Fierro's in-hull guide

There is a waaaay simpler, cheaper way to do this, i.e., shoot thru the hull. I can't claim credit for the original idea, as I found it in a long forgotten post on the internet some years ago. I did "perfect" the procedure since as stated the original solution had some problems.

I presently use this method with a dual beam fishfinder (Humminbird Matrix ) and it works just fine (third season), shooting thru the very bottom of a Deep V (non cored) Hull (1969 23 ft ChrisCraft Lancer)

Materials and tools:
 * Block of "Duct Seal" (Home Depot or your favorite other store)
 * Mineral Spirits (to clean inside of hull)
 * Paper towels
 * One pair disposable gloves
 * Coarse sandpaper
 * Sharp, sturdy knife (a box cutter will work)
 * piece of Aluminum foil

Best done on a warm sunny day...


 * 1) Find spot in hull where you want to mount transducer.
 * 2) Remove duct seal block from wrapper and wrap it in aluminum foil tightly. Put it in a  sunny spot to soften slightly.
 * 3) Have lunch
 * 4) Clean spot in hull (at least 4x4 inches) VERY VERY well with mineral spirits and use sandpaper to remove any fiberglass bits that may be sticking up.
 * 5) Clean again.
 * 6) Wait for Mineral Spirits to dry. Wipe away any residue.
 * 7) Remove block from foil
 * 8) Cut block in half (across the narrow dimension).
 * 9) Carefully trim off about 1/4 inch off of the other five surfaces of the block (block tends to form a skin that causes problem, you should see a non-shiny surface on all six sides).
 * 10) Form the block into a sphere with a smooth surface (wear disposable rubber gloves if your hands are the least bit dirty or oily to do this...very important!)
 * 11) Place duct seal sphere into hull in desired location.
 * 12) Locate transducer (make sure its clean and oil free!) on top of sphere. Some transducers have a fore and aft, make sure you are correct.
 * 13) Slowly and with a fair amount of force, press the transducer down until the duct seal sphere deforms and conforms to the hull and also wraps around the full active face of the transducer. When done the top surface will be somewhat "dished".
 * 14) Test Sonar,IF satisfied, you can now carefully press the outer edges of the duct seal block where it contacts the hull so that it meets the hull at a shallow angle. Pressing the block down initially leaves the block with a negative angle all around at the hull that can trap water and freeze.

If there is any skin on the duct seal or oil on the hull or on the transducer or your hands, it won't work. Its removeable at any time with no tools.

Yet another guide using Duct Seal, but final glue with epoxy
If you are going to go to a shoot thru-hull mount I'd ask your dealer where the best location is or do an on the water location search yourself. What has worked well for me is to make a dam with duct seal. Dam= a small circle of duct seal slightly bigger and higher than your transducer. Then add some water in the dam, then I set the transducer in the water and molded a Single thick band of duct seal over it so it couldn't move. Then I ran around the lake and watched my sonar unit for peak performance at all speeds. The duct seal is easy to relocate. Make sure you use a slight bit of oil when kneading your duct seal to make your dam and it will stick real nice to the hull, but you can still move it around if need be. Once I've located the sweet spot I take a permanent marker and marked the spot. Then when I got ready to mount I sanded that spot really good with a medium sand paper and then a fine. Then I vacuumed the area real good and damp mopped it. Allowed to dry thoroughly then applied my epoxy as per the Mfg instructions.