A chartplotter helps you understand where your boat is. Sonar adds another dimension: information about what lies beneath the water.
For anglers, that can mean identifying underwater structures, changes in depth and possible fish targets. For recreational RIB owners, it may simply mean having a clearer picture of the water beneath the hull.
Modern sonar equipment offers an impressive range of capabilities, from straightforward depth readings to detailed images of underwater features. But understanding what these systems actually measure is more useful than memorising their marketing names.
How sonar works
Sonar uses sound waves travelling through water.
A transducer sends acoustic pulses and receives returning echoes. The equipment processes those echoes to estimate distance and display information about underwater features.
Because sound travels differently through water depending on conditions, the quality of the result depends on the equipment, installation, environment and settings.
Sonar doesn’t produce an ordinary photograph of the seabed. It creates an interpretation of returned acoustic signals.
Depth sounder or fishfinder?
A basic depth sounder provides information about the depth beneath the transducer.
A fishfinder adds a graphical representation of sonar returns, allowing the operator to examine bottom contours, underwater structures and possible fish targets.
Many modern multifunction displays combine these capabilities.
If your main interest is coastal cruising, dependable depth information may be enough. If fishing is a regular activity, more detailed sonar can be worthwhile.
Neither system replaces appropriate navigational charts or safe navigation practices.
Traditional sonar
Conventional sonar generally displays echoes within a beam beneath the boat.
It can help identify bottom depth, changes in bottom profile and suspended targets.
The image is built up over time as the boat moves or the sonar continues scanning.
Understanding this distinction is important: the screen does not necessarily show a real-time side view of everything directly beneath the boat.
CHIRP sonar
CHIRP systems transmit a range of frequencies rather than relying on a single narrow frequency.
With suitable equipment and processing, this can improve target separation and the interpretation of underwater returns.
However, performance depends on the transducer, frequency range, depth and installation.
A system described as CHIRP is not automatically superior in every situation.
Down imaging and side imaging
Some sonar systems produce more detailed representations of structures beneath or alongside the boat.
Down-imaging systems emphasise features below the vessel, while side-imaging arrangements can provide information extending outward to either side.
These capabilities can be useful for anglers looking for changes in seabed structure.
However, their effective range and image quality depend on water depth, boat speed, transducer positioning and environmental conditions.
The transducer matters
The transducer is the connection between the electronic system and the water.
Its type, frequency capability and installation position have a major effect on performance.
Common installation arrangements include transom-mounted, through-hull and certain in-hull configurations, although suitability depends on the boat’s construction.
A poorly positioned transducer may experience aerated water, turbulence or interference, reducing the quality of readings.
On a RIB, installation should be assessed in relation to the hull geometry, trailer supports, engine and intended use.
Drilling or modifying the hull should never be undertaken without confirming an appropriate installation method.
{Illustration: A technical diagram comparing a conventional downward sonar beam, a down-imaging view and side-imaging coverage. Clearly identify the diagrams as conceptual rather than exact beam patterns.}
Reading the display
Sonar interpretation takes practice.
Strong returns, weak returns, suspended targets and bottom structure may appear differently depending on the system’s settings.
Not every mark is a fish, and not every fish will appear clearly.
Automatic settings can be useful, but understanding sensitivity, range and frequency selection helps users interpret the information more confidently.
The best approach is to become familiar with the display in known locations and compare what appears on screen with the actual environment.
Is sonar useful if you don’t fish?
For some recreational boaters, yes.
Depth information can help improve awareness of the water beneath the boat, particularly where depth varies.
However, sonar measures a particular area or direction relative to the transducer. It cannot guarantee that the water ahead is clear of hazards.
A depth reading beneath the stern does not necessarily describe the depth beneath the bow or farther along the intended route.
Charts, lookout and prudent navigation remain essential.
Choose the system for the activity
If you mainly cruise with family, a simple depth-capable arrangement may be sufficient.
If you fish regularly, a more advanced sonar system may provide worthwhile information.
For dedicated anglers, transducer choice and installation can matter as much as the display itself.
Buy the capabilities you’ll use, confirm compatibility and learn how to interpret the results.
Sonar can reveal a fascinating world beneath the boat. It becomes genuinely useful when you understand what the screen is telling you—and what it isn’t.