[Bird Dog]

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...

 

[StarTracker]

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.

 

[AWT2_Sail]

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?

 

[Barquito]

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.

 

[Bird Dog]

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.

 

[CapnBilll]

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.

Perko 0322DP8PLB Thru-Hull Connection - 1.5" Pipe : Boating Plumbing Fittings : Sports & Outdoors

Perko 0322DP8PLB Thru-Hull Connection - 1.5" Pipe : Boating Plumbing Fittings : Sports & Outdoors

www.amazon.com

 

[StarTracker]

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.

 

[colemj]

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

 

[deniseO30]

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!

 

[Bird Dog]

Comparability as listed for my plotter is the limiting factor.

Great insights and info from the team.

 

[teejayevans]

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.

 

[colemj]

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

 

[deniseO30]

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.

 

[Den]

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...

It’s called a helicoil. A threaded chunk with a threaded hole in it. Most are aluminum so bury it in Epoxy or it will sacrifice its self. Add a zinc nut on the inside on top of real fastener. It’s possible to 3D print the piece in a 20% poly carb in black white or grey and then bond it permanently to the hull. Kinda easy 20 minutes in software called fusion 360 and off to a local printer. Fusion 360 is free on the internet. When designing in a ASA or Poly Carb you’ll have to allow for .009% shrinkage. So when you draw both threads inside and outside tell the software to increase all dimensions by .009 %. The part will thread in tight. The part printed in ASA will be as strong as cast aluminum.

 

[Bird Dog]

Den, well familiar with helicoils on the smaller fastener scale. That's a fantastic idea for this application (and an excuse to someday get a 3D printer for my own additive manufacturing bright ideas). In this case it wouldn't need to be threaded on the inside since it just needs a properly sized inner diameter for the threaded shaft of the transduced to pass through, which is subsequently mechanically "clamped" by the nut. But this could be a very clever method to make a larger hole a smaller hole below the waterline on this scale, especially since in this case, inside the hull also has a 3/4" thick ply backing plate for added strength as a part of the transducer mounting location.

Stated conversationally, not trying to be a smarty pants, but wouldn't putting a zinc nut on the inside of the boat be utterly useless as an anode? It won't be in the water...

 

[Den]

Den, well familiar with helicoils on the smaller fastener scale. That's a fantastic idea for this application (and an excuse to someday get a 3D printer for my own additive manufacturing bright ideas). In this case it wouldn't need to be threaded on the inside since it just needs a properly sized inner diameter for the threaded shaft of the transduced to pass through, which is subsequently mechanically "clamped" by the nut. But this could be a very clever method to make a larger hole a smaller hole below the waterline on this scale, especially since in this case, inside the hull also has a 3/4" thick ply backing plate for added strength as a part of the transducer mounting location.

Stated conversationally, not trying to be a smarty pants, but wouldn't putting a zinc nut on the inside of the boat be utterly useless as an anode? It won't be in the water...

Quit making fun of my nuts.
I’ve have a couple soft metal parts left in my bilges where anything could have a current through it. In your house your stainless steel sink attaching your brass elbow is a zinc nut. 20 years from now your zinc nut will fail. Zinc nuts hold just about every switch in every marine dash. On a transducer made from machined aluminum I use nut which comes with it and a zinc lock nut.
If you decide to buy a printer buy the Prusa 3+ about $800.00 US as a kit from Czechia. They Chinese copies are getting better but not really a work horse. The materials used in Prusa are superior. Their new XL is $2,000 and above the casual use. For boats stick to ASA and poly carb. Learn his to use the printer with PLA. PLA is made from corn and biodegrades. You’re in for a shock if you think 3D printing is easy plug n play. Each item takes a different approach. I’d go as far as running a 2” corse thread into the boat to increase the Epoxy bonding surface. Also the hole has some cell loss. So in fusion 360 pick the thread for the transducer place in a cylinder add the outside thread. Increase all dimensions .009% and bond this threaded hole to the hull.
The corse thread will double my surface bonding area and prevent vibration a straight thin seem won’t.

 

[colemj]

No way would 3D printing a helicoil-like converter be easier or cheaper or better than either the OP's original thought, or just glassing it over and remounting.

This is being way over-thought for such a simple thing.

Mark

 

[Den]

Just the opposite. It would be the strongest repair possible. A solid threaded hole in carbon fibre vs a fibreglass patch!?
Easy and fast yes. Thread it in with a bolt in the sensor hole with a coat of systems west, paint it primer and bottom paint. Thread in the sensor in bedding compound and connect.
It’s the correct mechanical repair in just about every material for a stripped oversized or damaged hole.
I’ll take that bet.
For tricks I’m going to run the project on a 3D slicer program which will estimate the cost in poly carb.

 

[colemj]

Carbon fiber would be the wrong material here because of the metal fittings that need to sit in it and the water. The OP would need to buy a printer, figure out how to use it, design the part, and make it. Then insert it. A glass repair could be done in 30 minutes and be as strong as the rest of the hull, and not a mechanical repair. No comparing the ease and cost difference, even if you believe one approach is better.

A helicoil isn't even the best mechanical repair for anything. The best would be to repair the hole so there was no structural, physical, or material difference with the rest of the piece, redrill, and retap. The reason this isn't done in most mechanical cases is that it is either not possible because of the material, or extremely inconvenient.

Mark

 

[Den]

Carbon fiber would be the wrong material here because of the metal fittings that need to sit in it and the water. The OP would need to buy a printer, figure out how to use it, design the part, and make it. Then insert it. A glass repair could be done in 30 minutes and be as strong as the rest of the hull, and not a mechanical repair. No comparing the ease and cost difference, even if you believe one approach is better.

A helicoil isn't even the best mechanical repair for anything. The best would be to repair the hole so there was no structural, physical, or material difference with the rest of the piece, redrill, and retap. The reason this isn't done in most mechanical cases is that it is either not possible because of the material, or extremely inconvenient.

Mark

Every street light in your town is bolted into helicoils or is a burial pole leaning.
Thread inserts in all industries are common. Your cell phone prolly has 3.
I think you just like to get angry.
Poly carb will bond to the hull with its alike Epoxy. The receiving hole will equal cast aluminum without the possibility of corrosion.
The 3D society is way friendly and patient that you imply.
In software the insert would cost 2.40 Canadian if the hull is 3” thick. Likely it’s 2” in poly carb.
On a group called Cults 3D you can link to 3D artists world wide. You can get a quote for any 3D part in any material. The owner is looking at a Fat 22$ bill plus postage for two inserts in poly carb. ASA and Nylon great marine plastics would not be appropriate.
In every corner of the planet is a 3D printer networked. Its jam packed with leading engineers. The very ppl who build the boat you sail are on it.
18 of these printers are at leading Yacht Clubs. Hinckley Yachts has one in their office. All for private use.
A fibreglass patch is equal to hammering in a cork. Would indicate zero knowledge of fibreglass.