Ac­tive and pas­sive

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Sonar tech­nol­ogy is cer­tainly not new: sub­marines have been us­ing it since the 1940s, and it has been avail­able com­mer­cially now for more than 30 years. Sonar is short for ‘Sound Nav­i­ga­tion and Rang­ing’, and there are two types: ac­tive and pas­sive.

Pas­sive sonar only lis­tens and doesn’t trans­mit (so doesn’t give away your po­si­tion). Pas­sive sonar has a much longer range than ac­tive sonar and is utilised by the mil­i­tary to track sub­marines.

Ac­tive sonar, the type used on yachts, emits an acous­tic sig­nal into the wa­ter. If an ob­ject is in its path the acous­tic sig­nal bounces back off the ob­ject and re­turns an ‘echo’ to the sonar trans­ducer. By work­ing out the time be­tween the trans­mis­sion and the echo the trans­ducer can de­ter­mine the range of an ob­ject. Ac­tive sonar range is shorter and its ac­cu­racy de­pends on a mix of vari­ables such as the wa­ter tem­per­a­ture and the power of the trans­mit­ter.

So how can sonar help on a yacht when we have al­ready have GPS, echo sounders and chart plot­ters? De­spite all that, yachts do still run aground: only in June this year, two su­pery­achts hit the bricks when rac­ing off

Porto Cervo in Sar­dinia, ru­in­ing their event and cost­ing tens of thou­sands of eu­ros.

A for­ward-look­ing sonar maps the seabed ahead, usu­ally over a cone of trans­mis­sion of about 15° ei­ther side of the bow. When in­te­grated with other nav­i­ga­tion de­vices such as a chart plot­ter, they be­come a very use­ful aid to nav­i­ga­tion, though the range is still very short. The trans­ducer on the Swan 78 is a through-hull fitting 2m for­ward of the keel and ex­tends 30mm below the hull, so there is a tiny bit of ex­tra drag – but it can be man­u­ally raised if re­quired.

The range de­pends on the type of ob­ject ahead. A ver­ti­cal sea wall re­flects ‘pings’ bet­ter than shelv­ing mud, so the range may only be 25m to 90m but, in my view, any in­for­ma­tion about what lies ahead is worth hav­ing.

I have used For­ward Scan to great ef­fect short tack­ing against a strong tidal stream in the So­lent, gain­ing the con­fi­dence to tack back to the shore be­fore our com­peti­tors helped make sig­nif­i­cant gains. Con­fi­dence that you are clear for just an­other boat length as you ap­proach the shore can be gold dust in­for­ma­tion and al­low your boat to achieve a clear lane of clean wind and make gains.

Us­ing sonars takes some in­ter­pre­ta­tion – sim­i­lar to us­ing radar, you have to play with the gain con­trol and it takes time to learn how to use it most ef­fec­tively. For rac­ing at close quar­ters, it can be quite tricky to use while us­ing other nav­i­ga­tion equip­ment. The For­ward Scan is in­te­grated with the chart plot­ter and that helps a great deal, though it’s still worth re­mem­ber­ing that the most im­por­tant part of pi­lotage is the ‘mark one eye­ball’.

Sonar is only an ad­di­tional aid but should work well where there are iso­lated boul­ders off a shore­line, so in ar­eas like Porto Cervo it is valu­able and to re­spond to it you need to be ready to tack out again at short no­tice.

Would you be able pick out a semisub­merged con­tainer or ice­berg? The­o­ret­i­cally yes, al­though you are not go­ing to get much no­tice if rac­ing at 20 knots.

There is also the risk of the through-hull trans­ducer com­ing out of the wa­ter as you surf a wave. A blue­wa­ter cruiser mov­ing at a slower speed would have more time to re­act, so set­ting it up with an alarm may help.

When cruis­ing I have used For­ward Scan to clear an area of seabed to al­low me to get closer to the beach when an­chor­ing in a bay with sparse depth sound­ings.

Look­ing ahead of your yacht in clear wa­ter you can of­ten see rocks and avoid them, but one of the great things about sonar is it can ‘see through’ muddy wa­ter as well.

In­te­grated with a chart plot­ter, For­ward Scan gives an in­valu­able view of what lies be­neath

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