Sounders

The basic concept of depth sounders/fishfinders hasn’t really changed in the last 60 years. They operate by sending a sound wave through the water and measuring the time it takes the wave to reflect back from the target or bottom. Systems consist of two components – a transmitter/receiver and a transducer positioned under the boat. Think of the transducer as an antenna that converts electrical energy into sound waves and then translates the returning sound waves back into electrical energy. Since sound travels through water at a constant 4,800 feet per second, we can measure the time it takes the wave to return and calculate the distance. The distance the sound wave will travel is determined by the frequency of the wave. The lower the frequency the further the sound wave will travel. The advances in the modern sounder have been in the interpretation of the raw return and the presentation of the finished product. The simplest of these devices is the depth instrument. It bounces a signal off the bottom, determines the depth, and displays the result digitally. It does not seek targets in the water; it just looks for absolute depth. Fishfinders, on the other hand, spend their processor energy trying to identify targets and their relative locations. They present the information on the screen for us to view.

Fishfinders are the most popular piece of marine electronics, and they are available in the widest range of performance and price of all boating electronics. Whether you’re running a 16' bass boat or 60' sport-fisherman, there is probably a fishfinder onboard. Today’s fishfinders have benefited greatly from the advances in microprocessor technology, sunlight viewable screens, and waterproof construction techniques. They will display the bottom depth and density, identify the depth of the fish, and present the images or raw return arches on the screen. The absolute performance of a fishfinder is determined by its power and frequency. The more power the unit has, the more likely it is to locate smaller targets and subtle changes in structure. As we said, the lower the frequency, the further the sound wave will travel. Most fishfinders use a 200 Khz frequency, which provides excellent results to about 350 feet. Many are dual frequency with the most common alternate frequency being 50 Khz. Depending on the power applied, 50 Khz units can reach depths of several thousand feet. The fishfinder display gets all the attention, but the transducer is equally important. A key point to fishfinder performance is transducer installation. The best fishfinder in the world cannot overcome poor transducer installation. Please follow the manufacturer’s instructions, and remember to order a fairing block for your thru-hull applications.

There are many different features available on fishfinders. One of the key features is color. Color fishfinders display the intensity of the signal return in various shades of color and often with a number of user-selectable palettes. The experienced fisherman can use the colors to determine the hardness and structure of the bottom, find bait schools, and even identify what type of fish are passing under the boat. The fishfinder display shows a history of what has gone under the boat. Often we want to see exactly what is happening this instant. Many units will provide this instantaneous display with a feature referred to as an "A-Scope" display. The "A-Scope" name comes from the beam angle of the transducer because it shoots down like the letter "A". It provides immediate feedback that can be used to quickly identify targets as they pass under the boat. This is a great tool for trolling and can be used to alert you to activity as you move from one fishing spot to another.

The greatest benefit to fishermen is the ability to "see" the bottom. Fishfinders offer many features to enhance our understanding of what’s down there. Almost all units present some combination of bottom lock, bottom zoom, and white line capability. Bottom lock does just what it says. It shows the last number of feet to the bottom and the structure there. The user can normally define the number of feet displayed from the bottom. Standard bottom zoom is usually 2x or 4x magnification and is presented in a window adjacent to the standard display. It is very helpful to find small structural changes and bottom fish. The bottom is usually presented as a line of varying thickness referred to as a white line display. The thickness of the line is an indication of the hardness of the bottom. A thin tight line depicts a very hard bottom while a broad line indicates a soft muddy bottom. The vegetation on the bottom is incorporated into the white line and is displayed as part of that hardness. With some practice, you will be able to read your new fishfinder and locate the ideal fishing spots.

Selecting a Transducer

Your sonar or depth transducer provide the eyes and ears for your fishfinder or depth sounder. Each depth transducer contains a piezo ceramic element that works like a loud speaker and microphone. It converts the electric energy from your fishfinder's transmitter into a high frequency ultrasound wave. This sound wave travels through the water until it hits an object, which then bounces the wave back to the transducer. The time lapse between transmitting the signal and receiving it back is used to measure the distance to the object. The ultrasound waves are almost totally reflected at the boundary between water and air (i.e. the surface). Hence, the key to good transducer performance is to position it below the surface and in bubble-free water.

Transducers are available in three types: Thru-Hull, In-Hull, and Transom Mount.

Thru-Hull Transducers – Typically mounted on large nontrailered boats, thru-hull transducers are inserted through a hole drilled in the hull of your boat. Thru-hull transducers can be made from a variety of materials, including bronze, aluminum, and plastic. They typically have a long stem that slides through the hull and is then held in place with an equally large nut. If the hull is flat, this is the extent of the installation. However, if the transducer is to be mounted on one side of a V-hull, then a fairing block is required. The acoustic signal is transmitted directly from the transducer to the bottom without signal loss. Thru-hull transducers are typically used on inboard powered hulls so the transducer can be mounted in front of the rudders, propellers, and shafts.

Transom Mount Transducers – Transom mount transducers are installed on the boat's transom using adjustable brackets. They are directly in the water, typically sticking a little below the hull. Of the four housing types, the transom mount is by far the most popular as it is useful for boats that are commonly trailered or intentionally beached. A well-designed transom mount transducer will work on almost any hull (except inboard powered boats) and at high speed. The disadvantages of this popular mounting style is that the transducer performance can be adversely affected by engine turbulence, hull style, and trim tabs. Because of disturbed water, the transducer works great when standing still, but loses the bottom while moving. Careful selection of the mounting location and angle and depth of the transducer can minimize this turbulence.

In-Hull Transducers – In-hull transducers are epoxied directly to the inside of solid fiberglass boat hulls. Don’t attempt to shoot through aluminum, wood, or steel hulls. Sound can’t pass through air, so if there’s any wood, metal, or foam reinforcement, it must be removed from the inside of the hull before installing the transducer. The sound is transmitted and received through the hull of the boat – but at the loss of sonar performance. Even still, the advantages are considerable. One, it can't be knocked off by a stump or rock. Two, it generally works quite well at high speed if it is mounted where a clean flow of water passes over the hull. Three, it can't be fouled by marine growth. Four, the boat does not need to be hauled for installation.

Fairing Blocks – Most boats have a certain amount of angle (dead rise) to their hull. If a thru-hull transducer is mounted flush to the hull, placing it at the same angle as the dead rise, the transducer will not function at its best. For accurate readings, a thru-hull transducer needs to "look down" as straight as possible. The thru-hull illustration shows a fairing block that, when cut to the angle of the hull, produces a "sandwich" effect to allow the transducer to look straight down into the water. Fairing blocks are available for most of the transducers we sell.