We're only slightly kidding when we say that one of the simplest ways to pick a transducer is to decide just how big of a hole you want to drill in your boat. There is something of an inverse relationship between sonar performance and difficulty of installation. Match the frequency of the fishfinder and transducer.
In-hull works with solid fiber-glass hulls.
Style and shape
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.
Transom Mount: Adjustable-angle bracket is screwed or bolted to transom, with transducer hanging below and behind hull. Simpler installation, but may encounter more turbulent water flow. Popular for trailerables.
In-hull: "Shoot through hull" transducers do not need direct water contact. Glued to the inside of the hull with silicone or epoxy.
Trolling Motor:Clamped to outside or permanently installed inside propeller hub of trolling motor. Minn Kota Universal Sonar and MotorGuide Sonar Ready models include built-in 200kHz transducers that fit many popular fishfinders.
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.
Nylon transom mount with speed paddle wheel.
Frequency and wavelength
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.
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.
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.
Bronze semi-flush thru hull.
If you operate in shallow waters, bottom detail and a wide beamwidth may be your highest priority. 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.
The transducer must be submerged in smoothly flowing water, free from turbulence or air bubbles (which reduce its ability to transmit or receive signals). Make sure it is mounted vertically so it sees straight down, and positioned so it stays submerged if the boat is bow-up while planing or heeling while turning.
- 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, with the transducer's trailing edge tilted downward.
- In-hull and thru-hull transducers are often mounted well forward (about 1/3 of waterline length aft, and within 6" of the centerline) in displacement powerboats, and 1' to 2' forward of the keel and to the side of the centerline on fin-keel sailboats. Mount well aft near the centerline on planning hull powerboats, well inboard of the first set of lifting strakes so it will always be submerged. To install a thru-hull in a cored hull, cut out the core material, seal the edges with epoxy to prevent water seepage, and reinforce the hull.
- 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 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.
- Transducer cables are electrically "noisy" and should be isolated from other wires. They can also pick up noise from alternators, which can show up on the screen as little spots.