Active and passive
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.