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Selecting a Sonar Transducer


The transducer is the heart of a fishfinder system, changing electrical pulses into sound waves or acoustic energy and back again. It is the device that sends out the sound waves and then receives the echoes, so the fishfinder can interpret or “read” what is below the surface of the water.

Sonar Transducer

Bronze semi-flush mushroom head thru-hull.

Which style of transducer for your boat?

Thru hull: Threaded bronze, nylon or stainless steel shaft passes through the bottom surface of the hull. Choice of styles: external football-shaped head with water flow smoothed by a fairing block that also corrects for the deadrise (sideways slope of the hull); or round mushroom head thru-hulls, either semi-flush or flush mounted. Most challenging to install, but likely to provide best signal quality. Displacement power and sailboats generally use thru-hulls.

Tilted element transducers are a type of semi-flush thru-hull transducer. The ceramic elements are tilted inside the housing, which compensates for the boat’s deadrise. This aims the beam straight toward the bottom, resulting in stronger echo returns and more accurate depth readings. They’re a good choice for large, trailered center console and walk-around cuddy cabin powerboats that cannot accommodate a thru-hulls with fairing blocks. They provide virtually flush installation to the hull. For hull deadrise angles up to 25 degrees. Excellent high speed results over 30 knots.

Transom Mount: An adjustable-angle bracket is screwed or bolted to transom, with the transducer hanging below and behind the hull. Simpler installation, but may encounter more turbulent water flow. Versatile and popular for trailerables. Can be used on wood, fiberglass, aluminum or steel hulls, with single or twin I/O, outboard and jet drive propulsion systems. Moderate performance at speed.

Transom-mount transducers are usually mounted to starboard, because this is typically the side where the propeller blades are moving downward. The upwash from the propeller blades causes bubbles and turbulence. Mount at least 3" (75mm) beyond the swing radius of the propeller. For twin outboard or sterndrive applications, mount the transducer between the two drives, either on or just off of the centerline. Mount so that the front of the sensor is slightly higher than the back of the sensor and the sensor projects below the hull, otherwise aeration will occur.

Sonar Transducer

In-hull works with solid fiberglass hulls.

In-hull: Installed against the inside of the hull bottom, the in-hull transducer sends its signal through the hull. “Shoot through hull” transducers do not need direct water contact. They’re glued to the inside of the hull with silicone or epoxy. An in-hull transducer is a good choice for a trailered boat, a vessel with a stepped hull, and for other types of high performance hull designs, as there is no drag, hull penetration or potential for fouling. No integrated temperature sensor. Can be installed while boat is in the water. For deadrise angles up to 30 degrees. As with thru-hulls, the selected location should be aft and close to the centerline so that the transducer is in the water at all times.

In-hull transducers need solid fiberglass at the mounting location; no foam or plywood coring material, or air pockets. A typical 600W transducer can transmit through 1/2" to 5/8" (12-16mm) of fiberglass. To install inside a cored hull, find a location with no coring or remove the core material.

According to the folks at Airmar, who manufacture the vast majority of transducers, in-hull transducers are far more sensitive than mounting a standard transducer inside your hull, and perform as well as or better than comparable transducers installed outside the hull. They’re engineered to offset the signal-loss from passing through fiberglass, with built-in deadrise correction to aim the transducer’s beam straight down toward the bottom.

Trolling Motor: Clamped to outside or permanently installed inside the propeller hub of the trolling motor. A new product from Lowrance, the SpotlightScan™, clamps to your bow-mount, cable-steered trolling motor and lets you see picture-like images at 455/800kHz. You aim the transducer with your trolling motor’s remote pedal, and it has a horizontal range of about 150'.

Triducer: Contains depth and temperature sensors, plus speed paddle wheel.

Dual beam, QuadraBeam and phased array: Include two or more of the piezoceramic elements that change electrical impulses into ultrasonic sound waves. Unlike traditional dual frequency transducers where you must toggle between high and low frequencies (most commonly 50 and 200kHz), some new-style dual beam transducers display a composite image with a central lower-frequency cone, and two wider higher-frequency cones. QuadraBeam™ transducers, used by Humminbird, operate on three frequencies (83/200/455kHz) with cone angles of 20°, 60° and 90°. Interphase scanning sonars, the Twinscope, the iScan and the black-box SE-200 use two phased arrays to sweep their beams both horizontally and vertically.

Sonar Transducer

Nylon transom mount with speed paddle wheel.

Which material?

  • Plastic thru-hull housings should not be used on a wooden boat. Wood swells as it absorbs water, so it may crack the housing.
  • Bronze thru-hull housings should not be used in aluminum boats. The interaction between the aluminum and the bronze, especially in the presence of saltwater, will eat away the aluminum hull and/or bronze housing.

What frequencies?

For maximum depth, use lower frequencies. For the greatest resolution, the least background noise on your screen, or the best view from a fast-moving boat, use higher frequencies. We recommend 200kHz for water depths up to 200' and 80kHz or 50kHz for deeper waters. Dual frequency sonars combine low-frequency/high-frequency and wide/narrow beam operation to give anglers the advantages of both.

A sound wave will bounce strongly off something that is longer than its wavelength. If the object is shorter, then the sound wave will almost wash over it and the echo will be very weak. Wavelength of a sound wave is calculated by dividing the speed of sound in water by the frequency. One sound wave at 200kHz is slightly longer than 1/4", so a 200kHz sound wave will be able to detect fish with an air bladder as short as 1/4". One sound wave at 50kHz is slightly over 1", so a 50kHz sound wave will only detect fish if their air bladders are large, slightly longer than an inch.

Beamwidth

Lower frequency transducers generally send and receive over a wider cone angle so they cover a wider area, detect fish that aren't directly under your boat, penetrate deeper water, but don't show as much target separation. Adjacent objects may blend together and you see a blob, instead of a pair of fish. Anglers use low frequencies to keep track of a spread of multiple downriggers. Showing both frequencies on split-screen provides wide coverage plus enhanced detail.

How much transmitting power?

The strength of the "ping" is expressed in watts RMS (root mean squared). Power is directly related to how well you see in silt-laden water, view down to greater depths, and successfully resolve separate targets and bottom structure. More power is better, so some manufacturers juice up the numbers by rating their product using peak-to-peak watts, which is eight times the RMS number. To avoid mixing apples and oranges, we use watts RMS exclusively.

Water depth

If you operate in shallow waters, bottom detail and a wide beamwidth may be your highest priority. You’ll want high frequencies (200kHz, 455kHz or 800kHz). Along coasts or in deep mountain lakes, maximum water depth readings may be what you're after. For maximum depths, you want a powerful low frequency transmitter and a narrow beam angle. Maximum depth readings will be 25%–50% less in salt water than in fresh water.