"Through-hull" Transducer 
"Transom-mounted" Transducer
General description of a marine transducer: A marine transducer is best described as an electronic sonar device mounted to a vessel that converts input signals of one kind into output signals of another kind, such as sound waves to electrical waves. In the case of fish-finder or depth finder transducers, an electronic ping sounding is emitted from the transducer, bounces from an object which it intersects, and is instantaneously received by the transducer. An image below the boat is detected from the emitted and returned “pings” and is translated to an electronic image recorded by the onboard electronics, oftentimes in the form of an image. The result is generally made visible on the screen of an on-board monitor. Boaters and fishermen use transducers mounted on or in their boats to detect images of fish, bottom structures, topography, depth of water and generally aid in navigation by locating images below the bottom of a boat. And, of course, there are many different types and models of equipment to suit most any user requirement. Many boat owners and fishermen are now equipping their boats with multiple transducers and electronics which perform various functions. The method and type of mounting a transducer used on boats oftentimes becomes very confusing. There are typically only 3 types of methods of mounting a transducer in a boat. Transducer installation and optimization comprise the majority of all marine electronics troubleshooting issues.
The "transom-mounted" transducer is externally mounted on the transom whereby the face of the transducer makes direct, unobstructed contact with the water. Transom-installed transducers are the most effective, the easiest to service, easiest to adjust for an optimum sounding, easily replaceable/upgrade and, of course, the most popular. This type of transducer installation allows for maximum performance of your electronic equipment as designed by its manufacturer. To mount a transom-mounted transducer, screw-holes are required by drilling holes below the waterline of your boat. Review and understanding your boat warranty "Exclusions/Limitations"- "Alterations and Modifications" clauses and disclaimers made by your boat manufacturer and the electronics manufacturer is highly recommended before attempting installation of this type of transducer (regardless the age of your boat). Many boat owners and boat dealers do not consider the multitude of problems which are most likely to occur, once the transdcuer is screwed to the transom. Oftentimes they can be very difficult to adjust properly. There is an abundance of advice on "how to drill holes in the hull of your boat" to mount a transducer by this method. Drilling screw-holes in your boat hull is not sound advice. Should you suspect water in your boat hull, transducer scews will be the first to be removed by your adjuster. Then what? It is not uncommon to find many screw holes in the transom of a boat made by boat owners simply by changing equipment or seeking a "better" location.  It's a good idea to understand exactly what you're getting into before chucking up your favorite drill bit. That can be the beginning of  an unexpected journey to boat devaluation. Ask your dealer for his written liability statement before he begins installation....that will give him the much needed opportuity to change his mind. And that "ill feeling" you get in the pit of your stomach when you are about to begin drilling...well, it's telling you something... Very Important!
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"Shoot-through" the Hull Transducer  -  "In-hull" Transducer
The "Shoot-through" the hull type transducer is mounted inside of the hull, usually in a bilge, sump or keel, permanently adhered to the cockpit side of the hull with an epoxy or may be contained in a liquid reservoir (generally a density inconsistent with that of water). The sonar ping emitted from the transducer is expected to sound adequately by penetrating a thick solid unblemished hull material. Hull substrate material can absorb, muffle or distort much of the sounding from a "shoot-through" transducer. In the hull FRP layup, there may be open voids, uncured or unsaturated glass mat, resin bubbles, uneven/overlapping glass mat, etc., none of which can be detected prior or even subsequent to the installation of a "shoot-through".  A "shoot-through" sounding can be much like "shouting into a pillow". Adhesive medium (ordinarily a different density than the hull) used to secure the transducer in these types of installations, actually increase adverse acoutics through which the "ping" must penetrate. The return "ping" must also transmit on its return trip unfettered through the hull and adhesive medium. Your electronics can confuse, or even refuse, the return sonar ping. The transducer has no contact with the water outside of the boat. "Shoot-thrus" are generally installed to avoid screw-hole damage in a transom below the waterline, for transducer protection while beaching or fishing stumps, logs and submerged debris. Many "shoot-through" users will settle for reduced transducer sounding to avoid sacrificing the integrity of his/her boat hull and, of course, it may not work at all. Other compromises are made in regards to eliminating a speed wheel and recording water temperature. A "shoot-through" transducer cannot be adjusted, serviced or changed without the likelihood of destroying the transducer during removal. Eliptical designed external high speed transducers may be used as a "shoot-through". "Puck" type transducers are flat on the hull side and may be substituted for a transom-style transducer and generally incorporate identical electronics. There is an abundance of advice on "how to install a "shoot-through"installed transducer. Transucers are designed by the makers to sound at optimum when making direct, unobstructed contact with the water. All else is a compromise.
A "Through-Hull" transdcuer is permanently installed in a BIG "through-hull" hole made completely through the bottom of a boat hull. The face of the transducer is in direct contact with the water. A "through-hull" is usually “faired” to the hull to offset deadrise (angles) so as to cause its beam is caused to become parallel to the surface of the water. This transducer is fixed, cannot be adjusted, and generally not found on trailer boats. They are usually the most expensive to purchase, to install and to replace and are usually found on larger boats. When installed in a trailer boat hull (generally used in more shallow protected waters) the "through-hull" transducer may become vulnerable to damage from trailer loading/unloading, beaching and underwater obstructions.The "through-hull' transducer generally can be efffective when installed properly and faired true. These are the most permanent of all types of transducers. 
Boat hulls built of aluminum and other metals are dense and reverberate an echo out of tune from pulsations made and returned by a "shoot-through" transducer causing erratic feedback....something like "shouting into a cavern". The return signal can easily become "confused" and misunderstood when it strikes the metal hull and attampts to convert it to an electronic "language" your marine electronics will understand. The confusion created will be evidenced as "garble" on your onboard monitor.
Aluminum Boat Hull - "Shoot-through"  / "In-hull Transducers
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SternMate™ transducer mounting system can easily be installed on most all boats. Some of the types of boats that use transom-mounted transducers for fish-finders, depth-finders, sonar and fish locators are:

How  to mount a transducer on: All-Purpose Fishing Boats    Aluminum Fishing Boats    Fish and Ski   Bass Boats    Bowriders    Flat Boats    Bass boats    Center Console  Closed Bow Runabouts  Welded Boats   Center Console   Closed Bow Runabouts    Center Console    Closed Bow Runabouts    Cuddy Cabins    Deck Boats    Houseboats     Dinghies   Electric Boats    Multi-Hull Power Boats    Catamarans    Jet Boats   Performance Boats    Pontoon Boats    Stern Drive boats    Cruisers    Skiffs    I/O Inboard-Outboard Walkaround boats   Sailboats    Runabouts    Outboard boats    Electric boats    Row boats    Fishing boats     Fiberglass boats  Steel boats  Painted Wood boats   
Marine electronics manufacturers of fish-finder, transducers, depth-finder, sonar and fish locator, transducer mount equipment:​
EAGLE    Furuno    Garmin    Humminbird   Lowrance    Norcross Marine  Northstar TechnologiesSI-TEX    Raymarine    SIMRAD    Northstar    Tacktick    Uniden    Navionics    Standard Horizon   ​
Disclaimer: The aformentioned marine electronics manufacturers do not endorse SternMate products. 
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Nanticoke Maritime LLC

3 Types of Marine Transducer Installations

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