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Compass Marine How To | all galleries >> Welcome To >> Seacock Backing Plates / Alternate Method / No Through Bolts > Start by Drilling a Hole
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Start by Drilling a Hole

Many boaters 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 is going to be slightly less robust, in strength 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 out of personal preference. Don't get me wrong this is still a 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 decent option for those 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.

This article also assumes you are utterly opposed to through-bolting your seacocks to the hull.

For tapping 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.

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 HIGHLY recommended.

Nikon D200
1/60s f/4.8 at 42.0mm iso200 full exif

other sizes: small medium large auto
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Jim Gillmore3 17-Apr-2017 18:12
John, Can ( use Coosa Board instead of the G10 I have a surplus and was thinking of glassing and using it.
John 21-Dec-2016 03:12
What's your opinion of the pre-made Grace backing plates that fit their thruhulls? I'm concerned about my ability to machine the G10 material which is so much harder than normal FRP.