In Part One of this series on SONAR we discussed equipment based on transducers with no moving parts, which cannot be controlled by the end user. Furuno’s gear takes a different approach. We’ll cover their equipment in this section, along with that of FAR Sounder.

Furuno ( www.furunousa.com ) arguably "owns" the high-end SONAR market. Their gear is standard on most of the commercial and sportfishing vessels that use SONAR for finding fish. Steve Bradburn is the Furuno expert on SONAR, and he’s been giving us a tutorial on how their SONARs work, what the specifications mean, and how this gear can be used for navigation.

A couple of basics on their gear are in order. To begin with, rather than a fixed beam as with the units we’ve been looking at, the Furuno SONAR has a transducer beam which is controlled in both the horizontal and vertical planes. This means you can look at a full horizontal circle around the boat, or take a vertical slice at any angle from straight off the bow to astern or abeam.

The transducer, shown above in an extended position below the hull, is moved by a pair of motors – one for vertical motion and the other for horizontal. There are knobs on the SONAR panel which allow you to direct the transducer where it will do the best job. Some situations call for changing the horizontal and vertical angles as the situation develops. Their transducers output beams between 4 degrees and 14 degrees in width, depending on model.

For navigation you want a narrow beam width, which shows the most detailed underwater image. The higher the operating frequency, the shorter the wave length and the narrower the beam width.

Some of the Furuno models output dual frequencies, which makes for a nice comparison (above) to show the difference in target resolution. The 215 Khz signal creates a 4-degree beam width, while the 85 kHz signal has a 10-degree beam width. As you can see, the narrower beam width has the detail much more clearly defined.

The transducer head "steps" around or back and forth in the vertical or horizontal plane, sending out a signal, then waiting for the response before moving to the next position. These steps typically run between 3 and 6 degrees. These SONARs have to process a lot of data, which takes time. The faster the data is updated, the more useful it is for navigation. Output power is not of great importance.

If you are looking at a full circle around the boat, at 500′ (160m), on average it might take 13 seconds for a sweep to be completed. In many navigational situations we do not need to see a full 360-degree sweep. Something closer to 120 degrees – i.e. 60 degrees (or less) either side of ahead – gives us the required data. As the sweep angle is narrowed, the time required to update is reduced, generating more timely data. With this gear you might use a really narrow forward beam, of say 60 degrees, when entering an anchorage, and then switch to a full 360 when you’re at a stop and ready to drop the anchor.

Another factor is the angle of the signal being sent out. To look the furthest distance ahead, a flat angle is used – that is, one that is parallel with the surface. However, there are instances where looking at a more downward angle can be helpful, so there is an easy-to-use control of the transducer’s angle.

Because the Furuno SONARs have narrow, controllable beams in both horizontal and vertical modes, they can see further forward in shallow water than the wider beam-fixed transducers. Factors affecting range include depth of the water, surface conditions (waves), water cleanliness (dirty water reduces range), bottom obstructions, and the shape of the target. Here are a couple of range examples.

This is a 17′ (5m) deep canal, with vertical sides. The image on the left is the SONAR, while the right hand shows a depth sounder for comparison. You’ve got good definition out 500 feet (150m). The SONAR is set on 1000-foot (300m) range.

Here’s a combination of chart, photo through the pilot house window, and SONAR display. In this case the water is fresh, very clean, quite deep, with a calm surface, and most important – there is a vertical shoreline bouncing the signal back. The range in this case is 3300 feet (1000m).

You can also choose to look horizontally ahead, take a vertical slice of what’s ahead, or combine the two. And there are a variety of tuning controls, similar in many respects to what we’ve seen with our RADARs.

OK, now let’s take a look at some more Furuno-supplied images. We’ll start with this image of the Tacoma Ship Canal. In the upper right-hand corner you will see the angle from the surface and the range at which the system is set. In this case we are looking at 0 degrees (parallel with the surface) at a range of 2000 feet (600m). The black outline of the bank and text has been added to explain the image.

Here is a split screen display. The bottom right is a vertical slice looking off 200 feet (60m) along the green line to port. The larger image on the left side of the screen is a horizontal view of the bottom.

This is a simple vertical profile, looking out 150 feet (46m). The transducer has been angled five degrees off center to port, which you can tell by the circle with the boat in it at the center top of the screen.

There is one very important aspect of these systems which you need to understand. With both the fixed transducers and the moveable Furuno models the further away from the boat the outgoing signal pulse, the wider it is. Think about the eight degree wide pulse from the Furuno 270. At a distance of 180 feet this will be about 24-feet wide. Even though the vertical slice looks like something very narrow, the two dimensional image is actually a compressed cone of information. Taking the eight degree example here, if there was an obstruction within four degrees of center, it is going to show up. The further away from the boat this obstruction, the wider the area in which it will be picked up and displayed.

This is where the ability to control the angle of the transducer starts to really work in your favor. If you see a target at some distance off with a vertical scan, you can quickly adjust the transducer a few degrees off center to port or starboard and determine where this obstruction really lies.

When we discussed this with Steve at Furuno he said that most of his clients who navigate with SONAR use the horizontal scan mode 95% of the time. When they are getting into a tight area, they may switch to a split screen, looking at horizontal and vertical scans at the same time.

Steve arranged for us to go out on a sea trial with one of his SONARs from Oxnard, California, so we could see how all of this came together in the real world.

Here is an image taken at the dock. The dark red indicates hard targets, in this case the edges of the channel. The boat symbol (white shape more or less centered) is the transducer location. Notice the target behind us?

That target represents this crew boat.

Here is a photo looking forward down the channel, to put this into perspective.

Amongst the controls are the range, angle from center, and angle to the surface. In this case we are looking all the way around the boat, out a distance of 600 feet (180m) with the beam looking parallel with the surface of the water. Because this is a shallow channel, the beam begins to hit the bottom about 100 feet (30m) in front of the boat, which shows up as an obstruction. However, as long as the bottom is constant depth, or sloping away, the clear (white) space ahead of the boat will remain open as the boat moves forward.

Here the SONAR has been told to search 98 degrees in width ahead of the boat. This speeds up the scanning time. Range is set to 200 feet (60m), and because of the greater water depth and shorter range, the area ahead looks open.

Now let’s add a new element. The lower right-hand corner of the display now shows a cross section of the bottom shape. So we have a cut at 90 degrees to our path, plus the overhead view on the left. While this sectional/cut view is at 90 degrees, it can be oriented in any direction, for example from the boat forward.

All of these images are raw data, without using any of the user variable image controls which are built into the system. We forgot to take photos of how you can clean up an image, using a control like what Furuno calls "TVG".

So Steve sent us a couple of examples of an image with and without TVG control. An image without any filters applied is shown above.

This is a SONAR image of the same area, now with TVG applied. Think of this like turning up sea or rain clutter on your RADAR. The image is now cleaner and easier to interpret. On the other hand, just as with RADAR controls, excessive use of TVG can mask images you might want to see.

Furuno also give you control of power level and pulse length. The image above is of max power and a long pulse length.

This image is still at high power, but now with a shorter pulse length. Just as with RADAR, shortening pulse length gives better target resolution. Note the rows of pilings now visible, which were masked by the previous image with the longer pulse length. In general, for navigation with these sets, you select lower-power, shorter pulse lengths.

Furuno sells most of these units to commercial fishermen. They look for both bait balls and their catch. The photo above was taken recently off Grays Harbor, Washington, and shows a series of anchovy schools just ahead of the boat. Of course these look like rocks as well. If you know the area, or have a chart you trust, you can check to see that these are not an obstruction. But if you don’t know for sure, then the answer is to slow down or stop and see if the SONAR target is moving. If the target is motionless, odds are it is probably an obstruction. You can also look at different viewing angles to try and get a feel for what the target represents.

This image was taken off Santa Cruz, California. The strong echo returns ahead of the boat are columns of bait fish. Note the white trail behind the boat symbol (middle of screen). This is the plotted trail of the vessel relative to the SONAR targets.

One of the questions we have is about the usefulness of this gear in detecting icebergs. We’re thinking of a situation where a small sea is running, maybe 2 to 3 feet (60 to 90cm), and 90% of the berg is submerged, leaving us with perhaps 2 feet (60cm) of berg above and lots more below. We asked Steve about this and he indicated he had heard from Furuno techs in Norway, where use in ice is common, that SONAR does indeed work with bergs, although he did not have any ice images to share with us. (On the other hand John Harries says that his Echo Pilot does not work in ice and chop – possibly due to the fact that they are scanning vertically while the Furuno gear allows horizontal as well as vertical scans.)

This strikes us as the ultimate underwater navigation tool. Of course it would take some use before one became proficient with it. And it is significantly more expensive than some of the other solutions, about $11,000 USD plus installation. But if you are prepared to put the effort into learning how to interpret the information, and have the budget, it could pay for itself very quickly by avoiding prop and bottom damage, not to mention opening up some new places to visit.

But wait, there’s more. A company called FarSounder ( www.farsounder.com ) has a phased-array-based system that is really interesting. They use a very sophisticated transducer, which sends out a single pulse every second, and then builds a 3D image updated on the same schedule. They can look forward to eight times the water depth. So, in 30 feet of water you can see ahead 240 feet.

Apparently a number of mega-yachts are fitting this gear. It seems to offer some navigational advantages over Furuno in the faster update, fixed transducer, and perhaps easier interpretation with its 3D image. On the other hand, the exposed transducer is potentially subject to damage. The cost of US$60,000+ is another negative.

In the next segment we will discuss the decision-making process – whether this gear makes sense, and if it does, what to choose, and for what reasons.