Material

The first step is to determine what material the transducer should be composed of.

• Plastic housings are recommended for fiberglass or metal hulls.
• Stainless Steel housings are recommended for steel or aluminium hulls.
• Bronze housings are recommended for fiberglass or wood hulls.

Bronze is preferable to plastic for wooden hulls because the expansion of wood could damage a plastic transducer and cause a leak. Installation of a stainless steel housing in a metal hull requires an insulating fairing.

A metal housing should NOT be installed in a vessel with a positive ground system.

Mounting Options

How the transducer should be mounted on the boat is also important.

• Through-hull with fairing blocks offer the best performance, especially at higher speeds.
• Through-hull flush mounts are best for trailer boats where good performance is required and there are no protrusions from the hull.
• In-hull transducers do not penetrate the hull, but do sacrifice some performance.

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.

Types of Transducers

Transom Mount Transducers

As the name implies, transom mount transducers are installed on the boat's transom, directly in the water and typically sticking a little below the hull. Transom mounts are composed of plastic and tend to be less expensive than other transducers.

Transom mount transducers are recommended for planing hulls of less than 27 feet (8 meters), such as personal watercraft and powerboats with outboard, inboard-outboard and jet drives. They are not recommended for large or twin screw inboard boats because aerated water from the propeller reduces performance. They are also not recommended for operation at very high speeds.

Transom mounts adjust to transom angles from 3° – 16°. For angles greater than 16°, a tapered plastic, wood or metal shim will be needed. However, the transducer should be adjusted so it is angled slightly forward when the boat is in the water.

In-Hull Transducers

In-hull (a.k.a. shoot-through) transducers are epoxied directly to the inside of the hull. These are only used in fiberglass hulls. In-hulls will not work with wooden, aluminum, or steel hulls, or in foam sandwich/hulls that have air pockets. Any wood, metal, or foam reinforcement must be removed from the inside of the hull.

With an in-hull transducer, the signal is transmitted and received through the hull of the boat. As a result, there is considerable loss of sonar performance.

In other words, you won't be able to read as deep or detect fish as well with an in-hull transducer as with one that's transom mounted or thru-hull mounted.

Fiberglass hulls are often reinforced in places for added strength. These cored areas contain balsa wood or structural foam, which are poor sound conductors. The transducer will need to be located where the fiberglass is solid and there are no air bubbles trapped in the fiberglass resin. You'll also want to make sure that there is no coring, flotation material, or dead air space sandwiched between the inside skin and the outer skin of the hull.

Through-Hull Transducers

Through-hull transducers are mounted through a hole drilled in the bottom of the boat and protrude directly into the water. This type of transducer generally provides the best performance.

Through-hulls are recommended for displacement hulls and boats with straight-shaft inboard engines. You'll also need a fairing block that allows the transducer to be mounted properly. Through-hull transducers must be installed with a fairing to ensure proper alignment and a secure fit.

Through-hull transducers must be positioned in front of the propeller, rudder, keel or anything else that may create turbulence. They must be mounted in a position that is always underwater and angled straight down.