Marine Sonar Guide

Sonar is the most misunderstood part of modern marine electronics, and the reason is that different boats are asking the water completely different questions.

A cruiser asks:

How much water is beneath me, and what is the seabed doing?

A serious fisher asks:

Where are the fish, at what depth, around which structure, and how are they moving?

Those require different equipment, different transducers and different budgets. A great deal of money is wasted by people who bought the answer to the second question when they only ever needed the first.

#How sonar works

A transducer converts electrical energy into sound. The sound travels through water, reflects from targets and returns. The system estimates distance from the round-trip travel time.

Returns can come from:

• the seabed
• fish
• wrecks and structure
• rock
• vegetation
• thermoclines — layers of rapid temperature change, which reflect sound and often hold fish
• aerated water, which scatters sound and produces noise rather than information

Sound behaves differently from radio. It is slower, it is affected by water temperature and salinity, and it is scattered badly by bubbles. Those three facts explain most of what follows.

#The transducer matters as much as the display

Transducer characteristics determine:

• frequency
• beam angle
• power
• sensitivity
• mounting geometry

A premium MFD cannot recover information the transducer never detected. This is the single most important sentence on this page, and it is the reason the transducer guide exists as a separate article.

Specify the transducer from the water you actually work in. Then choose the sounder that drives it, then the display that drives the sounder. Working the other way round — picking a screen and accepting whatever transducer is bundled — is how boats end up with expensive displays showing mediocre underwater pictures.

#Basic depth

For a cruising yacht, a reliable depth sounder remains one of the most important sensors aboard. Not the most sophisticated — the most important.

Know the configured offset. A system may display:

• depth below transducer
• depth below keel
• total water depth

These are different numbers describing the same water, and all three are correct. Which one your display shows depends entirely on a configuration value that is frequently left unset.

The failure mode is specific and dangerous: nobody set the offset, the display reads depth below transducer, and the crew assumes it reads clearance below the keel. That is an error in the direction that puts you on the bottom.

Fix it in ten minutes: compare against a lead line or boat hook in calm water at a known state of tide, set the offset, verify it, and write the convention on a label beside the display. Then set shallow and deep alarms to values that mean something.

#CHIRP

CHIRP sweeps through a range of frequencies within a single transmission rather than transmitting only one narrow frequency, then applies pulse compression to the return.

Benefits can include:

• improved target separation — two fish close together, or a fish holding tight to the bottom, resolve separately instead of merging
• stronger bottom definition, which also helps distinguish hard bottom from soft
• better sensitivity, extracting more usable signal from noise

What it does not change is depth capability, which is set by frequency and power — both transducer properties. See CHIRP versus traditional sonar for the full comparison, including why a cruiser probably does not need it.

#Frequency

#High frequency

More detail, better small-target resolution, less deep-water penetration. Right for shallower work and finer discrimination.

#Low frequency

More depth capability, generally less fine detail. Right for deep offshore.

#Medium frequency

A compromise, and often the practical answer for mixed depths.

Serious offshore fishing boats choose transducers based on target depth and species, not on brand. A boat working 40 m of reef and a boat working 400 m of drop-off need different hardware, and no amount of processing bridges that.

#Side scan

Side scan looks outward from the boat, building a picture of the seabed either side of the track.

Useful for finding:

• reef edges
• wrecks
• bait schools
• rock and rubble transitions
• bottom composition changes

It is a major fishing tool and usually not a core cruising-yacht requirement. A cruiser wants to know the depth under the keel; mapping the reef 30 m to port is interesting rather than necessary.

#Live sonar

Live sonar provides near-real-time imagery of fish and structure — you watch behaviour rather than infer presence.

Garmin launched LiveScope 2 in July 2026, reporting improved resolution, noise reduction and coverage over the earlier generation. Lowrance ActiveTarget occupies the same space.

For certain fishing techniques this is transformative: watching a lure, watching fish react to it, fishing vertically over structure. The honest test is whether your technique involves watching what happens in real time. If you are bottom-bouncing in 200 m, it will not help.

For bluewater cruising it is not essential navigation equipment.

#Forward-looking sonar

Forward sonar is more interesting to cruisers, because it attempts to show underwater terrain ahead of the vessel rather than beneath it.

Potential uses:

• unfamiliar anchorages
• a rising seabed on an approach
• poorly surveyed approaches, where the chart is least helpful — see marine charts Australia
• reef exploration in good light

It should not be treated as underwater radar. Useful range depends heavily on depth, geometry and acoustic conditions, and it degrades in exactly the shallow, turbulent water where you most want it. A common pattern is useful range roughly proportional to depth, which means in 5 m of water you are not seeing far ahead at all.

Treat it as an extra input for choosing where to go slowly, never as a licence to enter water you would otherwise avoid. See anchoring electronics.

#Installation

#Transom mount

Good for trailer boats and outboard craft, but vulnerable to aerated water at speed. Position it clear of the propeller wash, strakes and hull steps, and expect to trial the vertical position — too high and it ventilates, too low and it drags.

#Through-hull

Often best for serious performance when correctly installed, which means sited in clean flow and faired so the transducer face presents square to the water.

#In-hull

Avoids a hull penetration, but only works through suitable solid hull construction and may sacrifice performance. It will not radiate through a cored or air-filled laminate.

All three are covered in detail, with the fairing-block and siting specifics, in depth sounders and transducers.

#Vessel recommendations

#Cruising monohull

Prioritise dependable depth. A well-sited transducer, a correct offset, meaningful alarms, and bottom tracking that holds at speed and on both tacks. Add forward sonar only if remote shallow-water exploration genuinely justifies it.

#Cruising catamaran

Same priorities, with more attention to flow: hull turbulence, tunnel effects and the wake behind a mini-keel all produce aeration in places that look clean. Consider a transducer in each hull for redundancy and cross-checking. See catamaran navigation.

#Offshore fishing boat

Start with target depth and transducer specification, then choose the MFD ecosystem. Commonly a 1 kW class CHIRP through-hull where the depth justifies it. See offshore fishing electronics.

#Trailer fishing boat

A CHIRP transom transducer plus side imaging delivers excellent value, with patience required on the siting. See trailer boat electronics.

#Current ecosystems

Garmin — strong in CHIRP, scanning sonar and LiveScope 2, with the broadest transducer catalogue.

Simrad — strong offshore integration with NSS 4 and high-power sonar modules.

Lowrance — strong in Active Imaging and ActiveTarget, and the best fishing value.

Furuno — a deep reputation for serious fishfinding and offshore sonar performance.

Raymarine — Axiom 2 and Axiom 2 Pro RVM offer strong integrated sonar for dual-purpose boats.

#Bottom line

Do not buy the most advanced sonar. Buy the sonar that answers the underwater question your boat actually needs answered — and on a cruising yacht that means a dependable depth reading with a correct offset, which costs far less than the alternative and matters far more.