Insisted that I wanted a flush thru-hull, nothing sticking out.
Needed a 1 1/2" seacock and decided on the reinforched nylon one.
Convinced self that there were no bronze seacocks made with an alloy that was corrosion free.
A 1 1/2" Forespar seacock is a massive true flanged foot valve.
Its three 5/16" flange bolts are widely spaced and a fair distance from the thru-hole.
Those bolts are common available silicon bronze and imco nearly inert in salt water.
A338's hull in the flat of the bilge aft where I wanted the seacock was only 5/16" thick.
Built up the area with glass mat and epoxy to about 1/2".
This allowed chamfering the thru-hull hole, and chamfering the bolt holes.
The flat head bolt holes were chamfered deeper so that the heads could be covered with putty.
This would allow finding those bolts again in case the installation has to be taken apart.
The Forespar flush head straight thread thru-hull is rather long. It screws into the seacock about an inch or so.
The interior nubs that are used to turn a bronze thru-hull (just inside the head) are molded into the marelon at the far end of the fitting. Couldn't believe it, damned New Zealanders!
I called Forespar about their placement and the guy said that the two nubs were not for screwing the thru-hull in - "they're left over from the molding process." There are four small cut outs around the rim that are supposedly used to turn the fitting. Did they have a tool available? NO!.
But I felt at the time that I shouldn't shorten the thru-hull from what they supply.
That meant I had to build up a thick block of ply padding for the seacock. Almost two inches if I remember. Used pieces of various thicknesses of Meranti glued with epoxy.
But it left me with installing the damn thruhull and pissed at Forespar. Because I had already messed up the bitty recesses with dry fitting.
But this also meant that the flange bolts had a lot of meat to seat in.
And being through bolted and caulked with butyl means that they can be driven out later.
With a massive backing block this turned to be and if the bolt holes are all accessible, we might lag the seacock on from the inside, as Tony suggests. With the large fastening holes in the 1 1/2" seacock we'd be using, what, #16 screws? Plenty of grip. But through bolting is undeniably stronger, easier to remove - and mechanically fastens the backing structure/pad to the hull along with the valve. With any substantial backing the thru-hull hole and the three bolt holes imco don't present a weakness problem.
After the flanged seacock* is installed with butyl tape (which allows squeeze-out but also resistance to complete squeeze-out that would happen with the tube stuff - in theory anyway - that there will remain some waterproofing under the flange after bolting.
The seacock effectively shuts the hole. The thru-hull screws into the seacock independently from outside.
Used a lot of white thread sealer paste on the part of the thru-hull that screws into the seacock.
The long length of the fitting in the hole through the block has tube butly only part way but the flange is seated in the stuff. Didn't seriously cram it full. Fully screwed into the valve with thread paste it won't leak and if it does it won't matter - as long as I have buttoned the seacock in place.
Cut out a crude installing tool from 1/8" aluminum sheet. It extends down into the thru-hull to the nubs and also registers into two of the small cutouts in the rim of the thru-hull. Screwing it in and seating it slightly below the surface of the hull is a piece of cake. Unscrewing it years from now will be a different story.
The bolts holding the valve can thoerctically be driven out by turning the nuts off and using a driver pin to knock them out. Only the heads in the hull are caulked. NOT GLUED. If I have left enough room the seacock could then be unscrewed from the thru-hull inside. You'd have to be on the hard, of course. Then the thru-hull can be hammered out as it is not connected to anything. If I felt that a polysulfide sealant had to be used it'd be only under the head, not on the threaded shank. Would not seat the thru-hull with urethane rubber sealant.
If this works out in practice, we have here a non-corroding system that can be taken apart easily for replacement or recaulk.
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*Using an in-line ballcock valve for a permanent underwater hole in the hull is 100% WRONG. Even if Forespar shows this as an alternative in their literature you must assume that a ballcock valve shutoff on the hull
MUST ONLY BE USED ABOVE THE WATERLINE.
But imco NEVER used this way in any Areil/Commender.
The Forespar guy on the phone defended the ballcock/hull use by saying, 'a lot of builders use them this way'. That doesn't make the cheap-skates right!
I would not have any hole under water that depends solely on the thru-hull for its strength. The threading makes the thru-hull even thinner. IMCO the threads can be considered as SHEERING POINTS that a whack from the side can break. So even a drain above the waterline hidden away in a locker, almost inaccessible, for which you have a shutoff can get broke if it is a ballcock riding on the thru-hull.
Don't think that a hose clamped to a thru-hull drain is very clever either. Hose has tobe supported so it doesn't become a lever arm to break the ballcock off.
[way later edit: Groco (not Forespar) now has a ballcock conversion kit that adds a straight thread flange to the inline valve converting Groco brand ballvalves to a proper seacock.]
What have I missed?
Yeah, I know, keeping it short!
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