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?

Answer:

  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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Meet the Author

John

John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 12 years, as co-editor/publisher of AAC.

4 comments … add one
  • Piotr Berlinski Oct 28, 2010, 7:15 pm

    Hi John,
    My friend is thinking about installing a forward scan sonar on boat for high latitude sailing. But there are rumours about its susceptibility to thermocline. Can you write something about it? Is it really a problem? How deep is a thermocline on arctic waters?

    Peter

  • John Nov 3, 2010, 8:17 am

    Hi Piotr,

    We have never had any bad problems with the forward scan being blanked out by thermoclines in the north. In fact the worst place we have seen for this is in Maine in the large bays. There, sometimes, the sonar will not read more than 20 meters. I suspect the problem is the large rivers running into these bays that set up layers of different salinity. Our own home area of Mahone Bay in Nova Scotia is bad for this too.

  • Tod Apr 18, 2014, 4:04 am

    Hi John,

    I know this post is very old, but I’m quite interested in something you wrote:
    “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. ”

    My questions are: why would gyro stabilization be needed? Just to ensure that the transducer is shooting perfectly level when the boat is heeled?
    Have you heard anything more since then about EchoPilot (or anyone else) talking about a product that would detect a container?

    -Tod

    • John Apr 18, 2014, 8:12 am

      Hi Tod,

      Yes, I think that was the reason for gyro stabilization, although I’m no expert. And no I have not heard of any further developments on that front.

      I guess I would not worry about it much because I think the actual risk of hitting a floating container at sea is very small.

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