For more than 20 years we’ve been investigating SONAR – a form of underwater radar – as a means of helping to navigate in tricky areas. We’ve installed units for several of our clients, but for one reason or another they have not done the intended job. However, these were all installed a few years ago, and a lot of progress has been made on the electronics front in the ensuing period.

We like to explore remote anchorages, where charts are of limited or no value. This means feeling our way in with the depth sounder, slowly looking for underwater obstructions. We’ve got a small sounder/GPS plotter in the dinghy which is intended for survey work. But when it is cold, raining, and windy, neither of us wants to be out in the dink! Hence our renewed interest in SONAR.

Today there are many approaches to solving this problem. These range from somewhat smarter depth sounders – which are often called "fish finders"; to simple forward-looking SONARs; to full-on controllable SONARs used by commercial fishermen and hard-core sportfishing types. In this first part of three sections we are going to looking at several of the simpler products, with costs in the US$1500 to $2500 range.

Echo Pilot is one of the pioneers in the forward-looking SONAR field. They have a relatively simple system that is in effect a SONAR transducer looking forward only, in the vertical plane. This reduces the cost and keeps the overall system quite simple.

Above is a screen shot from the Echo Pilot web site ( http://www.echopilot.com ). This is looking ahead 60 meters (200 feet). This is a gradually upward sloping bottom according to their literature.

Here is another example, in this case looking at a seawall that is directly ahead. Although the SONAR image appears as a thin line, this actually represents a 14-degree-wide beam.

The Echo Pilot manual does a good job of explaining the principles of forward-looking SONAR. The transducer emits a ping which travels forward until it hits something and bounces back. The further forward the ping travels, the more acute the angle at which it hits the sea bed. At a certain point ahead, the angle will be so acute that the ping will not be reflected back to the transducer, and then the sea bed information is lost. In a muddy, flat bottom, like a river estuary, Echo Pilot will show the bottom at three to five times the water depth ahead. With a bottom that is sloping upwards ahead, the operational distance is longer, as it is with hard, rocky bottoms where the distance can be nine times the water depth. Vertical walls, like coral heads or breakwaters, can be seen at the longest distances.

We asked Echo Pilot to give us some more details on range and capability.

"It depends on the water quality, temperature etc. – silty rivers, turbulence and sewage works are bad news, but cold clear water is good. Please use the manual if it helps. In deep water you will usually get 150 to 180 meters – again water quality matters and we notice when we sea trial (all of them go to sea) that one week it is a good sonar day and the next time it can be less good and it is hard to pinpoint the reasons for this."

And about transducers:

"Transducers are more vulnerable to damage because they protrude through the hull, and impact damage is the most common fault. They do not have a limited life span otherwise, there is nothing within them that fails at a specific time. I think propagation of ultrasonic signals in water is quite a complex business!"

And Echo Pilot claims to have a real-time image as opposed to their competitors, which take time to build their image.

John Harries, who spends a lot of time cruising the high latitudes on his Morgan’s Cloud ( www.morganscloud.com ), has used one of the Echo Pilots and likes it a lot. Here are his responses to a few questions we posed:

"We have their first model, the FLS II. See our web site for some of the problems we had with it. The bottom line is that we would not now be without it. It does take some practice to interpret but it has also kept us off the rocks for many miles, much of them in uncharted waters. It really takes much of the sting out of cruising places like the east coast of Greenland.

"The human interface on the FLS II is terrible, but they have fixed many of the problems we and others bitched about on the FLS II on their new models."

Our interest is primarily for checking anchorages and narrow passages, looking for obstructions, so we asked John about this.

"It will work well for this. We scan most anchorages for obstructions with the FLS II prior to anchoring and frequently explore totally uncharted narrow passages using it. You do need to keep in mind that although it will look out 200 meters in deep water, the forward distance that you can practically resolve an obstruction at is limited to about three to five times the depth of the water. Therefore in water say 5 meters deep you are only really looking out 25 meters, at best. So in shallow water you need to move very slowly with someone glued to the FLS screen juggling the range to make sure you will be able to stop before hitting something."

If you do use one of these, what do you do in a tight anchorage for checking what is around you – and is there a problem crossing your own wake at dead slow speeds?

"Yes, you will get interference from your own wake, although we have never found it to be a big problem. A larger issue is that if you reverse hard your prop wash will shut the system down completely until it clears. I suspect this would happen with almost any system."

In our case, we would be mounting this on center, 16′ aft of the bow (in our forward watertight compartment).We asked John where he had mounted the transducer, and was it vulnerable to ice and debris?

Had he used it in the tropics with coral?

"No, but it resolves boulders on the bottom of arctic anchorages well, so coral heads should be no problem. Vertical face obstructions like boulders and coral heads are what it does best."

When asked if he could tell anything about bottom composition, i.e. mud vs. rock or kelp, John’s reply was "No".

John is a highly experienced real-world user, and we value his input.

Interphase ( www.interphase-tech.com ) is another vendor with forward-looking SONAR. They have several different units, all of which have a 12-degree beam width, and are able to see ahead five times the water depth. One of their models offers both a horizontal and vertical slice of the bottom. This has a 90-degree horizontal width. Adding a second transducer gives you a 180-degree horizontal scan (the transducers are mounted opposite each other, with a minimum separation of 18" (450mm).

The next two images were copied from the Interphase web site. Above is a photo looking aft at a narrow passage.

Here is the SONAR image of this same passage. We’ve been hoping to see one of these units in the real world, or talk to an experienced user, but so far this has not happened.

We did get some feedback from Brent Woodword, the electronics guru at Yachtmaster’s Northwest in the Seattle area. Brent has a customer with one of these units and he said it is hard to make sense of. His very experienced customer does not use it for navigation.

Humminbird is yet another participant in this field ( www.humminbird.com ). They offer several models which employ multiple narrow beams for sending out and receiving their SONAR energy. Humminbird offers a variety of models designed primarily for fishing, none of which are forward-looking. To map the images below, you have to slide past the objects on the bottom. If you are looking for a good place to drop the hook, this might be a good answer.

Above is a representation from the Humminbird web site of their 3D SONAR output.

And this is their side-imaging SONAR.

When we discussed our navigation application with Mark Gibson at Humminbird, he suggested a product they have called "Smart Cast." This is a remote transducer which sends a wireless signal back to the boat, and could be used from the dinghy at distances of up to 150 feet (45m). Or we might just use the side-scanning model direct in the dinghy, as a tool for positioning GPS waypoints over obstructions or good anchoring points.

Click "Next" for Part Two of this look at SONAR, wherein we’ll discuss a concept from Furuno that takes SONAR navigation to a whole new level of capability.