Cruising with forward-looking sonar

Practical: Electronics

John and Janet Read give the verdict on forward-looking sonar after putting it through its paces on a cruise to SE Asia.

Janet looked up from her book. “Looks like a rock coming up, John,” she said. I put my coffee down, reached for the autopilot and executed a neat swerve to avoid the pesky obstruction. Yes, life with forward-looking sonar (FLS) was good.

If the above describes your vision of cruising with this relatively new piece of technology, you are likely to be disappointed. While FLS is, in certain circumstances, a useful aid to navigation, it has limitations that restrict its value to specific situations.

How it works

The main limitation of FLS relates to the laws of physics (Newton's laws to be precise). The system works by sending a series of sound pulses (similar to a standard echo pilot signal) in an arc between forward and under the boat. The system then monitors the time taken and the direction of any returning echo to give a graphic The result of this is that the systems available to cruisers will only display the shape of the bottom ahead to a maximum distance of around four times the depth of the water for a relatively flat area depending on the nature of the bottom. With a rising bottom ahead this distance can improve somewhat. While advertisements will mention the ability to look ahead for 100m or more, in reality, this will only be possible in depths of 25m or so. In that depth of water, you would probably be moving confidently at cruising speed so the distance will be used up pretty fast, taking into account the time needed to identify an obstruction then stop or redirect your course. In the more realistic scenario of moving into an anchorage with 5-6m of water, the FLS will generally only display information ahead to a distance of less than 20m from the transducer.

Despite this limitation, the device is a useful aid to navigation in certain circumstances. For the past three years we have been cruising on the west coast of the Malay peninsular. Apart from delightful scenery the area offers a combination of cloudy water, coral reefs and is often poorly charted making navigation quite challenging. We use our FLS when exploring new areas or entering anchorages when the sun is low and obstructions are difficult to see. To gain a benefit from the system in these circumstances requires constant monitoring of the display and an appropriate speed.

Two modes
There are two modes of operation for FLS units, one provides a display of the area ahead with a vertical sweep and the other utilises a horizontal sweep. The first indicates the existence of forward obstructions; the second indicates the direction of any area of deeper water. Top end units offer a combination of both modes of operation.

We opted for an entry-level system providing the vertical sweep described above and it has been a useful if not vital piece of equipment. When approaching an area where we need to use the system we travel at minimum speed and monitor the display constantly. Even with this approach, there is limited time to take action when an obstruction is identified. The unit includes an alarm function which helps but this can give false alarms due to fish, floating debris and even waves which limit its usefulness

In order to “see” ahead the FLS needs to be able to peer forward of the hull, necessitating a through hull transducer which protrudes several inches below the hull. In order to be in clear water, this needs to be some distance back from the bow, further limiting the effective range. The transducer sends a signal back to a display unit mounted close to the steering position. The need to protrude below the hull makes the transducer vulnerable to damage. While motoring out of a river in southern Borneo, an area littered with floating logs, we managed to break the transducer off level with the hull. Extra vigilance is required in such circumstances. The power requirements of the system are modest, requiring little more than that required by a standard echo sounder.


As with any piece of equipment, a decision to purchase must take into account the availability of support. We purchased our unit in UK but the support has been less than satisfactory. After our encounter with the floating log in Borneo, our attempts to purchase a replacement transducer led to a frustrating and expensive process while dealing with the English manufacturer. In retrospect we would have been better off paying more and dealing with a supplier with a record of good customer support.

The verdict
Despite the limitations mentioned above, FLS has been useful to us in the particular circumstances where we currently operate. For someone sailing in well charted areas with clear water it would be of limited value apart from providing a back-up depthsounder.

Janet and John Read are long-term cruisers, currently in SE Asia on their Martzcraft 35, Sapphire 4.

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