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I have 2" holes in my hull for the transducers. The B150M that goes to my plotter decided to quit. I can easily just swap in another next haul out, but I have been mulling the Garmin GT23M or similar scanning type transducer. Most of the water I plod is on the shallow side. The beam angle of the B150M is pretty narrow in shallow water, and while yes, I ultimately just need the depth info of what is immediately below my keel, having a larger picture for situational awareness to compare against charts could come in handy. Some of the shoaling around here can be a surprise. If I go this route, I'm faced with a 1" threaded stem on the GT23M for a 2" hole. Aside from glassing over the hole and redrilling it the proper size, is there a proven way to sleeve or collar the 1" stem? I have some ideas in my head such as a large backplate epoxied to the inside of the hull with a 1" hole and copious sealant. The backface of the GT23M transducer is wide and itself could be sealed against the outside of the hull since I think my deadrise is within tolerances to not need the fairing block.

This is just my initial thinking out loud. I may just end up swapping in a new B150M since that's worked for as long as I've had the boat and is the easiest and least expensive solution. But I am curious about what one does, aside from reglassing and drilling a new hole, when putting an undersized stem into a hole in one's hull...
That glass job should be <4 hours if you aren't chasing a perfect finish.
Remove fitting
Grind inside for a surface patch
Grind outside to inside lip at 12:1 bevel.
Mylar over inside hole with a few layers of tape to stiffen
glass outside proud in one shot, let it go to green cure stage
Remove mylar inside, lay up interior patch that spans repair and out onto the hull tapering off your layers.
Roll this more gently since gravity is now on your side and you don't want to stress the green repair.
add a heater in the locker, give the outside a few min with a heat gun to get it warm not hot.
Have lunch
Fair out the patch, drill, epoxy paint etc.
 

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I would be worried about the strength of this patch, given that the new material will only be a 1/4 inch rim of new glass around the inside of the previous hole. Is this a valid concern?
12:1 is based on hull thickness, so say hull thickness is 3/8", 3/8*12= the bevel width around the hole. beyond 12:1 you get diminishing returns, below 8:1 I look at epoxy.
The key is to do the steps as listed in the order listed, the internal patch which you simply make quite a bit larger than the hole and you have no real issue in thickness so you don't grind it back down to the hull thickness. You can push the bevel a bit smaller if needed this way, if you for example make this inner patch 6" across, that gives you about 28 square inches of bond surface, polyester on polyester lets call it 200PSI because prep might not be perfect. With no outside patch, just to delaminate that inside patch, you'd need an impact that can deliver over 5000lbs of force to delaminate it, so now it becomes a calculation of the strength of this inner layer.

By doing this inside and outside in essentially one shot, you now have a chemical bond between the inner and outer, both acting as plugs, bigger at the edge than where they meet. An impact that would drive the thru hull up and in must both push your outer repair through the hull, and delaminate this inside repair.

Long explanation, simple job.

Bushings made in plastic etc make me cautious. It doesn't take that much force to disturb them, and to go from 2" down to 1" you're looking at a very thick wall, and the outer edge of the transducer not being supported on the hull but entirely on the bushing. Doesn't take a massive impact to disturb that. Plus many buyers might find it peculiar down the road.
 
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