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Thru hull and seacock installation

3696 Views 11 Replies 7 Participants Last post by  hopcar
I recently contracted someone to do various work on my boat and among these things was the removal of and installation of all new thru hulls and seacocks, of which there are 9 total. I had two of them permanently closed up but the others were put in with just the threaded thru hull and backing nut.

This contractor, even with 20 years of experience, was dumbfounded at the suggestion that he use flanged seacocks with backing plates instead of simply screwing a valve onto a thru hull with only the backing nut. He is a motor yacht type, not an offshore sailor. The thru hulls were installed professionally, he used 5200, and they are strong, clean, and look good but they are still only held in with the backing nut that's included with the thru hull. He also prepped, dried, and epoxied the existing holes in the hull so that they are strong as well.

Here is my question: Since this person no longer works for me, and I am going to add backing plates and flanges to all of these thru hulls, do I need to completely remove them all and start from scratch or can I (would you) gingerly screw on and seal a flange adapter to the existing thru hull? They have been in for a number of weeks now and the 5200 is completely cured.

I will remove them and start over if I have to. I'll be finishing this project myself. But I am concerned about breaking loose the thru hull seal as they are when I screw down a flange and seacock. Are the thru hulls strong enough to withstand the torque of adding a flange and valve? Would you have someone outside holding the mushroom head with a spud wrench while making these additions?

Currently I am leaning towards just adding the flanges and valves to the existing new thru hulls. Any advice on this would be hugely appreciated. I really don't want to break all these things out and start over.

We sail extensively and cruise full time with children. Although we don't currently cross oceans we do make hops from the Bahamas to the USVI.

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I am trying to think of a way to save you lots of work. All I can come up with is this :

Make an FR4 backing plate with a hole in it, that will fit over your through-hull. It will need a recess in it for the nut, as that is already stuck on with 5200. Place a second nut over the backing plate, put plenty of 5200 between the plate and the hull, and tighten down. You now have the load spread over a backing plate, without removing the through-hull.
Yea I'm leaning toward this option.

Currently no valves are on the thru hulls. Just the mushroom head is installed. I have backing plates already made for all of them. I think what I'll do is countersink a hole for the thru hull nut, bolt the seacock flange adapter to the backing plate with countersunk bronze bolts epoxied in from the underside, then screw that on to the existing thru hull. I don't have to dog it down, and like the guy above said, 5200 is strong stuff. I doubt there'd be enough force in this to even come close to stressing the thru hull seal. Also the backing plate would be 5200'd in place on top of the thru hull. In addition I would use fiberglass tape and epoxy over the top of the backing plate to make it a permanent part of the hull. Once that is cured, I can just screw down the bronze ball valve snug to the adapter flange.

These seacocks are all over the boat. They are well protected so nothing would be hitting them. But still, I'd like them as strong as possible. The boat is a 1980 Gulfstar 50, so the hull is old school and pretty thick. No core in it. And like I said, the new thru hulls were done well even though they weren't done to the standard of the Compass Marine guy, whom I tend to trust implicitly. They would be fine without the flanged seacocks but I prefer things to be bullet proof. I think at this point, removing the thru hulls would just damage the hull and create more unnecessary work.
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What you have is what I call a ball valve on a thru hull. Not a proper seacock. It works but like you say is not the best solution. I have done a video series on replacing the valave on a lagoon 38 that may be of some help. This is a link to the first one
Not sure what type of boat you have or how you plan to use it. The one example of why a proper seacock should be used is a boat I was asked to look at that had stray current corrosion. The heads of the thru hulls were literary eaten off over a period of a week. The only thing left keeping the water out was the flanged seacock. If this boat had, had ball valves on thru hulls it would have sunk.
I've watched your video many times and it's a great resource for everyone.

The mistake I made was trusting a contractor. I ASSumed that everyone understood new thru hulls and seacocks needed bronze flanges and the like but this guy was a world away from that, even after I specified. Damage done. All of his other work was top notch though so I relaxed in my micro management. Lesson learned, again.
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