John 

MY BUDDY AND I BOWLED ALONG on the current, with a flat and featureless seabed rolling away beneath us. From time to time, a big thornback ray might startle up from the mud and rush away into the gloom, or the odd anemone would relieve our boredom.
At one point, I wrote a message on my slate and passed it to Mike. It read: At least were going back towards Selsey!
Once again, we had got the transits for the wreck wrong. It was a typical Holborn SAC inflatable-boat weekend dive during the 80s. My logbook would read In the vicinity of the Outer Mulberry. I often wrote In the vicinity of... in my logbook then.
Today, divers are much more likely to use
a professionally run hardboat, and complain bitterly if the skipper doesnt 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 whats down there up on the LCD screen - provided you know what youre 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. Its 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 its 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 its 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.
Theres 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 doesnt break the sacred 1000 barrier.
One gloriously wet day in September, we set off to sea in a little 6m borrowed RIB to see if it was as easy to use as the salesman said.
The rain fell in stair-rods, our woolly hats were drenched and our clothes under our drysuits were already damp from getting changed, albeit very slickly, in the car park before we set off. It was like a typical British dive-club outing.
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 coxswains 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.
We looked for a section of Mulberry Harbour in the Solent, and there were the angular concrete and steel sections, clearly shown on the screen. Magic! Modern divers dont know theyre born!


Divernet
Mulberry
Mulberry section recorded to an SD card

SPECS
PRICE
With transducer £999 (797c2) or £2650 (1197c, illustrated)
DISPLAY size 5in (797c2), 10.4in (1197c)
DEPTH RATING 50m (side-imaging) & 500m (797c2); 50m (SI) & 1000m (1197c)
CONTACT www.johnsonoutdoors.co.uk
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