The Offshore Voyaging Reference Site

Q&A: Forward Scan Sonar

Question: We are looking at sonar options for use in areas with poor charts and low visibility. Our interest is primarily for checking anchorages and narrow passages, looking for obstructions. We have a small plotter/sounder in the dinghy, but nobody wants to go out in the dink when it is cold and rainy (and warm and dry on the big boat!).

I have heard a rumour that you guys have an EchoPilot unit:

  1. How do you like it and how does it work?
  2. What do you do in a tight anchorage for checking what is around you?
  3. Is there a problem crossing your own wake at dead slow speeds?
  4. Where have you mounted the transducer and is it vulnerable to ice and debris? In our case, it would be on center, 16′ aft of the bow (in our forward watertight compartment) or centered fore and aft, offset to one side about 5′ and alongside one of the stabilizer fins (which reside in sealed coffer dams).
  5. Have you used it in the tropics with coral? I assume it would work well with the abrupt shape of the coral heads.
  6. Can you tell anything about bottom composition, i.e. mud vs. rock or kelp?
  7. Is it any good in ice?
  8. What do you do for a depth sounder? Use this, or something more specialized?


  1. We have EchoPilot’s first model, the FLS II.
  2. 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 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.
  3. 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 the wash clears. I suspect this would happen with almost any system.
  4. Ours is further aft than yours, almost amidships (an EchoPilot recommendation, to keep it away from bow wave turbulence), and next to the keel. We have had no problems, even in ice. I’m no expert, but I would worry about having it on the center line without keel protection. I would guess that the aft position next to the fins might work better from the protection point of view. The only thing that I would worry about here would be turbulence from the fins, but you would know better than I what the situation is there. Our transducer is quite close to the keel root and we have no turbulence problems. You may want to talk to EchoPilot about the forward position and see what they think. The good thing is that the transducers are built to sheer off at the hull if hit, so there is no potential damage problem, and a spare is easy enough to fit. You should carry a spare anyway as the transducers tend to lose sensitivity over time. They seem to last us about two years.
  5. No, we haven’t used it in the tropics 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.
  6. No, it does not tell anything about bottom composition.
  7. No, it is of no use in ice. Some years ago EchoPilot announced a high end unit with gyro stabilization capable of detecting a container at sea, but quietly withdrew it after several years of development. I spoke to the owner of EchoPilot and he said it was just too difficult to get stable and would have been hugely expensive. Bottom line, this technology is hard to do well. I think I’m right in saying that most other products are based on fish finder technology and that EchoPilot are the only ones to start with a blank sheet of paper and design a forward scan sonar. Bottom line, they are the product to beat.
  8. We also have a depth sounder since the FLS II numbers are hard to read, it has no surface offset function (fixed, on later models) and we don’t trust it to read accurately down to a few centimeters as we sometimes need to do when calculating whether we will bottom out on a low tide. The challenge is that you need to get a depth sounder that does not operate at 200 KHz, or it will interfere with the EchoPilot and vice versa. We solved this by getting a smart depth sensor from Airmar that works on 175 KHz, designed for fishing boats with fish finders. Also, B&G sounders operate at 175 KHz.
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