Transducer Tricks

Brian Naznitsky of Kings Park, New York, asks:

“I recently purchased a depthsounder and want to mount its transducer inside the hull of my boat. The manufacturer recommends I flood my bilge and move the transducer to different positions, temporarily holding it in place with a bag filled with sand, until I find the best spot to permanently mount it. Filling the very slack bilge of my boat with water is not something I want to do. The water will certainly lap over the cabin sole when the boat is slightly heeled. Is there a putty or temporary adhesive I could use instead? Also, how do you recommend the transducer be permanently mounted inside the hull?”

Gordon West replies:

Filling the bilge with water won’t be necessary. Here is a much easier way to find the “sweet spot” for an inside-the-hull transducer mount. First, you want a spot clear of the keel, preferably forward of it. It should also be a spot where the hull is solid fiberglass with no core or voids.

To test locations, go for a sail. Get into about 20 fathoms of water and temporarily dangle the transducer over the side to see if you’ve got a strong bottom reading. Once you get a strong reading, head for the bilge and put your transducer in a plastic baggy filled with sea water. Hold the transducer in its baggy so it points straight down and squish the bag against the hull. Move it around and have someone on deck report when it is reading best. Again, make sure the transducer is pointing straight down, not at the hull’s deadrise angle.

Once you find a sweet spot that yields good sounder readings, mark it and head back to the dock. Now build a simple chest for the transducer (a short length of PVC pipe cut at an angle often works well), bond it to the hull, and fill it with mineral oil. Install the transducer in the chest pointing straight down and go for another sea trial.

With this sort of in-hull installation, your depthsounder’s accuracy will not be affected, only its maximum range, which may be decreased by 25 to 45 percent.

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