Scott.
read the end of post #29 - follow instructions to the letter.
As post #28 intimates: THERE ARE NO THRUHULLS FOR INLINE BALL VALVES.
[ further research refutes this, at least for Marelon]
So you have to use the backing nut of the thru hull with suitable rubber compoubd to weld the fitting in place. Then you have to turn the NPT valve on to the NPS, which very soon stops turning. If the idiot keeps turning he or she screws up the threads on the thruhull fitting. And the glued in thruhull will start turning, And can possibly crack the threaded tube because it is thin and compromised by the very threads that make it a fitting. Remember, it is a fitting designed for a seacock. It is not designed to take a load.
Pipes and fittings come is thicknesses too. Schedule 20, 40, 80, 160 - same outside diameter but bigger number thicker wall. You might look for thicker wall thruhulls, if you will persist on screwing an inline valve on to your Schedule 20 thru hull fitting
It is possible that some manufacturer by now has a thru hull with NPT on top and a locking nut on NPS threads on the bottom. I don't know. It would be a no brainer to produce. NOW, if such an item does not exist, we have to assume some regulatory agency says they can't. If they can't then boat builders are installing inline valves illegally as seacocks, so is your yard manager, and your surveyor should go back to house painting. He's dead wrong too.
In a properly installed seacock, as specified above earlier post no b.s., The thru hull fitting can be said to FLOAT inside the seacock. There are NO loads on the fitting. There is NO backing nut on the thruhull when the seacock is there. You cannot compare the two installations, they are apples and oranges.
The seacock is installed first with the flange either lagged onto the backing block or thru bolted thru the hull.* The thruhull is then screwed into the unit and seated onto the hull if mushroom - or into the hull if flush. One merely screws the fitting in dry and out again for shortening until it fits perfectly. This is how it has been done for decades. NO strength is imparted to the seacock installation by the thruhull.
*If that mound under Bill's seacock is epoxy-chopped strand-cabosil mix and permanently affixed ie molded to the hull and if the seacock is glued on there with polysulfide or 5200, then IMCO it is good enough to screw some lags into the backing thru the designated holes - bolts being unnecessary.
Thank you.
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