Whenever and wherever we go offshore, it's a fair bet that the closest solid object will be directly underneath us. To determine just how far away it is, we rely on fishfinders and depthsounders. But both suffer from one big drawback: They can't tell us the depth of water over a rock or sandbar until after we've gone over it. That's where forward-looking sounders come in.
Conventional downward-looking depthsounders and fishfinders work by transmitting short squeaks of ultrasound downwards and then measuring the time it takes for each one to return as an echo from the seabed or underwater object. The main difference between a sounder and a fishfinder is that a sounder has to separate the echoes coming from the bottom from those coming from fish, bubbles, and debris, and from those ricocheting between the seabed and the surface. Conversely, a fishfinder is designed to show all those fascinating "extra" echoes that the depthsounder struggled to ignore on its graphic display.
As you have probably already guessed, the difference between a downward-looking fishfinder and a forward-looking sounder is that the latter transmits its ultrasound pulses forwards instead of straight down. Because the job of a forward-looking sounder is to tell you whether something it has detected is shallow enough to be a hazard or whether you can safely go over it, a graphic display is absolutely essential; that's why the display of a forward-looking sounder looks so much like that of a fishfinder. But the technology that produces that display is very different and considerably more sophisticated. That's because when it detects an underwater object ahead, a forward-looking sounder can't simply measure its distance from the surface, as a fishfinder does. It has to measure the angle between the object and the surface.
There are two companies producing forward-looking sounders, and each has chosen a different way to do this. Both methods have their advantages and drawbacks. Interphase uses directional transmission, which scans up and down or side to side like an underwater radar. It's a very efficient system, allowing the company to claim an effective range of 1,200 feet forward from a 450-watt transducer. But ultrasound is much slower than radar, so it takes a half-second for each ultrasound pulse to travel out that 1,200 feet and then return, and as much as 20 seconds more for the unit to build a complete picture-20 seconds in which a 30-knot boat will have traveled almost as far as the forward-looking sounder can see.
Another problem is that although this beam is focused to some extent, it is nowhere near as narrow as the beam of a good radar. With the beam covering a vertical arc of only 12 degrees, it cannot show whether an object 200 feet ahead is on the surface or 20 feet down.
Instead of using directional transmission, Echopilot floods the water ahead with ultrasound, then measures the angle from which each echo returns to the transponder. Like a floodlight compared to a spotlight, the broad ultrasound beam doesn't have the penetrating power of the Interphase, so even the top-of-the-line Echopilot FLS Platinum can only see 200 meters (656 feet) ahead. But every pulse produces enough echoes to build up a complete picture, so it can update the screen image every few seconds and produce a more detailed image than the Interphase.
One problem common to both units is that the distance at which echoes can be received from the seabed or object depends on the depth. At a distance of more than about four or five times the depth, the pulse will hit a level seabed at such a shallow angle that it won't be reflected back to the transponder. But upwardsloping bottoms can often be seen at ten times the water depth or more, so the edges of dredged channels show up clearly, as do rocks and vertical surfaces.
The message that comes out of this is that forward-looking sonar is no substitute for prudent navigation; it is simply not capable of warning you that you are about to plow into something when you are doing 30 knots. Boaters who have bought forward-looking sounders with that expectation have invariably been disappointed. But there are many more for whom a forward-looking sounder has been a godsend, allowing them to creep into uncharted anchorages or rocky bays.
And if you can shoot beams of ultrasound forwards to build up a picture of the seabed ahead, why can't you send them sideways, too? Echopilot is doing exactly that, with several sideways-looking versions of their forwardlooking sounders.
Humminbird and Lowrance have developed a different kind of side-imaging sonar that uses the fact that sound waves that meet a nearly vertical obstruction are likely to produce stronger echoes than those that are reflected from a nearly horizontal seabed or object, and that when they are obstructed by something, they aren't reflected at all. Using light tones to represent strong echoes and progressively darker shades of grey for weaker ones, these units produce a stunning representation of the underwater landscape-almost like an aerial photograph but with shadows and highlights giving a three-dimensional effect.
Compared to a $100 fishfinder, none of these technologies come cheap. But the wow factor of sidescan sonar is almost irresistible, and for the serious fisherman, it could easily be indispensable. Forward-looking sonar doesn't have quite the same immediate appeal, but it could pay for itself if it saves a single dinged prop in a shallow marina.
This article originally appeared in the November 2009 issue of Power & Motoryacht magazine.