Experience replacing legacy transducers

[Patches]

I hauled out to replace my circa 1990 transducers which went to a B&G "Focus" speed/depth display sold as original equipment with the boat.  To upgrade to more modern transducers (200 HZ) you need to enlarge the hull opening diameter from 1 3/4" to 2".  I'm replacing with a basic Raymarine i50 depth sounder.

In thinking this through beforehand, I thought I would be able to remove the transducers from their thru-hull housings, back off the nuts inside, give them a whack with a mallet from above, and they (and their ancient sealant) would pop out. Then, my plan was to double up two hole saws (1 3/4" inside to act as the new "pilot", 2" for the new hole) and just drill through the bottom.

The plastic nut for the speed transducer backed off with some taps with a hammer and a short piece of hardwood to the flanges on the nut.  The nut on the depth transducer housing would NOT budge. No amount of persuasion with the mallet from above would dislodge the speed transducer housing despite removing the nut.  Thus my whole plan for using the doubled up hole saw was foiled.

I went up in the boat and got two wooden plugs which are normally tied off to their thru-hull valves in case of emergency.  I inserted these in the housings from below, gently tapped them in until they were seated, and then cut the protruding portion off (a little proud) with a multi-tool saw.  Then I used the 2" hole saw—which now had a solid "center" for the pilot bit to grab—to cut the new hole.  I prepared the new hole (sanding back bottom paint, smoothing the hole sides, and removing left over reside from the backing plate inside the hull), added the sealant to the new housing flange and bottom part of threads, and then pushed it in and tightened from above.  You will need a helper to add a bit of pressure to the new transducer housing from below (to prevent it from spinning) while you tighten the new locking nut from above.

For anyone thinking of doing this, I recommend this technique (inserting the wood plugs into the old/legacy 1 3/4" housings from below so the pilot bit from your 2" hole saw can center the new cut). Don't even bother with trying to remove the locking nuts from the old housing.  It will take two minutes with the hole saw and you're ready to prep and secure the new transducer housing in your boat.

Patches