Revised by BoatUS editors in April 2012
More and more boaters are using the electronic device they purchase for a fish finder also as a depth finder. This is because most fish finders provide the depth as an integral function of their operation. However you can still purchase electronic devices (depth finders) dedicated to pinging for depth. These usually display the information as a numerical digital readout. Some also provide other information, such as water temperature. But today's advanced fish finders do this and much more; therefore many boaters rely primarily on that type of device.
Fish Finders have been evolving quickly. While you can still purchase the simpler versions that essentially give you depth, basic bottom information and “fish” symbols in somewhat obscure black and white, you can now buy much more complex units with high resolution color displays that give incredible details of bottom contour and everything in between. They can give you information, for example, about the type of bottom (soft or hard) and growths or rocks or obstructions. As the quality of fish finder technology improves, the list of features available continues to grow. For example, some can show you sophisticated bottom topography. Some can show levels of significant water temperature changes, often important for some types of fishing. You can also get devices that include GPS chart plotters. It seems that each model year these devices can do more and more. Different units have different cones of sight in which they collect information. Some more advanced fish finders even have transducers that can "look around" or ahead to give you information beyond the scope of merely what's directly underneath your boat. Clearly, it's important before you buy to study the latest and greatest of what's available, carefully considering the manufacturers’ recommendations as to the products, your needs and your budget.
The Fishfinder's "Eye" Under Your Boat
A transducer converts electronic impulses from the fish finder to sound waves, which travel down through the water. When the sound waves bounce off the bottom or a fish or other target, the returning echo is picked up by the transducer, which converts it back to electrical impulses. These impulses are then sent to the fish finder where they are converted to a picture (symbol) of the bottom and other information. There are different frequencies available for different purposes. Fish finder manufacturers provide information as to range and other features.
The transducer must be mounted carefully in accordance with manufacturer instructions if you wish to get the full benefit from your equipment. The more complex the information provided by the device, the more important it is to get the transducer "just right."
Installed properly, a transducer mounted flush with the transom works well for many boats. If it isn't mounted flush, it creates turbulence that will interfere with the signal — you'll see bubbles and not fish. Benefits of using transom-mount transducers include accessibility for cleaning and maintenance, not having to drill holes through the bottom, and being able to swap transducers easily if you change fish finders. Transom-mount transducers usually work best with outboards; inboard boats typically have turbulence from struts and rudders as well as prop wash. Transom mounts can easily be installed by the average handyman. However be very careful to seal any screw holes in your transom so water can't migrate into the coring and eventually cause rot or delamination or core failure.
Through-hull transducers for simple depth finders can usually be used with shoot-through or drill-through mounting. Mounting a shoot-through simple depth finder transducer requires finding an area in a solid fiberglass hull (it won't work with wood, steel or cored hull) with has no bubbles or voids or coring in the fiberglass and which is an area free of turbulence or air in the water. Make sure the transducer is installed perpendicular to the water. Typically people build water boxes over a solid hull section in an appropriate area (where there is no turbulence outside) and mount the transducer in the water box so that it can best "see" through the hull without interference from air and other impediments. This type of installation seldom has the range and accuracy as a transducer looking through water alone, but many find it more than adequate for a simple depth finder. Some manufacturers specifically recommend against this type of mounting. However some transducers with such negative recommendations nevertheless work this way, just not as well. But use of this method is seldom if ever successful for fish finders which give all that information in addition to depth alone.
Drill-through style transducers come in many varieties. They require that you cut a hole in the bottom of your hull for permanent installation. Installation is obviously more difficult than with transom-mount transducers, but the benefit is that a through-hull transducer can be mounted on any hull. Various factors must be taken into consideration. For example, on planing V-hulls, the through-hull should be mounted aft, on a relatively flat area that won't be lifted out of the water or be affected by turbulence. On curved hulls, a fairing block may be necessary to keep the transducer pointing down (slight angles can be tolerated). For the average handyman, we'd recommend having the yard install a through-hull transducer. There are also transducers available which have a mounting flange that you install through the hull. These have a hole in the middle which contains the actual transducer typically with O ring seals. With this you can quickly insert or remove a transducer or blank plug. This comes in very handy when you have a transducer failure. With the boat in the water you'll get a quick influx of water, but most people have no problem with this.
Installation and Maintenance Tips
- Don't trim the fish finder's transducer cable to fit your boat. If it's too long, coil it. Cutting the cable will interfere with the accuracy of the fish finder's signal. Some manufacturers supply cable if you need a longer cable.
- Immediately after haulout,, clean the bottom surface of the transducer to remove slime and barnacles. If necessary, sand very lightly with 240-grit sandpaper.
- Never paint a transducer with bottom paint. Special paints inhibit marine growth without interfering with the signal.
Different fish finders have different displays and symbols and you should practice with yours, looking at known targets, so that you know how to read it quickly and well.