Backing pads for thru-hulls

scenarioL113 · Post by **scenarioL113** » Jan 21st, '21, 08:01

I have to replace the backing pads on my raw water intake thruhulls and was looking for suggestions on what material to use etc...

I searched some older posts and many were older posts. I am sure they are still good but just wondering if any material have changed more recently with the availability of more composite material in the marketplace.

I am thinking wood? I just am not sure tho.

Teak, Mahagony, pvc board???

regardless, do any material get fiberglassed into the hull???

Thanks for any input

Frank

Snipe · Post by **Snipe** » Jan 21st, '21, 08:31

I am going with these of making my own from garlite g 10 from McMaster Carr. https://dreamgreen.org/boat-parts/182-t ... ing-plates

Ironworker · Post by **Ironworker** » Jan 21st, '21, 09:53

I used some thick stainless sheet metal but snipe may have a better solution. I would not use wood of any sort.

Rawleigh · Post by **Rawleigh** » Jan 21st, '21, 11:37

Captain Pat had a section on this. Whatever you use, use thickened epoxy to bed it in to fit the contour of the hull. The upper edges are cut at a 45 degree bevel so you can glass over them and incorporate them into the hull. If you are using composite you can probably skip the glassing in part. it is not like they are going to take the stresses a rudder or strut would.

Tony Meola · Post by **Tony Meola** » Jan 21st, '21, 21:44

https://www.mcmaster.com/fiberglass/str ... nd-strips/

We used 1/2 thick fiberglass sheet, cut round sections out and glassed them to the hull. The original wood ones were starting to go when we replaced them. Thickened epoxy to set them to the hull then woven mat over them.

scenarioL113 · Post by **scenarioL113** » Jan 22nd, '21, 10:36

Ok so no matter what material I use should the thruhull be bolted down and then glassed over OR does it get glassed in and then after it is cured you come back and drill holes and bolt it in tight?

I never did it before, so I dont know any better, plz excuse my ignorance on this subject. Fiberglassing is not my thing (but I am too cheap to pay so I gotta learn)

Ironworker · Post by **Ironworker** » Jan 22nd, '21, 13:11

I've installed a bunch. I just bed them in 5200 or 4200 with no glass.

Rawleigh · Post by **Rawleigh** » Jan 22nd, '21, 14:07

If you are using a wood pad, you can drill the thru hull hole slightly oversize and use a stub of PVC pipe epoxied in to isolate the wood from any possible water intrusion. You can also use 5200 on the PVC. Grind the PVC flush and then do your glassing over to waterproof the rest of it. I have done this when mounting outboard and brackets with good success in preventing water intrusion into the transom wood.

Carl · Post by **Carl** » Jan 22nd, '21, 14:08

Ironworker wrote: ↑Jan 22nd, '21, 13:11 I've installed a bunch. I just bed them in 5200 or 4200 with no glass.

Same here, just 4200 and I have reused the old wood pads that were there, also have used UHMW for the old transducers that needed a wedge for proper angle. Most important is to clean surfaces so it all seals.

bob lico · Post by **bob lico** » Jan 22nd, '21, 19:14

i have been installing thru-hull fittings for about 55 years and nothing absolutely nothing beats E-Pay. the wood has numerous names in different parts of the country and is used for outside decks for expensive homes usually 5/4" planks ,it grows in the Amazon rain forest and is actually heavier then water ! yes that dense !! i would apply a little 3m5200 on the bottom with a drilled hole slightly larger than the fitting and 6" by 6" should be good for any fitting up to 1 1/4" for 2" i would use 8' by 8" good for a thousand years . trust me----------

scenarioL113 · Post by **scenarioL113** » Jan 22nd, '21, 20:02

My raw water intakes are partially in bilge water most of the time. I think composite (FRP etc) material or epay (like Bob suggested) is how I am leaning.

Carl (and Ironworker) you mention to not glass in. Is that referring to wood or composite as the padding?

When we say to cut hole slightly larger than the thruhull fitting, OK, what happens with the gap between the bedding and the thruhull? goop 4200/5200 in there?

Thanks for all the replies and suggestion (keep them coming plz)

Tony Meola · Post by **Tony Meola** » Jan 22nd, '21, 21:56

Frank

The backing plate gets glassed to the hull. You will have the bottom (muchroom) with threads that get screwed into the Upper part that is in the boat. The mushroom section gets sealant. I know the guys say 5200, but that is way overkill. Plus removing it means a ton of work being careful you do not take part of the hull with it.

I would use 4200 or Life Caulk. I use Life Caulk.

After you have this all together you will see on the flange on the Thru hull, holes that are for bolts or studs, depending on what you want to use. What I actually did, since I used the Half inch structural glass that was glassed in with epoxy to the hull then matt, I drilled a hole into the backing pad. Then I Taped it so it was threaded and then screwed in a silicon bronze bolt.

The other option is to drill thru the hull and the head of the bolt is on the outside of the hull and then you put a nut on the inside. If you go that route, I would also caulk the bolt so it is sealed at the head.

I hope that makes sense.

Snipe · Post by **Snipe** » Jan 23rd, '21, 11:18

I saw this a while ago maybe this will help this is how I made mine. https://marinehowto.com/seacock-backing-plates/
Sounds like exactly what Tony did.

Tony Meola · Post by **Tony Meola** » Jan 23rd, '21, 22:17

Reading the article I guess I took a chance. Do not have a drill press, so I drilled with cordless drill. Actually you are drilling straight since you are drilling through the holes in the flange which act as a guide.

I did it after we had the backer and the thru hole installed. I just made sure I used the right length bolt and I put tape on the drill bit so I knew when to stop drilling.

I don't remember what size bolt I used, but they are cheap enough.

https://www.boltdepot.com/Tap_bolts_Sil ... _4-20.aspx

Here is 1/4-20 but even going up to 5/16 they are still less than a $1.00 a piece. For the number you need not worth cutting rod, too much trouble.

The other thing I did, is that when I tapped it, I used an attachment to my ratchet. Made it so much easier.

Snipe · Post by **Snipe** » Jan 24th, '21, 10:57

Tony here are some pictures of mine. i bought Groco flanged sea cocks with built in flush/crash valves.this is my first time trying to put a picture up figured i would try considering i am stuck inside freezing in the northeast

Tony Meola · Post by **Tony Meola** » Jan 24th, '21, 20:43

Nice work. I have the same ones.

The only issue I have with them is, I have one plug that fights with me every fall when I try and winterize the boat. Makes me worry about what if I really need to pull it in a hurry.

I still think a regular crash valve is needed just in case.

By the way, looks like you have stainless mixed with bronze. Put some tef Gel on that.

bob lico · Post by **bob lico** » Jan 27th, '21, 23:01

i should have mention the most important reason for E-Paid or mahogony is that it swells up that fraction of a inch when water in the bilge comes in contact as opposed to a fiberglass backer.i have crash valves on both raw water intakes rather essential for easy application of 'SaltAway" at the end of season and antifreeze for winter storage.rather foolish to store a boat with salt water in all those cooling tubes of gear cooler, heat exchanger,intercooler.

ktm_2000 · Post by **ktm_2000** » Jan 28th, '21, 16:40

I'm curious here....

I thought the whole purpose of the backing pads for thru-hulls were a place to mechanically bolt the seacock to the hull without putting bolts actually into the hull.

The goal being to protect against the possibility of you going hard aground against something and it scraping off the outside of the thru-hull leaving the seacock still firmly bonded to the inside of the hull?

In my case I've used a piece of 3/4" meranti plywood, beveled the top sides and bedded it in a bunch of epoxy and thickener, then glassed it to the hull bottom with 4 layers of 1708 set in epoxy. The piece went over the original thru-hull hole so I will drill it out from the bottom to make sure it is exactly right. The seacock will be bolted to the pad with bronze wood screws.

looking backwards, the single pad on the starboard side - Glassed in before fairing
https://photos.app.goo.gl/DnYrVw2wXRYdFfzu6

Tony Meola · Post by **Tony Meola** » Jan 28th, '21, 21:31

Actually the bolts are supposed to serve two purposes. One is to keep the valve from turning on you for what ever reason.

But from what I have been told is that the most important, it prevents the valve from being broken should someone step on it or if something else puts a strain on it.

I have yet to figure out how breaking off a thru hull can happen but hey what do I know.

Carl · Post by **Carl** » Jan 29th, '21, 07:55

Tony Meola wrote: ↑Jan 28th, '21, 21:31 Actually the bolts are supposed to serve two purposes. One is to keep the valve from turning on you for what ever reason.

Get a stuck valve and people bring out the big guys and big tools...

Tony Meola wrote: ↑Jan 28th, '21, 21:31
But from what I have been told is that the most important, it prevents the valve from being broken should someone step on it or if something else puts a strain on it.

Get a stuck valve and people bring out the big guys and big tools...

And if you add a big ol' sea strainer directly on top with a raw water hose coming off the side, maybe plumb in a fresh water wash out and...
Lots of leverage. Especially when the raw water hose has been on for a long long time and its time to change. Shut valve and start yanking, pulling...

Tony Meola wrote: ↑Jan 28th, '21, 21:31 I have yet to figure out how breaking off a thru hull can happen but hey what do I know.

Get a stuck valve and people bring out the big guys and big tools...

And if you add a big ol' sea strainer directly on top with a raw water hose coming off the side, maybe plumb in a...Lots of leverage. Especially when the raw water hose has been on for a long long time and its time to change. Shut valve and start yanking, pulling...

As was mentioned shearing off from hitting something. Not a big issue with the flush mount fittings, but remember those clamshell strainers that screwed in from the underside into the seacock. That would be all that would be holding that strainer in place with bolts.

ktm_2000 · Post by **ktm_2000** » Jan 29th, '21, 13:27

When I removed the one I had in my boat originally it lived well protected under the pump for over 10 years. I highly doubt it got hit hard any time based upon where it was located.
https://photos.app.goo.gl/TGdcAajq4Sq6Wr6b9

When removing the motors, I took the seacock off and then took out the strainer, the thru-hull strainer had a hairline crack near the base. I was able to bend it (it took some force) and cracked the tube off the strainer base. I had the seacock bolted to the wooden piece which was epoxy'd down to the inside of the hull, even if it had cracked and failed when I was on the water I believe that the seacock would have held and I would not have had a problem.
https://photos.app.goo.gl/LQFs7NT3R7CQJTXUA

I would have to guess years of freeze thaw over the winter had weakened the metal and it cracked, it may have been cracked for years without me knowing but me personally I would never take a chance with a below the waterline thru-hull which does not have a seacock on it.

a belt and suspenders approach would also be to have conical wooden dowels strategically placed so one could easily open a hatch and jam the dowel in the opening.

Ironworker · Post by **Ironworker** » Jan 29th, '21, 13:59

Thanks for that post. Next time I put the Reaper on the hard, I'm going to change one of my thru hull fittings!

Tony Meola · Post by **Tony Meola** » Jan 29th, '21, 22:27

Carl

Even if I broke out the big guns to remove a valve or Thru hull, it probably would not mater much since I would not do it unless I was on the hard. So if broke it would just be one of those Oh' @#!$ moments. No harm no foul except to my wallet.

I tend to tread lightly on items that could sink the boat while it is in the water.

Carl · Post by **Carl** » Jan 30th, '21, 15:51

You me and many here might not go yanking on the plumbing when in the water, but some would.

scenarioL113 · Post by **scenarioL113** » Feb 1st, '21, 08:06

Thanks for ALL the replies. This will help more people than mean down the line. Lots of good info here. I was down for the count most of last week so I was off the PC (my wife would not let me touch it with my germs). I was sure I had Covid but both tests were NEGATIVE. Fever Chills headache for most of the week.

Now I have a few ideas on how to handle the backing pads....although I am still not sure which one I will use....smh

Whaler1777 · Post by **Whaler1777** » Feb 12th, '21, 12:56

I used coosa bluewater 26 for all thru hull backings, colloidal silica mixed with adhesive filler as a putty between the hull. On the main seawater intakes the edges radiused then covered with a layer of 1708 biaxle.

scenarioL113 · Post by **scenarioL113** » Mar 30th, '21, 20:17

Taking this a little further now,

I did not mention earlier that this thru hull is for my engine intakes. I have intake strainers like this:

https://photos.app.goo.gl/TcT4hxaexSLHNvmD6

Right now I have the old gate shutoff valves and want to install new 1/4 turn valves. My existing valves do not have a flange like a seacock valve.
Basically my intake strainer threaded pipe comes up thru the hull and there is a nut that tightens it down to the hull (or hull backing pad).

If I install the new style seacock with flange then I have no idea how it would connect to the pipe coming up thru the hull...
I KNOW it will thread onto it ....BUT.... the intake strainer is connected to it and will not spin therefore tightening the new seacock down will not allow me to epoxy the pad to the hull beforehand.

The intake strainer is "fixed" as opposed to a "mushroom" intake that can be spun into place from outside the boat into the seacock that is already bolted to the ...

Am I making any sense??? Plz somone smack me and tell me I got this wrong in my head bc I just am not getting it...

Unless the intake I pictured above is NOT compatible with a flanged seacock...

Tony Meola · Post by **Tony Meola** » Mar 30th, '21, 20:40

They are all basically the same.

You remove the nut, screw the new valve down until it makes contact with the backing plate.

Now if the issue is that the thru hull is in the way of installing a new pad, al you need to do is remove the nut, and then clean the bottom paint off the outer edge of the thru hull. Doing this will expose the screws that keep the thru hull from spinning. Just remove the screws and drop the thru hull.

I would fill the screw holes with Marine Tex since the screws are probably just into the hull. This will make sure that when yo put the screws back in you are getting a good bite and the screws do not strip out. The other option is depending how the Thru hull flange lays out, is to use a long screw and go right up into the boat and put a washer and nut on the inside.

Since the screws are there just to keep the fitting from spinning no reason to go with the long screws.

Hope this helps.

scenarioL113 · Post by **scenarioL113** » Mar 30th, '21, 20:55

Tony

Thanks for the quick reply. I believe the intake strainer (the clam shell looking part) is actually thru-bolted as well as the south bay strainers. (yes that is 10 bolts thru the hull) per side. The south bay strainers are obviously over the intake strainers.

Yes I am understanding it better now. I am thinking of buying groco seacocks and the groco backing pad that they recommend. My intentions would be to 4200 it down to the hull.

The groco backing pad has insert nuts that are tapped into a precut hex-shaped hole so they dont spin when they are put in. This kind of requires the seacock to be snug down into position that may not line up with these nuts that are in the backing pad. That was kind of my concern.

I guess I could 4200 the pad down (while the 4200 is still wet)and then spin the seacock into position and allow the backing pad to possibly turn that last little bit until tight. Then install the bolts to the flange and after that point the 4200 will dry...not the ideal solution but that is all I can think of how to do it with what I got.

Carl · Post by **Carl** » Mar 31st, '21, 06:11

That's pretty much it...dry fit snug. Do not crank tight as you'll be "caulking" it in place and that sealant takes some space.

FYI- You can go with a straight thru-hull intakes if your running eel grass / south bay strainers. The clamshell adds complexity along with another possible restriction. Also unless those eel grass strainers have the removable screens they make it almost impossible to clean the clam shell pickups.

scenarioL113 · Post by **scenarioL113** » Mar 31st, '21, 14:21

I was able to remove everything and incredibly nothing broke. A testament to quality parts. I ordered the flanges that are threaded on both ends and the shut-off valve will be added on after. (it is good bc it is serviceable but prob not in my lifetime).

It will also allow me to have an easier time mating it to the thru hull and gooping the backing pad/plate to the hull, snugging it and lining up the embedded nuts all at the same time.

Any recommendation on "goop"....3m 4200, boat-life,...something else.

I just pulled 2 transducer today and one of them I used 4200 about 20 yrs ago and I got it out and it was a struggle so I know that crap works good...

Tony Meola · Post by **Tony Meola** » Mar 31st, '21, 20:44

Frank

I lean more towards life caulk. A little easier to remove. Not sure 4200 or 5200 has a purpose on something that needs to be removed at some point.

scenarioL113 · Post by **scenarioL113** » Mar 31st, '21, 22:09

I removed my south bay strainers. They were bolted thru the hull with stainless bolts/nuts. They was NO backing pad on the inside of the hull and looked to have been sealed only with possibly boat-life. They never leaked (I have had the boat over 20yrs)

There are 3 bolts on each end of the south bay strainers and the raw water intake comes up thru the hull between them.

My new seacock flange has its own backing plate.

I was thinking of taking an old cutting board (plastic-type) and making like maybe a 2" wide strip to use as a backing pad that will cover all three bolts and caulk them. I would make 4 separate ones for the 2 south bay strainers I have.

This would be better than just bolting them thru the hull like they were correct?

Yannis · Post by **Yannis** » Apr 1st, '21, 09:41

For what it's worth, I changed my engine throughull water intakes 5 years ago.
I had read ALL threads available on this board on “how to” etc.
I never understood why there should be any flange or backing plate or anything else in the equation.
I slid the through hull through the existing hole and secured the nut from the inside, making sure there would be abundant sikaflex on all surfaces, that would leak outwards as I would tighten the nut.
So I tightened the nut and forgot about it.
Never leaked, never dared to leak...

Tony Meola · Post by **Tony Meola** » Apr 1st, '21, 20:30

Yannis

The backing plate is there for added structural support, as well as making sure you get a tight seal, and also so that you can install bolts to keep the fitting from spinning.

I believe Grady White does not use backing plates, but they use an oversize washer to spread the load.

scenarioL113 · Post by **scenarioL113** » Apr 1st, '21, 21:04

Tony, would you say life caulk for any of the bolts that go through the hull. and also for a thruhull transducer too? (fairing block etc...)

On a side note I got all the intakes removed and I was also able to drop my port oil pan and get it out WITHOUT lifting the engine....YAY
The 2 middle oil pan bolts in the rear by the bellhousing will probably take me a day each to put back in when my new pan arrives....Ill save that for another thread...SMH

Yannis · Post by **Yannis** » Apr 2nd, '21, 01:30

Tony,

I'm not a naval architect, but I fail to understand what additional structural support a 3cm glass thickness needs.
Then, to avoid spinning, I had a friend counteracting from below; once it was tightly screwed we arranged so it was straight (to the boat direction) and left it there to cure.
And, yes, I also used an oversize washer too!
The next day I cut around all spilled sikaflex from top and bottom.

Tony Meola · Post by **Tony Meola** » Apr 2nd, '21, 23:02

Frank

Anything that penetrates the hull should be caulked. I Would do the transducer also.

Tony Meola · Post by **Tony Meola** » Apr 2nd, '21, 23:10

Tennis

It's to help keep from crushing the hull , and add lateral structural strength should something or someone come down on them, causing the fitting to want to bend.

As for spinning, it can happen. Vibration, pounding, hitting something.

Tony Meola · Post by **Tony Meola** » Apr 2nd, '21, 23:14

Tennis

The pad helps create lateral strength, so it it is harder to bend it should something hit it.

As far as spinning, trust me it happens.

scenarioL113 · Post by **scenarioL113** » Apr 3rd, '21, 07:13

Tony Meola wrote: ↑Apr 2nd, '21, 23:02 Frank

Anything that penetrates the hull should be caulked. I Would do the transducer also.

I meant to use "Life Caulk" for these. As opposed to another product....Thanks for the replies

Preston Burrows · Post by **Preston Burrows** » Apr 5th, '21, 07:53

Good info / views here, don't forget bonding of the through hull valves as well, I've not noticed any mentions of bonding in this discussion unless I skimmed over reading any comments of bonding too.

scenarioL113 · Post by **scenarioL113** » Apr 26th, '21, 20:13

Yes, of course I wll be bonding the new hardware.

BTW, I opted for new mushroom intakes and scrapped the clamshells. They were only going to be redundant and not needed. I marine-tex'd the bolt holes. The Groco backing pads sit on top of the old bolt holes and are 4200'd down so it is doubly-good...
I used 3m 4200 on the Groco backing pads and used the mushroom with the flange-nut to hold the backing pad clamped snugly for 24hrs while the 4200 setup. Next day I removed the mushroom and 4200'd them and the seacocks (as well as gooped the inside of the hole) I also used some basic thread sealant on the straight threads but dont think that it is necessary...either way I did it and I aint takin it back apart.

I also did a Garmin transducer and used the 4200 on tha as well...might as well get my moneys worth out of a $25 tube!

Just need to reinstall the south bay strainers and the thru the hull work is dont (for now)

Moved my batteries inboard of the engines and I did this not only for center of gravity but I built a nice shelf that sits on top of where the thru-hull transducer is. This will also serve to protect the thru-hull from being stepped on. ( always seem to step where I shouldnt when I am in the engine compartment which seems to be ALWAYS)....and did I mention shortening the battery cables and soldering new lugs...etc smh It has been some month....still not done yet and still have to do my favorite thing in the world...bottom-paint smh more... I am sure I left out about 12 other things as well, like barrier coating repairs...lol I know I know we all been there...