There are seacocks with tapered bronze plugs, and seacocks with rubber plugs and seacocks with ball valves integral to them. Seacocks have big husky bottoms with flanges and well-protected internal threads and they squat down against your hull or preferably against a backing plate leaving not a single thread of the thru-hull shaft exposed. Seacocks may be made of bronze or Marlon.
Usually, seacocks have three bolt attachments points on the flange. Usually seacocks have straight (NPS) threads on the bottom (seaward) side, which allows them to fit nicely, and deeply over the straight NPS threaded thru-hull shaft. On the top end they have tapered (NPT) threads, which permit the attachment of NPT threaded components of various types. A seacock is a seacock whether the integral valve within it is a bronze tapered plug, a rubber plug, a ball valve, or some other type of valve.
But there are also just plain old in-line ball valves such as one might install somewhere inboard of a seacock as part of your plumbing system. These aren't seacocks at all, but they are being installed in the place of seacocks regularly, and for some marine yards they have supplanted seacocks.
When you install one of these in lieu of a seacock, you leave an exposed section of the threaded shaft below the valve. groco doesn't recommmend that you use these in-line valves in tehplace of seacocks. Groco also warns that leaving more than one half inch of the threaded shaft exposed is not a good thing. (See previous post on this topic for a link to Groco information) These valves have tapered (NPT) threads on both the bottom (seaward) end and also on the top end. If you attached one of these in-line ball valves to a standard straight NPS thru hull fitting, the NPT threads will not provide a secure connection because of a mismatch of the threads with the NPS thru hull threads and the resulting minimal thread engagement.
Now that I have restated the obvious, allow me to also say that I learned today from the engineering department of large marine supplier that there is yet one more possibility. This supplier provides thru hull fittings with what they call "bastard threads". These "bastard threads" are somewhat similar to an NPS “M” mechanical thread used for assembly of construction scaffolding, but are somewhat different even than an NPS “M” mechanical thread. The “bastard thread” is designed to allow a minimum of three and one half turns until it is hand tight in a female NPT threaded fitting such as an in-line ball valve. They also commented that the industry standard specifies that the threads of the two components must turn three and one half turns by hand and be capable of being further tightened by a mechanical device thereafter.
The marine supplier attests that their thru-hull to in-line ball valve connection meets the industry standard, and that these installations are done regularly at marine yards in lieu of installing seacocks. They sell the same thru-hull with its bastard threads for use in a seacock that they also supply with NPS threads.
So just because your thru-hull has straight NPS fittings doesn't necessarily mean that the threads are standard NPS threads. When you are matching seacocks to thru-hull fittings, the NPS thread design on the two components may be somewhat different. This does not mean that the two threads won't go together, but you have to wonder a bit about whether or not you would want two different NPS thread designs in a bronze-to-bronze connection below water level, and particularly in a thru-hull. The marine supplier sees no problem with this as long as you really seal the connection.
As to whether all of this makes a darn bit of difference, my marine yard reports that they have been installing in-line ball valves in lieu of seacocks for over twenty years without a single reported failure, and that they rarely install seacocks anymore at all. My yard uses the above referenced supplier. The licensed marine surveyor that I used to assess my recent accident damage agrees with my yard that in-line ball valves are just fine, and says that they now represent the standard installation.
However, I still get a very warm and snuggly feeling when I crouch down behind a chandlery plumbing shelf and slip a 1 and 1.2 inch straight standard NPS thru-hull fitting all the way into a bronze Groco seacock, and I don't care how heavy that sea cock is. It feels good and just watching the threads spin down together puts the mind at ease. Nightmares about flying toolboxes subside instantly.
With simple shackles costing forty bucks, and the basic no-chart GPS running $200, why would anyone voluntarily select an in-line ball valve over a seacock to save forty bucks. A two-inch hole in the bottom of a boat is a formidable thing to face at sea.
And Bill, I had a long-sealed, tapered bronze plug, green-with-age seacock that was mounted directly to the hull with no backing plate and no bolts or screws to hold it in place. I assuem that it was original equipment. I wanted to rehabilitate the head upon haul-out. Bad idea. T'would have been a far far better thing to do and a far far cheapert thing as well to buy a porta-potti.
That seacock was so badly stuck that I could not free it. I finally did free it, but that was after the yard had cut it off the through hull from outside the hull, and after I stuck it in a vise on my work bench and tried all of the tricks in the book, including taking off all of the correct parts and using the wooden hammer on the backside trick. What finally did free the valve was a solid swack on the backside with a framing hammer. This is not recommended by any responsible party, but it did work. My guess is that the thing had probably been struck for more than a few years. Anyway, that started me down the long painful road to SEACOCK REALIZATION and the discovery all of the above factoids. As painful as it has been, I suppose that this is all in the way of education
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