Boating NZ - - Feature -

A con­ven­tional fish fin­der is use­less for lo­cat­ing a static item like a pro­pel­ler on the bot­tom. The sin­gle sonar beam will sim­ply show the bot­tom as be­ing frac­tion­ally shal­lower as you go over an ob­ject.

What you need is tech­nol­ogy pro­vid­ing a 3D image – a for­ward-scan­ning or side-scan­ning so­lu­tion – which de­liv­ers de­tailed, high-res­o­lu­tion im­ages ca­pa­ble of show­ing an out­board prop stick­ing up out of the sand.

Side-scan­ning tech­nol­ogy works be­cause the ping goes out to ei­ther side of the boat. As the echo from an ob­ject is de­tected so too is the ‘shadow’ be­hind it, which when dis­played on the screen will read­ily show the ob­ject’s shape. The dis­tinc­tive blades of a prop will show up, un­less it fell into an area that can­not be seen – such as a crack be­tween boul­ders or other bot­tom de­bris.

ABOVE Side-scan (above) or for­ward­scan­ning sonar can show ob­jects on the bot­tom con­ven­tional sonar can­not. LEFT Side-scan shows the ob­ject and the ‘shadow’ be­hind it.

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