Airmar B75M vs B175M: does your fishing justify 1 kW?

Choose by fishing depth, sounder support and hull installation, not wattage alone. B75M can be a practical medium-band starting point for a smaller fishing installation. B175M deserves investigation when the fishing brief and supported sounder justify the larger job. Neither transducer produces a usable screen without compatible electronics.

#The acoustic comparison

Shaded rows contain different values. On a small screen, scroll the table; model names stay visible within it.

Sources: Airmar's B75M and B175M pages. The B75M page's feature section describes the medium band, but its generic specification block contains conflicting low-band and maximum-depth entries. We use the clearly labelled medium-band features above and omit a maximum-depth contest. Ask for the current model-specific acoustic sheet before ordering.

#What the narrower beam changes

As a geometric illustration, a 20° cone covers roughly 35 m across at 100 m depth, while a 12° cone covers roughly 21 m. Those figures use 2 × depth × tan(half the angle); they are not measured footprints from these products. They show why more coverage and a tighter view are different objectives, rather than a simple good-versus-bad ranking.

For an angler looking beneath the boat, ask what target and bottom detail are actually needed. For an owner mostly wanting depth while cruising, a high-power fishing sensor may solve a different problem from the one aboard. The B175M versus B175HW comparison explores another coverage decision.

#Example: a planned offshore upgrade

Imagine a boat with a working sounder whose owner wants to fish deeper. Before choosing B175M, obtain confirmation that the exact sounder or module supports the proposed transducer, frequency band, power and connector. If a module and display upgrade are also required, include them in the decision now.

An installer should measure deadrise at the proposed position, confirm the correct tilt variant and assess access inside the hull. Do not treat the old transducer's mounting as reusable until dimensional drawings and hull construction have been checked. A higher-rated sensor installed where aerated water interrupts it can still deliver disappointing results.

#The buying verdict

Retain or shortlist B75M when it meets a documented fishing brief with supported electronics and a sound installation. Investigate B175M when the added capability has a clear purpose and the complete system supports it. Do not infer a proportional increase in usable depth from the power ratio.

Request a quote covering the sensor, module if required, connector arrangement, hull work and commissioning at relevant operating speeds. Have the supplier resolve the B75M documentation discrepancy in writing. We have not verified Australian installed prices or run these sensors side by side; this comparison identifies the questions that should decide a purchase.

#When extra power is the wrong first purchase

Suppose a fishing boat loses bottom information at speed but works at rest. Replacing a 600 W sensor with a 1 kW sensor without diagnosing the installation may leave the original fault untouched. Ask the installer to investigate the water flow, mounting, connections, supply and supported configuration. A higher power rating is not a remedy for every source of poor information.

Conversely, an owner whose fishing brief genuinely needs a different acoustic capability should compare the complete sounder and sensor combination. If the current sounder cannot drive the proposed unit as intended, the upgrade quote must include that consequence. The B75M documentation ambiguity noted above is a reason to obtain the exact acoustic sheet, not to invent missing performance values.

#Example: a staged fishing-boat refit

Start by recording present fishing depths and the condition in which useful returns are obtained or lost. List the current sounder, sensor and connector by model. Have the supplier propose the retained installation after repair, a B75M route and a B175M route only where each is technically supported. Put the full installed scope beside each, including any haul-out and housing work.

Shaded rows contain different values. On a small screen, scroll the table; model names stay visible within it.

A budget owner may find the existing system adequate after a correct installation repair. A no-limit owner should still ask the same questions before buying a more complex system. Put the diagnostic work into the project scope so it is not skipped in favour of replacing boxes.

#What a useful acceptance record contains

Record boat speed, approximate depth, relevant sonar settings, transducer selection and conditions during the agreed checks. The record helps distinguish repeatable installation issues from a one-off observation. It is not proof of a universal maximum working depth. If the system fails the agreed brief, log the defect and corrective plan before declaring the refit complete.

The B175M versus B175HW comparison addresses a different question—band and coverage—and should be read before assuming all B175 models solve the same fishing problem.