so we have the option of looking forward. However, we were interested to see what sort of forward-looking data we could obtain with the transducer in its retracted position.

navigation with forward-looking SONAR

The image above shows three conditions. At the right the transducer is looking straight down. In the middle it is looking forward at a 75-degree angle (15 degrees off of vertical). And at the left it is tilted to 65 degrees. Bottom line is that the depth and bottom type data are slightly different, but the trend information would be about the same, except that with the 65-degree angle, it is slightly advanced over a straight down view. It is a small advantage, but we’ll take all we can get – and we are now using the depth finding feature at the 65-degree angle.

navigation chart errors

There is one other issue to consider. We’ve been referring to comparing the depth display to what’s charted. But there are lots of situations where charts are wrong. This type of data gives you another chance to catch that charting error, before it catches you, as shown in the photo above of a fishing boat that made the ultimate error.

As we mentioned in a pervious article, fish finders can also be used to discern bottom condition, by interpreting second and third echoes.

Bottom line, if we were outfitting a boat today we’d be sure to have a good depth finder with graphical display fitted.

Post Script: It is not necessary to have those large, clunky, high-drag external transducers. Airmar produces round, flush mounted units for most of the electronics suppliers. These will often retrofit into the B&G-style through hull fittings.

Posted by Steve Dashew  (February 26, 2007)

2 Responses to “Depth Finders for Navigation”

  1. bincy Says:

    please suggest me a look ahead sonar camera which will provide depth of the water logged area as well as down profile of that area lying 5 meters ahead of me.

    Steve Dashew Reply:

    We have been using the Furuno CH 270 Sonar.