KEY POINTS:
Side imaging "3D" sonar, usually the preserve of far bigger vessels, is now available in a size, and more importantly a price, suitable for trailer boat owners.
Standard fish finders or depth sounders basically provide a two-dimensional picture of the bottom. They also, and this is the bit that interests most of us, provide an indication of any fish targets between the sea floor and our boat. How good an indication that is depends on the quality of the sounder.
Most operate using one or two frequencies (normally 50 and 200kHz) and these are transmitted in different beam widths, varying anywhere between about 9 and 45 degrees. These "torch beams" shine straight down, getting continually wider the deeper they go.
It's a tried and true method of identifying fish targets, the sea floor and bottom structure. However, it is, it must be said, still pretty difficult for most users to work out exactly where these "points of interest" are in relation to their boat. (How many of us have spotted an interesting looking reef structure on the screen and then spent what seems like ages circling around trying to find it again, this time from the direction in which we wish to anchor?).
There is also an issue that, given that the beams get wider as they go deeper, the deeper the water, the greater the area of uncertainty must be. In 50m of water and using a wide beam, therefore, a target appearing on the screen could actually be anywhere within an area of about 50sq m. Switch to a narrow beam and things are not much better. You might only be seeing an area of 15sq m and missing what is outside that. Of course, some modern sounders have split-screen capabilities to address these issues and the new digital sounders can also make a big difference.
There is also a new kid on the block: side imaging sonar. The first of these models, from manufacturer Humminbird, will be on display at the BLA NZ stand at this week's Hutchwilco New Zealand Boat Show.
According to Cameron Burch, Humminbird brand manager at BLA, the scenarios above are where side imaging sonar really excels.
"Side imaging allows you to look out either side of the boat and see picture-like detail of structure or fish targets exactly where they are in relation to your vessel," he says. "You can see these up to 100m on either side and in real time.
"That sort of seafloor coverage allows users to pinpoint targets and bottom structure and sound out huge areas of sea floor that would normally take them hours using a conventional 2D fish finder."
