NPT = Not Playing That game on my sea cocks
Ebb,
I am not sure that I care to find out. In my still very uneducated opinion, it make no sense to have an NPT to NPT connection between a thru-hull and a sea cock. This does make sense when using an in-line valve, but a seacock must fit snuggly and bolt through or screw into its backing plate. The length of the thru-hull will depend on the measurements A + B in the above drawings. So unless they made a series of these thru-hulls in various lengths, one would be in the position of having to cut off part of the NPT fitting on the thruhull and rethread these things.
When I spoke to the Groco rep, we did discuss the NPT to NPT fittings on their thru hull to in-line valve option for above the water applications. He told me that they design NPT threads to turn by hand 2.5 to 3.5 turns. Having more turns is not OK, and having less turns is not OK. Then you get about another three turns mechanically (as I recall) with your wrench.
That makes a total of 5.5 to 6 threads engaged, and that ain't enough turns in my opinion for a below water and below valve application, despite the fact that many marine yards install in-line ball valves on thru-hulls in exactly this manner, and Groco themselves show you the best way to do exactly what they do not recommend which is to hook one of these NPT ball valve to NPT through hulls. Indeed even some surveyors accepot the use of these things as valid applications, and the ABYC may actually allow them (according to Buck Algonquin). However, Groco and many marine writers strongly recommend against doing this NPT to NPT ball valve to thru-hull thing below water, so why design a seacock that replicates this silly use of NPT fittings even if the NPT to NPT joint would be tucked away within the seaock body with all of it's glorious six threads engaged (you hope).
The idea behind an NPT fitting is that the taper in the male and female threads coming together creates the necessary compression to hold the joint together....provided that the joint will not be stressed laterally. And by the way, plumbers are not supposed to even do this NPT to NPT thing below ground in a fuel tank installation. They are supposed to use something called a swing joint to reduce the potential of ground-movement-caused lateral sheer severing these NPT to NPT joints. This taper is a good thing for most plumbing applications. The Groco rep says that finely machined NPT fittings if tightened correctly will hold without teflon tape or pipe dope. Pretty cool, huh? Well yes, but not were there might be lateral forces such as flying bodies, tool boxes or storage containers, boxes of canned beans, or spare parts that ypou might store beneath the cockpit in the vacinity of your cockpit thru-hulls.
Now in a traditional flanged sea cock, a different approach is used: A sandwich consisting of a sea cock flange with backing block on the inside and a thru-hull flange on the outside, fully engaged male and female threads along the full length of the A + B dimension (The depth of the threads in the bottom of the sea cock plus the distance from the top of the backing block to the outside of the hull less 1/4 inch), and lots of bedding compound between all of the invloved parts creates the compression necessary to hold the system together. The NPT Threads in this application would offer no advantage that I can determine, would make installation more difficult, and actually serve to weaken the joint by reducing the number of engaged threads.
Now having said all of that, the Groco rep also advised me that the industry is changing rapidly and this means that using conventinal wisdom is somewhat dangerous without consulting your manufacturer and installer (or yourself) about the design of the parts and the installation procedures and material to be used. For instance, Groco has gone from straight thread thru-hulls to something approximating "bastard" NPS threads and finally at this point in time to their current combination thread, but their literaure doesn't overtly state that. And then there is my developing expertise which is based partly on reading writers who published books that predate the changes in the industry relating to thread depth... and also one manufacturer's strong recommendation with is current with their product.
For another instance, this business of which bedding cocmpound to use and coming to understand the cohesive and adnesive characteristics of various products such as 5200, 4200 and various polysulfides is not that simple. You can download the detailed 3M technical sheets and still not undertand what you are dealing with. I had to call 3M to dissover that:
1. 4200 does not act as an adhesive on bronze, but only as a sealant.
However, getting it off fiberglass is another thing all together.
2. The jury is still out on the interaction between 4200 and certain plastics such as PVC and nylon. 3M told me to test the product before using. Prudent advice, but what are the standards for that test? Does your marine yard know the answer? Do you? Wanna be a guinnea pig?
Back to threads, your installer (or you) could very easily assume that a part has one sort of thread and find that the truth is far different. So consult the manufacturer and your yard before you install. If you are using a yard to do the installation, and if you get the answer, "Because we have always done it this way," then take your boat and sail rapidly to another yard, assuming that your original thru-hulls and seacocks are still installed. It's a bit late to do so otherwise.
Certainly there is no substitute for hands-on experience when it comes to working with boats, but when the manufacturing environment is changing, being up to date with the technology is equally important.