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.