Choosing a Transducer

Mounting Options 

First, you’ll want to determine where you intend to mount your transducer. Smaller boats with outboard or sterndrive motors can often hang a transducer right off the transom of the boat. This location is great for smaller vessels, and boats that are trailered regularly. 

Boats with inboard propulsion and high-speed boats will likely want to consider through hull transducers for even better performance. Modern through hull transducers can be installed nearly flush to the hull to minimize drag and ensure great performance at higher speeds. Internally tilted or offset elements compensate automatically for the deadrise angle of the boat’s bottom, making sure your transducer’s beams are optimally positioned.  

Larger displacement boats can also consider transducers with high-speed fairing blocks. This gives you access to larger and more sophisticated transducer options that can’t sit flush against the hull.     

For those not wanting to penetrate the hull of a vessel, in-hull transducers are another option. Basic and advanced transducers can be installed in an in-hull configuration where they transmit right through the fiberglass boat bottom. If your boat’s bottom is not solid fiberglass though, this option won’t work for you.  

Material 

Through-hull transducers are typically offered with plastic, bronze or stainless-steel housings. The material your boat’s hull is made from will be the determining factor in which you will need. 

Plastic transducers are often the most economical option out there, though today’s plastic transducers can also offer the same level of performance as their metal counterparts. Plastic transducer housings are engineered structures designed to withstand the forces and conditions of the underwater, saltwater world. Plastic through hull transducers can be used in hulls made from fiberglass, carbon fiber, plastic and metal. Plastic transducer housings cannot be installed in a wood hull. Wood swells when it becomes wet, and can generate enough force to crack a plastic transducer housing.      

Bronze transducers are normally considered an upgrade over their plastic counterparts. Bronze is a long-lasting material that is well suited for the marine environment and is utilized in many other underwater boat fixtures.  Bronze transducers can be used in boats made from fiberglass or plastic, and are required on boats with wooden hulls. The exceptional strength of the bronze through-hull fitting prevents it from being cracked or crushed by the wood hull as it naturally swells.   

Bronze transducers are specifically NOT recommended for use on any boat with a metal hull to prevent electrolytic corrosion. The interaction between the metal hull and the bronze transducer, especially in the presence of salt water, will eat away the metal hull and/or the bronze housing 

Stainless Steel transducers are recommended for use on steel or aluminium hulls, and are also compatible with all other hull materials. These transducers will sometimes come with special isolation sleeves or gaskets, or other special installation procedures to keep them from coming in contact with the metal hull. Stainless steel is a very stable material, but can still be subject to electrolytic corrosion if it comes in contact with a dissimilar metal in the presence of saltwater.   

One last thing to keep in mind if you have a vessel with a positive ground system, DO NOT install a transducer with a metal housing.  

Display 

Ensure that the transducer you select has the features that you want to see displayed: depth, speed, temperature, or a combination. 

Transducer Power 

Power refers to the strength with which the transducer sends the sonar "ping", expressed as watts RMS. Higher power increases your chances of getting a return echo in deep water or poor water conditions. It also lets you see better detail, such as bait fish and structure. Generally, the more power you have, the deeper you can reach and the easier it is to separate echoes returning from fish and bottom structure from all the other noises the transducer detects. 

Transducer Frequency 

The accuracy with which your fishfinder detects bottom and other objects is also determined by the frequency selected for the depth you are viewing. Raymarine depth transducers can be tuned to two different frequencies: 200 kHz (high) or 50kHz (low). 

200 kHz works best in water under 200 feet/60 meters and when you need to get an accurate reading while moving at faster speeds. High frequencies give you greater detail to detect very small objects but over a smaller portion of water. High frequencies typically show less noise and fewer undesired echoes while showing better target definition. 

For deep water, 50 kHz is preferred. This is because water absorbs sound waves at a slower rate for low frequencies and the signal can travel farther before becoming too weak to use. The beam angle is wider at low frequencies, meaning the outgoing pulse is spread out more and is better suited for viewing a larger area under the boat. However, this also means less target definition and separation and increased susceptibility to noise. Although low frequencies can see deeper, they may not give you a clear picture of the bottom. 

Mud, soft sand, and plant life on the bottom absorb and scatter sound waves, resulting in a thicker bottom image. Rock, coral and hard sand reflect the signal easily and produce a thinner bottom display. This is easier to see using the 50 kHz setting, where the bottom returns are wider. 

A rule of thumb would be to use the 200 kHz setting for a detailed view to about 200 feet and then switch to 50 kHz when you want to look deeper. Better yet, display both views side-by-side on a split screen for both perspectives. 

Cone Angles 

The transducer concentrates the transmitted sound into a beam. In theory, the emitted pulse radiates out like a cone, widening as it travels deeper. In reality, beam shapes vary with the transducer type and typically exhibit "side lobe" patterns. The following figures give a graphic representation of the transducer's actual transmit radiation patterns.

For the scope of this discussion, however, the idea of a cone works just fine. The signal is strongest along the centerline of the cone and gradually diminishes as you move away from the center. Wider angles offer a larger view of the bottom, yet sacrifice resolution, since it spreads out the transmitter's power. The narrower cone concentrates the transmitter's power into a smaller viewable area. Cone angles are wider at low frequencies and narrower at high frequencies. 

To simplify it, a wide cone angle can detect fish around the boat and not just those directly under it while exhibiting less target separation. A narrow cone concentrates the sound output enabling it to better detect small details, such as fish or bottom structure, but only scans a small amount of water at a time. 

Learn More 

For more information on sonar/fishfinder technology, check out this episode of Raymarine Live that explores the different kinds of sonar, transducer technology, and explains what you are seeing on-screen.