Seacock Backing Plates

Start by Drilling a Hole

Many readers have asked me how to install stronger, flanged seacocks without also drilling extra holes in your hull for the bolts that hold the flange. It can be done, as you will see below.

Though this method is going to be slightly less robust, than through bolting, I would have no qualms about using this method on my own boat. I do admittedly through bolt on my own vessel but this is out of personal preference.

Don’t get me wrong this is still a very robust installation, if done with care & patience, and it is stronger than the hundreds of thousands of vessels out there with noting more than a valve threaded to a thru-hull.

This is also a good option for those owners with cored hulls, where thin external skins are to thin to take the countersink depth of the bolt head.

This article already assumes you’re using GPO-3 polyester/fiberglass or G-10 epoxy/fiberglass sheet for the backer blocks, and have already made the circle/donut. It also assumes you are utterly opposed to through-bolting your seacocks to the hull or physically can’t due to external skin thickness on a cored hull..

For tapping the backers I recommend a thickness of about 5/8″ for your GPO-3 or G-10. In testing I have exceeded 1000 pounds of direct pull on 1/2″ home made polyester fiberglass board. G-10 and GPO-3 are far stronger than my home made fiberglass sheet and the extra thickness yields even more strength. In testing with G-10 just one single 5/16″ X 18 thread bolt, drilled & tapped into 1/2″ G-10 far exceeds the ABYC strength requirements for seacocks..

Unlike the other method, where you glass the backing plate to the hull first then drill the hole for the thru-hull, with this method you drill the thru-hull hole first. To do this project a drill press is strongly recommended.

Make A Shim

Before you drill and tap the backer block you need to tighten the flanged adapter to the backing block using the thru-hull. Because you’re missing the thickness of the hull you will need to create a spacer that takes the hulls place.

I used some PVC exterior trim lumber and drilled the same diameter hole as in the backing block. Then all I did was center & tighten the flange and thru-hull together with the backing plate and hull thickness dummy spacer sandwiched in between.

Drill & Tap

Please don’t cut corners on the quality of your tap and always use the proper size drill for the tap you are using.

This is an “F” drill for a 5/16″ X 18 thread tap. You could also use a 17/64″ drill with a 5/16″ tap.

IMPORTANT: Fiberglass or epoxy sheet is susceptible to off-center tapping so be careful to not work your tap crooked or wobble your tap side to side. It will make the tapped hole bigger than you want. Practice in a scrap piece of fiberglass first if your not comfortable with this.

Once tapped through use an air gun to blow the flutes of the tap out. Do this before reversing it out of the fiberglass.

Cutting The Studs

Silicon bronze threaded rod is expensive but silicon bronze carriage bolts are cheap. Simply install two nuts onto the bolt and clamp only one of the nuts in the vice while tightening the other back onto it. This essentially locks the bolt into the vice so you can cut it without it spinning or moving.

Why Studs?

IMPORTANT: The reasons for using studs or even epoxy captured nuts, if the backer is thick enough, is for future removal. During removal the studs or bolts into epoxy inset nuts can be removed fully and the seacock can be peeled off with a guitar string or pried at an angle without the studs interfering. With fully captured non-removable bolts, it makes future removal and nightmare project.

Picture yourself trying to remove a seacock in the future with three captive bolts and you can’t get under it with a guitar string or to even rock it back and forth due to the studs being permanently  in place. You can also accidentally mar, gall or damage the threads and then the bolt snaps or strips out and begins spinning in the substrate with no way to fix it other than MAJOR surgery. The reasons for drilling and tapping for a stud or burying a bronze nut in the plate, are more about the future and less about the now.

Cut Off The Head

As you get good with the Dremel you can make some amazing bolt cuts with threads that work as good as factory. Of course you could use a hack saw too but the Dremel is nice & fast. Always chase the fresh cut with a thread die.

Insert Studs

With the studs made you simply thread them into the backing block. If you got a little aggressive with the tap it’s not the end of the world. Mix up some slightly thickened epoxy and apply with a syringe into the hole, wax the stud and thread it in with the thickened epoxy and let it cure. This will create a nearly perfect thread interface between the stud and backing plate. Even with a good tap I still dip the stud into epoxy, coat the inside of the hole and thread it in.

IMPORTANT: If using the epoxy trick I just mentioned please wax the stud well or you won’t get it back out once the epoxy has cured.

Side View

This photo shows a side view of the assembly. The hull & sealant are the only pieces missing.

Don’t Forget to De-Wax

You’ll want to de-wax the backing plate. Polyester finishing resin has added paraffin to aid in the cure process and this won’t yield a good secondary bond if the wax has not been removed. This wax, and the wax coating from what you laid it up on, id not using GPO-3 or G-10, must be removed prior to installation or nothing will adhere well.

Over the years I have found the dedicated, but overly priced, cocktails of chemicals sold by Pettit or Interlux as de-waxers definitely do work. I only wish I could figure out what the chemical mix is because I bet I could mix it up for considerably less than what they sell it for.

Sand The Hull

Before you begin sanding the hull you should remove any surface wax. Gelcoat, like polyester finishing resin, is also an air inhibited cure so any gelcoat on the interior of a hull has a waxy surface. In order for proper adhesion any interior gelcoat needs to be removed.

This are still needs a bit of sanding to get rid of the rest of the gelcoat.

Installation

Once your dry fit is confirmed parallel, and all sides of the thru-hull head sit flush, on the hulls exterior, you will need to do one more round of de-waxing of both hull and backing block.

After that is done you need to wax both the flange (not the backer block) and the thru-hull fitting with copious amounts of mold release wax, butchers wax or even Collinite. The wax should be a paste type carnuba.

After you have waxed it cut a scrap board to length and wedge the thru-hull into place. If you are on gravel or dirt lay a board on the ground to wedge the upright board against.

Contrary to popular belief you really do not need two people to install a thru-hull, especially if you can be a little creative.

Mix Your Resin

For illustration I choose to make a formula to show the consistency better. Whether epoxy, polyester or vinylester all should be plenty strong for this application. Epoxy is considerably stronger than polyester resin, in a secondary bond, and is what I personally use.

My mix for illustration consisted of chopped strand fibers, West System 406 Coloidal silica or Cabosil, some West 404 and polyester resin. The same mix can be made with epoxy. When I shot this I was not ready to lay it up so used the less expensive polyester resin because it was going to waste anyway.

I like making my own “cocktail” of putty because it gives me more control over consistency and how it fills & fillets. Kitty hair is another product I sometimes use but it will not fillet as well as your own custom mix will.

For a catalyst in the illustration I used some red dyed MEK-P. I like dyed MEK-P so you know you have it thoroughly mixed.

Wear rubber gloves and apply a thin coat of the mix to both the hull and backing block, then slather a large amount onto the backing block and thread it onto the thru-hull about three threads. I then used vise grips to grip the flange to prevent it from spinning and went out side the boat and used my step wrench to tighten down the thru-hull and seat the backing block.

Note: Epoxy will yield a much stronger bond than polyester resin but even polyester should give you about 500 PSI adhesion. ABYC standards call for the seacock to support a 500 pound static load for 30 seconds at the inner most hard fitting or hose barb. Remember that 500 PSI is per square inch. Your backing block is considerably larger than 1 square inch…