# Thru transom bolt holes, best way mount transducer bracket?



## Shallows (Mar 29, 2020)

My skiff has two small 11/64 holes all the way through the transom for a basic screw on plastic Humminbird transducer bracket - the last owner mounted very poorly, so I am re-doing everything.

I have cleaned up the holes, drilled them out a bit, so I can seal everything good - but not sure the best way to do this; I am thinking maybe put bolts facing out - so the nut is on the transducer side and then epoxy the bolts into the holes? But I also have 3M 4200 and 5200 if better. What would be the best way to do this?


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## castagain (Apr 7, 2020)

Have you consider this - https://www.seaworthyinnovations.com/sternpads


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## Shallows (Mar 29, 2020)

castagain said:


> Have you consider this - https://www.seaworthyinnovations.com/sternpads


Thanks, that is interesting - but there are already holes drilled, so not sure it would benefit.


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## TheFrequentFlier (Feb 17, 2016)

Shallows said:


> Thanks, that is interesting - but there are already holes drilled, so not sure it would benefit.


Maybe plug the holes with 5200 or something more robust if you want to do a larger glass/gelcoat repair job, then put the plate over it... at least has to be better than just using 5200 alone and exposed directly to saltwater which will break down over time.


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## JC Designs (Apr 5, 2020)

Thicken up some epoxy, over drill the inside skin and core leaving the outside hole as small as it is now. Tape over the hole on the outside then fill from the inside with thickened epoxy. Once the epoxy cures, re drill your mounting holes being sure to not drill all the way through again! This will create an epoxy barrier around your fasteners that won’t leak. You can now use the 4200 to seal the water from your mount holes


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## Shallows (Mar 29, 2020)

JC Designs said:


> Thicken up some epoxy, over drill the inside skin and core leaving the outside hole as small as it is now. Tape over the hole on the outside then fill from the inside with thickened epoxy. Once the epoxy cures, re drill your mounting holes being sure to not drill all the way through again! This will create an epoxy barrier around your fasteners that won’t leak. You can now use the 4200 to seal the water from your mount holes


That seems like a good idea, but my transom is very thing - maybe only 3/4", so this is how the holes happened in the first place as previous owner used 3/4" screws. I am not sure that 1/2" depth screws would have enough strength to hold the transducer - its like 4" pretty big, but perhaps someone can comment on this. 

The other option would be to do what you are saying and then just drill all the way through again, which at least would hopefully seal water from entering transom.


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## Shallows (Mar 29, 2020)

tgjohnso said:


> Maybe plug the holes with 5200 or something more robust if you want to do a larger glass/gelcoat repair job, then put the plate over it... at least has to be better than just using 5200 alone and exposed directly to saltwater which will break down over time.


I have thickened marine epoxy and epoxy cloth, I can fill the holes properly if I have to.

What I am unsure about, is how to seal the holes properly with a stainless bolt inside of them - especially one which I may need to loosen it's nut at some point to adjust transponder, its a bit tricky!


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## JC Designs (Apr 5, 2020)

Shallows said:


> That seems like a good idea, but my transom is very thing - maybe only 3/4", so this is how the holes happened in the first place as previous owner used 3/4" screws. I am not sure that 1/2" depth screws would have enough strength to hold the transducer - its like 4" pretty big, but perhaps someone can comment on this.
> 
> The other option would be to do what you are saying and then just drill all the way through again, which at least would hopefully seal water from entering transom.


A half inch of screw in epoxy will hold a lot more than you think. You could also build a little dam on the inside and backfill with thickened epoxy if you feel the need but I wouldn’t go through all the trouble just to drill all the way through again. I would do as I said, then drill and tap using machine screws.


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## State fish rob (Jan 25, 2017)

I used super glue to seal wood inside hole .3/4 transom ,then carry on. Saturate each drill hole , it will draw into raw plywood. May want to redrill to clean out old wood . I filled holes w 5200. Been over a decade w no problems.


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## lemaymiami (Feb 9, 2007)

An alternate repair is to drill out the holes - all the way through to accept short hardwood dowels (just larger than existing holes) then epoxy them into place just below the level of the gelcoat... allow to harden a day or so then come back, lightly sand the area around the repair(s) and use matching gelcoat to complete the repair (thank you John Greviskis since it was shown on one of his shows years ago...)... Once that's done you can install your new transducer wherever you prefer (if it was me I'd not want to drill anything into the repaired area). As already noted 3/4 inch laminate is a lot sturdier than you'd think and there will never be a lot of stress on that transducer so you could use short screws into a 5200 caulked pre-drilled holes to mount that new unit... 

You can always, if you prefer - build up the interior of that transom where the mount will be before doing any transducer mounting if you prefer... 

Hope this helps - and "aren't boats fun" ?


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## DuckNut (Apr 3, 2009)

Read your owners manual and see if it can be mounted to the hull on the inside rather than the outside.


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## Shallows (Mar 29, 2020)

I looked at this "Stern Pad" again which was mentioned in this thread and I am going to consider it, but it also may be perfect (as its around 4.5") for mounting accessories inside my boat like my gps/depthfinder - though I'd prefer to just buy a similar composite block like this and epoxy it on - rather than use VHT tape; does anyone sell little finished blocks like this for mounting accessories that you can epoxy in place?


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## Shallows (Mar 29, 2020)

JC Designs said:


> A half inch of screw in epoxy will hold a lot more than you think. You could also build a little dam on the inside and backfill with thickened epoxy if you feel the need but I wouldn’t go through all the trouble just to drill all the way through again. I would do as I said, then drill and tap using machine screws.


So I looked at things closer today and actually the transom in this area is only 1/2" thick - a few inches higher its 1.5" thick - they just taper it down thin in the lower area, I assume so the Gardboard plug fits easier. Therefore, my only real option is thru-hull bolts - I like you're ideas though, want to do something along the lines of coating with epoxy first, then perhaps 5200 the washers on the bolts.

Another option is this "stick on" stern pad idea, I just don't know how much I would trust VHT tape - may not be opposed to just epoxy, or 5200 on something like that though.


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## Shallows (Mar 29, 2020)

lemaymiami said:


> An alternate repair is to drill out the holes - all the way through to accept short hardwood dowels (just larger than existing holes) then epoxy them into place just below the level of the gelcoat... allow to harden a day or so then come back, lightly sand the area around the repair(s) and use matching gelcoat to complete the repair (thank you John Greviskis since it was shown on one of his shows years ago...)... Once that's done you can install your new transducer wherever you prefer (if it was me I'd not want to drill anything into the repaired area). As already noted 3/4 inch laminate is a lot sturdier than you'd think and there will never be a lot of stress on that transducer so you could use short screws into a 5200 caulked pre-drilled holes to mount that new unit...
> 
> You can always, if you prefer - build up the interior of that transom where the mount will be before doing any transducer mounting if you prefer...
> 
> Hope this helps - and "aren't boats fun" ?


Thanks and what you said makes sense, unfortunately I measured today and the lower transom is only 1/2" thick in the lower area - so my only option is thru-hull bolts, or some kind of stick on stern pad; and if I go the thru-hull bolt route I mine as well just use the existing holes.


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## Water Bound (Dec 12, 2018)

I’ve used the Stern Pad on 3 different boats now, they are great. When the transducer is going back in the same place, I’ve filled old mounting holes with epoxy and then covered with the Stern pad, looks very clean. The 3m adhesive they use is no joke. They have some YouTube videos demonstrating the strength you should check out before worrying about epoxying something. That being said I do think there is one called the Stern Saver which is an epoxy version.



Shallows said:


> I looked at this "Stern Pad" again which was mentioned in this thread and I am going to consider it, but it also may be perfect (as its around 4.5") for mounting accessories inside my boat like my gps/depthfinder - though I'd prefer to just buy a similar composite block like this and epoxy it on - rather than use VHT tape; does anyone sell little finished blocks like this for mounting accessories that you can epoxy in place?


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## JC Designs (Apr 5, 2020)

Shallows said:


> So I looked at things closer today and actually the transom in this area is only 1/2" thick - a few inches higher its 1.5" thick - they just taper it down thin in the lower area, I assume so the Gardboard plug fits easier. Therefore, my only real option is thru-hull bolts - I like you're ideas though, want to do something along the lines of coating with epoxy first, then perhaps 5200 the washers on the bolts.
> 
> Another option is this "stick on" stern pad idea, I just don't know how much I would trust VHT tape - may not be opposed to just epoxy, or 5200 on something like that though.


I would personally, build a dam and fill with epoxy to make a small backer section before I through bolted. That said, 1/2” plywood on average has 270lbft of pull out strength with a #10 screw and 240lbft of pull out strength with a #8 screw. So unless you plan on using the transducer for a ladder you’d be fine. Even so, the plastic would most likely break before the screw pulled. Carry on, James


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## lemaymiami (Feb 9, 2007)

Remember as well that one of the great advantages of building a fiberglass boat is that you can always reinforce (build up from the inside) any area that's not strong (or thick enough...). Not hard at all to reinforce that area properly so that decent screws mounting a transducer to your transom actually have something to bite into...


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## Shallows (Mar 29, 2020)

JC Designs said:


> I would personally, build a dam and fill with epoxy to make a small backer section before I through bolted. That said, 1/2” plywood on average has 270lbft of pull out strength with a #10 screw and 240lbft of pull out strength with a #8 screw. So unless you plan on using the transducer for a ladder you’d be fine. Even so, the plastic would most likely break before the screw pulled. Carry on, James


I agree that a 1/2" #10 screw would be fine - but a 1/2" screws tip will just barely pierce the entire transom; I could build up the inside though like you say - I did buy some epoxy cloth mat and I have marine epoxy, so could throw some layers of epoxy inside - will just have to read up on this, worked with fiberglass cloth and rebuilding in the past, but not epoxy.


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## Shallows (Mar 29, 2020)

lemaymiami said:


> Remember as well that one of the great advantages of building a fiberglass boat is that you can always reinforce (build up from the inside) any area that's not strong (or thick enough...). Not hard at all to reinforce that area properly so that decent screws mounting a transducer to your transom actually have something to bite into...


Yes, I agree, my last boat was welded aluminum - really love being back to fiberglass where you can modify things more; have worked a lot with fiberglass in the past, rebuilt boats stringers & decks etc.; though, epoxy is new to me.


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## JC Designs (Apr 5, 2020)

Shallows said:


> I agree that a 1/2" #10 screw would be fine - but a 1/2" screws tip will just barely pierce the entire transom; I could build up the inside though like you say - I did buy some epoxy cloth mat and I have marine epoxy, so could throw some layers of epoxy inside - will just have to read up on this, worked with fiberglass cloth and rebuilding in the past, but not epoxy.


All you have to do is prep the surface with some 50-80 grit, tape off a dam, and thicken the epoxy with some fumed silica, the pour the thickened resin into your little mold. The epoxy will bond exceptionally well to the prepped surface! You can then scuff and put a layer of cloth over your “casting” but would honestly be over kill.


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## Shallows (Mar 29, 2020)

JC Designs said:


> All you have to do is prep the surface with some 50-80 grit, tape off a dam, and thicken the epoxy with some fumed silica, the pour the thickened resin into your little mold. The epoxy will bond exceptionally well to the prepped surface! You can then scuff and put a layer of cloth over your “casting” but would honestly be over kill.


I think I will do this and then either use short screws like you mentioned, or some kind of stern pad block. Today I filled in the transom holes with West Six10 thickened epoxy, it seemed pretty thick so that is all I used (even though I have mat). In the next few days, I think I'll lay some mat down on the inside to reinforce.


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## DuckNut (Apr 3, 2009)

If you want a pad, anytide can make you one.

Think about this. 90% of all transom problems are a result of screw and bolts.


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## Shallows (Mar 29, 2020)

DuckNut said:


> If you want a pad, anytide can make you one.
> 
> Think about this. 90% of all transom problems are a result of screw and bolts.


I wonder what they would make it out of though? I read up on the "Stern Saver" version of a transom mount pad and its made out of Starboard and uses epoxy as adhesive - this is an absolute no go according to King Starboard themselves.


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## DuckNut (Apr 3, 2009)

Shallows said:


> I wonder what they would make it out of though? I read up on the "Stern Saver" version of a transom mount pad and its made out of Starboard and uses epoxy as adhesive - this is an absolute no go according to King Starboard themselves.


Hmmmm

https://www.kingplastic.com/using-adhesive-with-king-starboard-2/

Or

https://www.westsystem.com/specialty-epoxies/gflex-655-thickened-epoxy-adhesive/

And a host of others will bond it just fine.


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## DuckNut (Apr 3, 2009)

Shallows said:


> Yes, I agree, my last boat was welded aluminum - really love being back to fiberglass where you can modify things more; have worked a lot with fiberglass in the past, rebuilt boats stringers & decks etc.; though, epoxy is new to me.


I went back and read the entire thread. There have been excellent points made by all and yet there seems to be an issue.

You say you have pat experience with fiberglass and I am assuming you mean using one of the ester resins. Epoxy is used in the exact manner as the ester resins, no difference in usage.

What I see happening is overthinking.

Using JC's idea, take an appropriately sized disposable food container and fill it 1/2" with epoxy to make your puck. Epoxy that to the hull and mount the bracket to this. That would eliminate the worry over starboard.


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## JC Designs (Apr 5, 2020)

DuckNut said:


> I went back and read the entire thread. There have been excellent points made by all and yet there seems to be an issue.
> 
> You say you have pat experience with fiberglass and I am assuming you mean using one of the ester resins. Epoxy is used in the exact manner as the ester resins, no difference in usage.
> 
> ...


Simple as that, Done and done!


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## Shallows (Mar 29, 2020)

DuckNut said:


> Hmmmm
> 
> https://www.kingplastic.com/using-adhesive-with-king-starboard-2/
> 
> And a host of others will bond it just fine.


Yes, I read that King article before - that special 3M adhesive is around $40 alone, you have to prep the starboard with a blow torch to an exact degree, etc. etc. - the entire process sounds like a nightmare - and even after all of that I am un-convinced that it would have anywhere near the bond of just a fiberglass or epoxy sheet; just seems like I would be over-complicating life using it when there are other options - I like you're epoxy puck idea, that just makes more sense.

Do you know, does anyone sell a somewhat small finished epoxy or fiberglass sheet - preferably something with layered cloth or mat, in similar thickness to starboard (1/2-3/4") that you can cut to size for little things like this? I am just not sure what to search for, or if there is a specific name for this. Would just be easier than pouring my own as have a few accessories I want to mount inside the boat too using something like this.


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## JC Designs (Apr 5, 2020)

Shallows said:


> Yes, I read that King article before - that special 3M adhesive is around $40 alone, you have to prep the starboard with a blow torch to an exact degree, etc. etc. - the entire process sounds like a nightmare - and even after all of that I am un-convinced that it would have anywhere near the bond of just a fiberglass or epoxy sheet; just seems like I would be over-complicating life using it when there are other options - I like you're epoxy puck idea, that just makes more sense.
> 
> Do you know, does anyone sell a somewhat small finished epoxy or fiberglass sheet - preferably something with layered cloth or mat, in similar thickness to starboard (1/2-3/4") that you can cut to size for little things like this? I am just not sure what to search for, or if there is a specific name for this. Would just be easier than pouring my own as have a few accessories I want to mount inside the boat too using something like this.


Look up G10 board, you can probably find a small piece like you need.


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## DuckNut (Apr 3, 2009)

Shallows said:


> Yes, I read that King article before - that special 3M adhesive is around $40 alone, you have to prep the starboard with a blow torch to an exact degree, etc. etc. - the entire process sounds like a nightmare - and even after all of that I am un-convinced that it would have anywhere near the bond of just a fiberglass or epoxy sheet; just seems like I would be over-complicating life using it when there are other options - I like you're epoxy puck idea, that just makes more sense.
> 
> Do you know, does anyone sell a somewhat small finished epoxy or fiberglass sheet - preferably something with layered cloth or mat, in similar thickness to starboard (1/2-3/4") that you can cut to size for little things like this? I am just not sure what to search for, or if there is a specific name for this. Would just be easier than pouring my own as have a few accessories I want to mount inside the boat too using something like this.


Cut cloth into a bunch of little pieces and lay them on each other to get the thickness you need. 1 yard of 1708 would be plenty.


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## Shallows (Mar 29, 2020)

JC Designs said:


> Look up G10 board, you can probably find a small piece like you need.


Thanks, just looked this up - gonna read more, but seems like it may work.


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## Shallows (Mar 29, 2020)

DuckNut said:


> Cut cloth into a bunch of little pieces and lay them on each other to get the thickness you need. 1 yard of 1708 would be plenty.


Yes, 1708 cloth is what I am using now; with epoxy do you generally wait for each cloth layer to cure before laying a new layer though, or can you lay say 5 or 6 layers of 1708 at once? The issue i have is the Six10 epoxy I have takes 5 hours to cure, so I could only lay one layer a day if wait to cure.


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## DuckNut (Apr 3, 2009)

Shallows said:


> Yes, 1708 cloth is what I am using now; with epoxy do you generally wait for each cloth layer to cure before laying a new layer though, or can you lay say 5 or 6 layers of 1708 at once? The issue i have is the Six10 epoxy I have takes 5 hours to cure, so I could only lay one layer a day if wait to cure.


Six10 is not laminating resin and not to be used with cloth. You need to get a quart of laminating resin. You could do 3-4 and let them kick for a half day then go add another 3-4 and keep going like this until it is thick enough. Sand it to look nice and then you can use the Six10 to stick it to the hull if you don't have any resin left. Just need to thicken the resin with something or it will run out when you stick the block to the hull.

Without buying any other products save your sawdust from sanding and mix that with the resin to thicken it.


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## Shallows (Mar 29, 2020)

DuckNut said:


> Six10 is not laminating resin and not to be used with cloth. You need to get a quart of laminating resin. You could do 3-4 and let them kick for a half day then go add another 3-4 and keep going like this until it is thick enough. Sand it to look nice and then you can use the Six10 to stick it to the hull if you don't have any resin left. Just need to thicken the resin with something or it will run out when you stick the block to the hull.
> 
> Without buying any other products save your sawdust from sanding and mix that with the resin to thicken it.


Actually West System says you can use Six10 to wet out lighter cloth, but I get what you are saying - a thinner epoxy would be easier to work with when it comes to laying down cloth and plus one that dries faster may be more ideal so you can take breaks between layers if that helps the strength at all?

It's also a catch 22 because the 1708 is 17-ounces; though my guess is that its more of an issue that its not ideal to work with - not that it won't work, as I have already filled several large roughly 3" x 1" holes on my bench seat areas using multiple layers of 1708 layed at once with Six10.

From West System:

"Six10 will wet out fabrics up to 12 oz per sq yd. For heavier fabrics, a 105 Resin-based epoxy with its low viscosity should be used to ensure thorough fabric wet out."


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## JC Designs (Apr 5, 2020)

Heat up your a/b components prior to mixing to about 120-130*f and mix in small batches and it will wet out better. But I would just do as @DuckNut said and buy some laminating resin.


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## DuckNut (Apr 3, 2009)

CSI you answered your own question and solved the problem. I knew you could once you became unfrozen.


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## Shallows (Mar 29, 2020)

DuckNut said:


> Using JC's idea, take an appropriately sized disposable food container and fill it 1/2" with epoxy to make your puck. Epoxy that to the hull and mount the bracket to this. That would eliminate the worry over starboard.


If I went this route the puck would essentially be epoxy'd to the Gel Coat surface - do you think the Gel Coat would be strong enough to hold this weight/stress, or is there a possibility of the whole deal falling off with Gel Coat attached?

The other option is to just screw right into the transom directly and build up the transom thickness with epoxy so the screws are covered on the inside - rather than poking through. I'm kind of on the fence as to the best route right now.


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## Shallows (Mar 29, 2020)

JC Designs said:


> I would personally, build a dam and fill with epoxy to make a small backer section before I through bolted. That said, 1/2” plywood on average has 270lbft of pull out strength with a #10 screw and 240lbft of pull out strength with a #8 screw. So unless you plan on using the transducer for a ladder you’d be fine. Even so, the plastic would most likely break before the screw pulled. Carry on, James


JC, could you go into a little more detail on how the dam would be shaped, sizing, etc? I am about to re-install my new transponder which has a different hole layout, so thinking I may just use screws and then just build up the inside of the transom with your epoxy dam idea or cloth layers to thicken it from screw tips poking through, but not sure on the shape you were thinking exactly. I have already filled the old holes and re-inforced on the inside with a single layer of 1708.


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## JC Designs (Apr 5, 2020)

Hot glue some pieces of plastic or cut an old sour cream container or the like and hot glue it down. Scuff the surface of the transom and hull with some 36/50 grit first. Tape up any openings and pour in epoxy to fill. Shape and size is on you, I don’t have to boat or the transducer in my hands. The epoxy will bond to the hull and transom and after it sets just pull the plastic out.


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## DuckNut (Apr 3, 2009)

Shallows said:


> If I went this route the puck would essentially be epoxy'd to the Gel Coat surface - do you think the Gel Coat would be strong enough to hold this weight/stress, or is there a possibility of the whole deal falling off with Gel Coat attached?
> 
> The other option is to just screw right into the transom directly and build up the transom thickness with epoxy so the screws are covered on the inside - rather than poking through. I'm kind of on the fence as to the best route right now.


If the gel is on properly it should stick just fine. There is not a lot of strain on it. If you are worried you could always drill holes and mount it with screws from the inside, attaching the ducer on the outside.

Ok, last thing I'm going to say on your thread.

Stop overthinking it, get the job done and go fishing. The best part is if you screw something up it can easily be fixed.

Post the pics this weekend on the finished job.


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## No Bait / Lures Only (Apr 10, 2011)

Shallows said:


> Yes, I read that King article before - that special 3M adhesive is around $40 alone, you have to prep the starboard with a blow torch to an exact degree, etc. etc. - the entire process sounds like a nightmare - and even after all of that I am un-convinced that it would have anywhere near the bond of just a fiberglass or epoxy sheet; just seems like I would be over-complicating life using it when there are other options - I like you're epoxy puck idea, that just makes more sense.
> 
> Do you know, does anyone sell a somewhat small finished epoxy or fiberglass sheet - preferably something with layered cloth or mat, in similar thickness to starboard (1/2-3/4") that you can cut to size for little things like this? I am just not sure what to search for, or if there is a specific name for this. Would just be easier than pouring my own as have a few accessories I want to mount inside the boat too using something like this.


Piece of coosa


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