Newbie installing a new thru hull transducer

[mforum]

Hi,
I have an ‘86 Catalina 27.
I removed what is likely the original thru-hull depth transducer.
I now am installing a plastic Airmar DST 810 thru hull transducer.
The nut on the old transducer appears was mounted on top of what looks like 3/8 or 1/2 inch plywood.
Reading the airmar DST 810 Manual it shows the washer should go right next to the hull.
I don’t like the idea of plywood (35+ years old) sitting between the washer and the hull and am thinking to remove it, but am uncertain if it was put there to give a flat level (not curved surface directly from the hull) for the old nut.
The red arrow in the picture is where I think the inside of the hull surface is and above is where the plywood starts.
The blue arrow is what I’m not sure on (is this laminate?). I don’t think I have a cored hull but am not clear.
I’m thinking to take the plywood out put the washer and nut directly against the inside of the hull and seal everything with 3M 4200 white sealant.

thanks

 

[Don S/V ILLusion]

i would be surprised the thickness at the bottom of the hull on any boat is only 1/4 inch.

If the hull is flat where you want to install the puck, a backing plate is superfluous. And you are correct that plywood isnt a good material even if sealed with epoxy. There are preferable inert materials which won’t/can’t rot.

 

[Ralph Johnstone]

Looks like the hull is 1/2" and the plywood ring is 1/2".

1. Neatly remove the rotted ply and sand the FG hull smooth.
2. Get a small 1/2" thick square panel (2" inch bigger than the dia. of the wooden ring) of FR4 Epoxy Fiberglass Composite Sheet Panel and sand it to the inner contour of the hull.
3. Glue the FR4 to the hull with DC4200.
4. Run the nut down to the FR4 and tighten while the 4200 is still soft.

The epoxy FG will not rot as did the wood.

 

[mforum]

Looks like the hull is 1/2" and the plywood ring is 1/2".

1. Neatly remove the rotted ply and sand the FG hull smooth.
2. Get a small 1/2" thick square panel (2" inch bigger than the dia. of the wooden ring) of FR4 Epoxy Fiberglass Composite Sheet Panel and sand it to the inner contour of the hull.
3. Glue the FR4 to the hull with DC4200.
4. Run the nut down to the FR4 and tighten while the 4200 is still soft.

The epoxy FG will not rot as did the wood.

Yeah, I think the hull is 1/2” thick and the plywood backing is the other half.
Is FR4 the same as G10 fiberglass sheets? It may be difficult for me to find locally or get in a timely fashion.
Does it need to be 1/2” or is 1/4” sufficient?
What about high density polyethylene? too soft?

 

[Ralph Johnstone]

Is FR4 the same as G10 fiberglass sheets?

Yes they are similar. Someone please correct me if I'm wrong but I believe FR4 is an epoxy reinforced glass fiber while G10 is a polyester reinforced glass fiber. I would think the polyester is cheaper but still more than strong enough for the job at hand.

Does it need to be 1/2” or is 1/4” sufficient?

Depends on the inside radius of the hull. The tighter the radius, the thicker the composite needed. I would imagine as long as the edge doesn't become paper thin, it's good to go. The composite is also acting to some extent to spread the force of the nut over a larger area which makes the life of the hull a little easier.

What about high density polyethylene? too soft?

Otherwise known as Starboard. Not a good idea as it creeps with pressure and time.

It may be difficult for me to find locally or get in a timely fashion.

See what Amazon.com says if there's nothing local.

 

[Ward H]

First, your hull is solid fiberglass.
The donut ring of plywood is a “backing plate”. Its purpose is to reinforce the hull as Ralph said, to prevent flexing of the hull around the hole. Plywood was used because builders didn’t envision these boats lasting 30, 40 years or more.

I used 1/4” G10 to make backing plates for all my thru hulls. I’ve sourced it from eBay, Amazon and McMaster Carr.
Some online marine stores are selling G10 backing plates in various sizes but you will still need to drill any holes needed.

I cut my backing plates out of a sheet of G10 by using a good quality hole saw, then drill the center to size.
I put a bed of thickened epoxy reinforced with chopped strands of fiberglass around the hole and set the backing plate into it. You could also glue the backing plate to the hull with 5200 adhesive.
I keep the backing plate square to the hull by installing an old or new cheap thru hull, tighten it down, then remove before the epoxy sets.

I’ve read of others who install epoxy soaked plywood as backing plates figuring they will last another 30 years.

Whatever you use, do install a new backing plate.

 

[Hayden Watson]

Yeah, I think the hull is 1/2” thick and the plywood backing is the other half.
Is FR4 the same as G10 fiberglass sheets? It may be difficult for me to find locally or get in a timely fashion.
Does it need to be 1/2” or is 1/4” sufficient?
What about high density polyethylene? too soft?

FR4 is flame resistance Class I in accordance with NEMA test method 7.11. Most often what you find now will be dual rated G10/FR4. It is extremely hard and stiff so 1/4" would work for a backing plate. It is very hard on saw blades because it is so dense, so it is not the easiest to work with.
As you surmised, one of the main reasons for the backing plate is to create a flat surface perpendicular to the hole bore. Given that, on my boat, I cut a square piece of FR4 that was 1½" bigger than the bore. After cleaning and sanding the inside of the hull with 60 grit sandpaper I bedded it into thickened epoxy with sufficient depth to fill the curvature of the hull. After that was done, I use the old bore as a drill guide to drill the hole through the FR4 making sure that the hole saw was flat against the FR4.

 

[dlochner]

A good layer of thickened epoxy is the best adhesive as it will fill in any unevenness in the hull, from a mild curve or the pattern from the cloth.

Starboard and similar materials are not good because they are soft and will compress.

Ease the edges of the backing plate where they come in contact with the hull, this helps to avoid point loading.

Do not let 5200 come any where near the backing plate or through hull. There is a special place in Dante's Inferno for this who do.

 

[Ralph Johnstone]

FR4 is flame resistance Class I in accordance with NEMA test method 7.11. Most often what you find now will be dual rated G10/FR4.

I understand that G10 is polyester & glass. Any idea what FR4 is ?

 

[dlochner]

I understand that G10 is polyester & glass. Any idea what FR4 is ?

Wikipedia thinks G10 is an epoxy laminate and FR4 is a flame resistant epoxy laminate.

https://en.wikipedia.org/wiki/G-10_(material)

https://en.wikipedia.org/wiki/FR-4

 

[Scott T-Bird]

If you haven't already, be sure to verify that your new transducer has the same diameter as the hole for the old one. I purchased a package with the DST 810 transducer recently and I will need to drill-out the original hole to make it fit.

 

[Hayden Watson]

I understand that G10 is polyester & glass. Any idea what FR4 is ?

the FR4 G10 that has been tested to meet the flame resistance requirement of the NEMA test method 7.11.
G10 and FR4 are the same thing and most suppliers list it as dual rated, in other words, what they sell meets the structural requirements of G10 and the flame resistance rating of FR4.

 

[Ralph Johnstone]

Hmmmmmmm. A tiny piece of flame resistant backing plate inside a fiberglass shell which supports fire in a most spectacular fashion.

Every little bit helps .

 

[mforum]

If you haven't already, be sure to verify that your new transducer has the same diameter as the hole for the old one. I purchased a package with the DST 810 transducer recently and I will need to drill-out the original hole to make it fit.

You are right. The DST 810 is a little larger in diameter than the older transducer. The current hole is 50 mm (2 inch) and the manual calls for a 52 mm hole.

 

[Hayden Watson]

You are right. The DST 810 is a little larger in diameter than the older transducer. The current hole is 50 mm (2 inch) and the manual calls for a 52 mm hole.

The best way to drill the hole out to a larger diameter is to use the proper hole saw to cut a hole in a piece of plywood and then use that plywood as a drill guide. The last time I did this, I did used butyl tape to help hold the ply in place and it worked great.

 

[Ward H]

Do not let 5200 come any where near the backing plate or through hull. There is a special place in Dante's Inferno for this who do

Dave, I fully agree with the advice and warnings on not using 5200 on a boat.

But, in this use, adhering a backing plate to the fiberglass hull, why is it not recommended? It's considered a permanent water proof adhesive. So is thickened epoxy, is it not?

I only suggested it in case the OP felt more comfortable with it than with epoxy or if 5200 was more readily available. If he rounds/tapers the edges of the backing plate so there are no hard points, as you suggested, why wouldn't it be acceptable?

 

[dlochner]

Dave, I fully agree with the advice and warnings on not using 5200 on a boat.

But, in this use, adhering a backing plate to the fiberglass hull, why is it not recommended? It's considered a permanent water proof adhesive. So is thickened epoxy, is it not?

I only suggested it in case the OP felt more comfortable with it than with epoxy or if 5200 was more readily available. If he rounds/tapers the edges of the backing plate so there are no hard points, as you suggested, why wouldn't it be acceptable?

Good question. The inside surface of the hull is probably not completely flat or smooth. Thickened epoxy will fill the gaps and unevenness better than 5200 and provide a solid base for the backing plate. Even when cured 5200 remains slightly flexible, it is an adhesive sealant, this flexibility may have some negative consequences if a thick layer is necessary and make it difficult to keep the through hull tight against the hull.

For about the same cost as a tube of 5200, a cartridge of TotalBoat Thixio or West SixTen can be purchased, both of which are more versatile than 5200.

 

[mforum]

I’m reading a lot about different viewpoints on how to adhere backing plates.
I’ve not worked with epoxy resin such as the west system and and don’t mind trying it but feel a little challenged with it as I’m not sure on what consistency/filler/timing/setting it in place is needed and if I screw it up (as has been known to happen) creating a bigger problem.
What are the disadvantages to using 3M 4200 FC to adhere and seal the backing plate as RJ describes above?
3M says it can be used above and below the waterline and its “flexibility allows for dissipation of stress caused by shock, vibration, swelling or shrinking.”

 

[mforum]

The best way to drill the hole out to a larger diameter is to use the proper hole saw to cut a hole in a piece of plywood and then use that plywood as a drill guide. The last time I did this, I did used butyl tape to help hold the ply in place and it worked great.

I’m thinking a long the same lines however re-reading the manual it’s actually 51 mm not 52 diameter which means taking off an additional 0.5 mm radius. Current hole is 50 mm as best I can measure without calipers or a micrometer, which makes me wonder if I can’t just take off that 0.5 mm with sanding.

 

[dlochner]

I’m reading a lot about different viewpoints on how to adhere backing plates.
I’ve not worked with epoxy resin such as the west system and and don’t mind trying it but feel a little challenged with it as I’m not sure on what consistency/filler/timing/setting it in place is needed and if I screw it up (as has been known to happen) creating a bigger problem.
What are the disadvantages to using 3M 4200 FC to adhere and seal the backing plate as RJ describes above?
3M says it can be used above and below the waterline and its “flexibility allows for dissipation of stress caused by shock, vibration, swelling or shrinking.”

Its a sailboat, its an Internet forum, there will be conflicting advice almost by definition.

The backing plate needs to become one with the hull, essentially you are making the hull thicker at that point. Using epoxy will firmly bond the plate to the hull. It should not move or flex. Thickened epoxy will fill any gaps or unevenness due to the weave of the fiberglass. 4200 and 5200 will not do that as well.

4200 should be used on the threads of the through hull and on the mushroom that comes in contact with the hull. It should also be used on the threads on the inside to secure the nut.

3M uses numbers, obviously, to brand their sealants and adhesives, the higher number products have better adhesion and less flexibility than lower numbered items, while lower numbers have greater flexibility and less adhesion. 5200 is an adhesive with some flexibility, 3000 is a flexible sealant with limited adhesion. 4200 and 4000 are somewhere in the middle.