Starting out fishing from your own small boat can be a minefield of things to learn and do. This guide explains the differences between the four types of sonar available to recreational sea anglers and how to select each one depending on intended use.
CHIRP is the most common type of sonar that all fish-finders will use as a minimum. CHIRP sonar uses a band of frequencies to ping the sea floor and detects the differences in returns between each frequency. The fish-finder uses these returns to build up a more accurate “image” that you see on your screen. Typically using frequencies between 50-200khz, the bottom definition is usually pretty good, and usually good enough to distinguish between rock and muddy sea floors, presence of weed growth etc. The view generated by CHIRP sonar is that of whatever is directly underneath the transducer which is represented as a side-on view on your fishfinder screen.
CHIRP is generally effective at all boat speeds but transom-mounted transducers can suffer from image disruption at higher speeds because air bubbles and water turbulence under the transom make it harder for the transducer to send and receive sonar pings. Due to this I find CHIRP to be most effective at higher speeds when using a through-hull or in-hull transducer, where the water flow under the hull is cleanest and as such, turbulence and air bubbles are not an issue. The images above are taken using a through-hull Airmar M260 1kW transducer.
Effective depths of operation will vary with frequency – lower frequencies penetrate deeper than higher frequencies but for most UK coastal and offshore fishing any of the available frequencies will work fine. It’s only blue-water fishing in places like the US and Australia where sonar range can become an issue.
I tend to use CHIRP sonar when under way at speed and I'll keep an eye on it for any interesting drop offs or structure that I'll then do a slower pass over using down scan to get a higher definition view of what's there.
Down scan sonar
Down scan sonar uses higher frequencies than CHIRP – typically in the 455-800kHz range – and it’s the higher frequencies that give a higher definition image on your screen. Down scan sonar can distinguish between the various sea floor compositions and is also great at detecting structure (wrecks & wreckage, boulders, pilings and other submerged features). As a general rule, where there is structure there will be small fish, and where there are small fish, there will be bigger fish! The name down-scan is slightly confusing as the image on your fishfinder screen is a side-on view of whatever is under the transducer – best to think of it as scanning down under your boat.
I find that down scan sonar tends to work more effectively at slower boat speeds, up to 10-12kts max, and for highest image quality then 3-5kts is best.
Down scan sonar transducers are generally only available in transom mount configuration. There are a couple of through hull transducers that I’m aware of but the price of these is prohibitive when a perfectly good transom mount will do the job just fine.
The higher frequencies used for down scanning don’t penetrate to the same depths as CHIRP sonar, but again, for the type of fishing we do in the UK this isn’t a problem.
Side scan sonar
Like down scan, side scan uses the same high frequencies of 455-800kHz but this time the sonar pings are sent out sideways either side of your boat, and the sonar returns are represented on your fishfinder screen as a birds-eye view of the sea floor and water column either side of your boat. This view in particular is great for scouting for sea floor features, wrecks and structure. Most side scan sonar units have ranges of many tens of metres, often 60m or more each side. This means you have the ability to quickly search a swathe of sea floor 120 metres wide to find features that might hold fish.
Again, like down scan, slower boat speeds are best for side scan. I find that 3-5kts is perfect to give high quality, clear images of the sea floor. Transom mount transducers are most common for side scan, and I’ve found it important to locate the transducer on the transom in a spot that benefits from clean water coming out from under the hull. Any turbulence or bubbles will quickly degrade the quality of the image that the transducer is able to generate.
Also like down scan, the higher frequencies used for side scan sonar don’t penetrate to the same depths as CHIRP sonar, but this hasn’t prevented me from using side scan effectively for locating wrecks in the waters I fish off the south coast of the UK.
Live scanning sonar
A relatively new offering for recreational sea anglers, live scanning sonar interprets sonar returns in a similar way to hospital ultrasound scanners, presenting a view of the sea floor in real-time. This allows the angler to see his or her lure jigging under the boat and see fish move in to hit the lure in real-time. Various view configurations are possible, from a forward-looking birds-eye view of the sea floor in front of the transducer in wide and narrow configurations to a more traditional side-on presentation of a downward view.
This style of sonar was primarily designed with shallow-water lake fishing in mind, but I’ve heard reports from blue water anglers that live scanning technology can be used at impressive depths as well as in the shallows. Take a look at the Lowrance promo video of their Active Target sonar, and the Garmin promo video of their Panoptix Livescope sonar to see what's possible.