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Pearson 28-2
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Discussion Starter · #1 · (Edited)
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...
 

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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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‘77 Pearson 10m
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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?
 

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Barquito
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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?
I think Star Tracker was suggesting a 12:1 bevel. That would make it plenty strong.
 

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Pearson 28-2
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Discussion Starter · #5 ·
I read it the same as Barquito and the scarf is there for that reason. But I am no fiberglass expert by any measure of delusion.
 

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If you seriously don't want to reglass, then getting a machine shop to make you a bushing with a flange is your next stop.
OD 2", ID 1", Bronze, Aluminum, Stainless are your options. Or a high density plastic.
I'm sure somebody makes a through hull with these dimensions.

 

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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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With that transducer, it is possible to do like you suggest with an inside glassed backplate and a 1" hole drilled through it and still be safe. The transducer width will span the 2" hole by quite a lot, and the two 3/8" mounting bolts 7" apart will make it strongly fast to the hull. There won't be a safety or structural issue with this, as long as you get the backing plate well-glued/glassed to the inside hull and fill the 2" hole from the outside with thickened resin before drilling the 1" hole.

However, the right way to do this is to glass it over as suggested above. A 12:1 bevel is the "correct" way, but really is overkill for glassing a thruhull hole inside and out. Particularly so for this application. So if you can't get 12:1 because of deadrise curve, chine, or some other reason, don't sweat it. Like mentioned, this is a fast and easy job.

It doesn't look like there is a good way to make a bushing for that transducer that would be quicker or cheaper than the two options above.

Mark
 

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One of None
Hunter 34
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Aren't there optional transducers for the unit you're thinking about getting many other manufacturers have choices of transducers you may be able to find something with the same operating parameters that actually fits the opening!
 

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Pearson 28-2
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Discussion Starter · #10 ·
Comparability as listed for my plotter is the limiting factor.

Great insights and info from the team.
 

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You could just glass in the hole and mount transducer in the inside if hull is solid fiberglass. I did this when I had problems with my built in one, I bought a cheap one and mounted the transducer with 4200 and temporarily mounted the instrument while my old one was sent to the factory to be repaired. You can test it this will work by filling a plastic bag with water (absolutely no air) and rest the transducer on the bag.
The key is there can be no air pockets, so if hull is not solid fiberglass it won’t work, that’s why the plastic bag test can’t have any air whatsoever, best to fill it by submerging the bag.
 

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You could just glass in the hole and mount transducer in the inside if hull is solid fiberglass.
Aren't there optional transducers for the unit you're thinking about getting many other manufacturers have choices of transducers you may be able to find something with the same operating parameters that actually fits the opening!
This is a specialized CHIRP transducer that is more than just a depth sounder. It can't be mounted inside, and other manufacturers sell the same one (with different plugs).

Mark

 

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One of None
Hunter 34
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Well assuming the bottom of the boat has a slight curve to it even where the transducer holes are and you will most likely use epoxy and cloth and maybe even g10 on the inside given all that and if the substrate is actually dry it would be pretty hard to fail making an inside overlay plus a feather edge filler with the new hole for the transducer size. I mentioned the curve because there is great strength in curves.
 
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