Side-Scan Sonar Review
797C2 & 1197C SI COMBO
Today, divers are much more likely to use a professionally run hardboat, and complain bitterly if the skipper doesn’t put the shotline into the centre of the wreck. We take it for granted that he knows where it is.
Echo-sounders go a long way to relieving this problem, but they need careful interpretation. The clever technical divers of previous times could find a wreck using a magnetometer, towing a “fish” behind the boat and looking for magnetic anomalies in the seabed. It was never that reliable, and you needed to be dedicated, systematic and have plenty of manpower available to use one efficiently. Today we have the magic of side-scan sonar, which simply puts a picture of what’s down there up on the LCD screen – provided you know what you’re looking at.
Side-scan sonar has until now been applied in the same way as a magnetometer, using a fish to get the transducer away from the turbulence caused by the hull moving through the water. Mounting the transducer on the hull always caused too much “noise”.
But we are undergoing a digital electronic revolution. It’s not just a question of getting the “ping” right. Software writers have learned how to turn the “ping” into information that almost any of us can understand, and it’s the software that makes the difference.
HUMMINBIRD, A DIVISION of the US giant Johnson Outdoors, produces a range of echo-sounders (or “fish-finders”) that give a sort of picture of the seabed. For use in small boats, these can be combined with a GPS chart-plotter. And now you can get an economically priced dual-beam side-imaging sonar that can really tell you whether it’s a wreck or a mound of sand. Its transducer is readily mounted on the transom of the RIB, or can be included as a through-hull unit on larger vessels.
There’s never much space in a RIB, which makes the new compact 797c2 SI Combo a good prospect. Hardly bigger than a typical fish-finder, it sits on a standard Humminbird mount and can be detached quickly when leaving the boat moored overnight. Complete with transom-mounted transducer and everything you need to make it work, it still doesn’t break the sacred £1000 barrier.
One gloriously wet day in September, we set off to sea in a little 6m RIB to see if it was as easy to use as the salesman said. Our boat had been temporarily kitted out with the Humminbird 1197c SI Combo which, besides having four additional buttons for user-selected short cuts to the different functions, works in exactly the same way as the 797c2 SI Combo, but has a screen twice the size.
In fact it was rather big for the boat and occupied most of the space available on the console. It also costs twice as much, but it made photography easier in the appalling conditions.
It also allowed us to have both chart-plotter and sonar functions working simultaneously and still let all three of us see what was going on without crowding the unit and obscuring its view of the GPS satellites, but I will describe the functions assuming it to be the smaller unit. Naturally, the GPS function gives speed over the ground and course. You can look at it as a conventional chart, with a perspective that matches the coxswain’s view, or with a real image from Google Maps laid over it.
This makes interpretation of where you are, in relation to landmarks and navigation buoys, for example, exceedingly easy. It also gives time of day, distance travelled and actual position co-ordinates, and can be linked to a VHF radio to meet the new small-boat navigation and positioning requirements.
For temporary installation on a borrowed RIB, we used a transducer mounted to the transom with a suction cup. The side-imaging function takes in a view of 180°, and you can alter the range according to depth of water.The dark part represents depth to the seabed. Then the software writers have unravelled the signal to take account of perspective and give you a picture (complete with sonar shadow) of objects that might be lying proud of the seabed.
Spot something, and you can not only mark the spot but record the image to an SD card.Magic! Modern divers don’t know they’re born!