Depth Sounder/Fishfinder Installation

Project date: 2007

The next logical step in the development of the Navigation system is the installation of a digital depthfinder/fishfinder. The DSM-300 is a matching component produced by RayMarine. Since this installation, RayMarine has made available a DSM-30, which is a bit less expensive, but has about the same feature set. While many devices in this project are network attachable (i.e. NMEA2000), the data throuput requirements of the depth finder are such that it must be directly connected to the display unit.

We're continuing to add items to the wiring board under the helm Here you can see the grey DSM-300 depth sounder box mounted to the board as shown on the left side of the photo. Whenever you can, you should try to dress all of the wiring off so that it is neat and organized. While this is not always possible, the goal here is to leave the wiring in better condition than it was when you started.

I actually ran out of mounting board space, so I added a panel to it. It attaches with thumb screws so that I can access the area behind the panel if I need to.



The depth sounder is wired to one of the switches on the Aux switch panel at the helm.

Whenever you are running DC power wiring, you must be mindful of the potential voltage drop and size the wiring accordingly. This is especially true with electronic equipment. If you do not understand this concept, please review my article on High Power DC Wiring:

High Power Wiring

While the DC power wiring to the depth sounder may not technically be classified as high power wiring, the concept holds true.

The depth transducer used for the DSM-300 is an AirMar P79 In-Hull model.

While I am adept at drilling holes in the boat, I still cringe if I have to drill holes through the bottom of the boat. The AirMar P79 transducer is designed to transmit through the hull of a non-cored boat with little loss of performance as fiberglass is "transparent" to ultrasonic signals. The P79 is a very popular transducer, and is used on several different brands of depth finders.

One advantage to the P79 is that if the location is not ideal, you can easily move the transducer to another area in the hull as there are no bottom-holes to patch.

To install the transducer's mounting ring, locate an area on the hull, then sand the area to smooth out the bottom. Then coat the bottom of the ring with a liberal amount of GTE Sealant I or II as called for in the installation instructions, and glue the ring to the boat hull.

Here the transducer's ring is glued to the bottom of the hull. The manufacurer recommends GE Silicone I or II rather than 3M 4200/5200 sealant, as you will be filling the ring with either mineral spirits or Pink RV Antifreeze. The transducer will then fit on the ring. The liquid acts as an acoustic coupler between the transducer and hull bottom. If you use mineral spirits, it can attack the 3M 4200/5200 sealant.

The manufacturer also recommends against epoxy as well, as it is brittle and can crack over time. In either case, if the liquid leaks out, the transducer performance will diminish.

I used Pink RV Antifreeze as I had some left over from last season's winter layup.

The next step is to place the transducer on the ring according to the installation instructions. One nice feature of this transducer is that by rotating it on the mounting ring, you can match the deadrise angle of the hull so that the signal is "shooting" straight down. This is not always the case with thru-hull transducers, and they may introduce some error introduced by the angle of the hull bottom.

The next transducer to install is the AirMar ST69, which is a transom mounted combination speed paddle wheel and temperature probe. As it mounts below the waterline, I used a technique to waterproof the transom by drilling the mounting hole large, filling it with epoxy, then drilling the correct size hole. You can see here the "ring" around the screw threads is in fact cured epoxy. This is an absolutely safe and watertight method.

At this point, you may wish to review my article on drilling holes in fiberglass (and more importantly, properly sealing them):

Drilling holes in fiberglass

While it is against the manual's recommendation, I used a transom-mount paddlewheel on an inboard boat. But I had no choice as there was no other suitable location. The reasoning behind this is the speed paddle wheel may give a false reading as the propeller's thrust may be added to the boat speed. At any rate, I can compare the speed sensed by this transducer with the GPS speed. Otherwise, the temperature probe will still be accurate.

After everything is interconnected, its time to connect the DSM-300 to the C-80 MultiFunction Display and make sure it works. If it does, you should see a screen similar to this. We're sitting in 5ft of water (under the transducer).

Note that the transducer is measuring the depth below it, not the true depth of the water. Since the transducer is about 2ft below the waterline, the actual depth is closer to 7ft.

To obtain an accurate water-depth reading, you would have to add the distance from the waterline to the depth of the transducer to the transducer's reading. Fortunately, the MultiFunction Display has provision to enter in an offset distance so that the depth reading will add the distance the transducer is below the waterline and show the true depth. The offset is typically manually entered, so consult your owner's manual.

(Click on the drawing for a full sized version)


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