No-Nos and Good Practice with Thru-Hulls and Seacocks
Clark October 27th, 2013
A common boatyard misdeed is to install a thru-hull like this,
All images courtesy of Groco
then screw on a ball valve like this,
with an appropriate tailpiece connected to some item of plumbing below the waterline. This is done all the time, but it’s bad practice for several reasons:
1. The threads don’t match. I’m not the first to write about this: If you go here, on Compass Marine’s excellent technical blog, he has even cut a fitting in half to show the difference in the threads.
The thru-hull has straight threads (NPS, National Pipe Straight) and the ball valve has pipe threads (NPT, National Pipe Taper). They’re not meant to go together, but if you force them you can get 2-3 turns and a watertight seal. Two or three turns isn’t enough overlap for a strong joint.
2. The threaded part of the thru-hull isn’t meant to be exposed, at least not when it’s below the waterline. It’s meant to have a seacock surrounding it. Left naked, the grooves where the threads are cut make for thin spots, lots of them.
3. If you use this method you’re not spreading potential loads on the the thru-hull to enough of your hull. Even if you’ve got a good backing plate, the thru-hull should still attach to a flange of some sort, which is then thru-bolted to the hull. By the cheesy method, the only thing spreading the load of the thru-hull, and holding it to the hull, is the bronze nut that came with it.
The ABYC standard is for the assembly to withstand a 500-pound lateral load for thirty seconds.
The proper solution is something like this, a seacock:
It’s beautiful, isn’t it? That big, heavy slab of bronze, with its wide base. So seamanlike. It also retails for nearly $300 for a 1-1/2″ version, which is why so many people cram ball valves onto straight-threaded thru-hulls.
Condesa had two lingering ball-valve-on-thru-hull seacocks that I got rid of during my last stint in the boatyard. Mind you, both of these stood up to at least 12 years and a circumnavigation, but the longer I live and the longer I own my boat, the more serious I get about eliminating weaknesses that could sink her.
I’d been eyeing Groco’s flanged adapters for some time, and they ended up working as advertised:
The idea is to give you the same solid, strong base as a proper seacock, but with pipe threads, which will accept a standard ball valve:
The combined cost of the flanged adapter and a quality ball valve is about half the cost of a bronze seacock of the same size. (A 1″ seacock retails for $180. A 1″ flanged adapter is $40; a good 1″ ball valve $40-$50.)
When the valve wears out, you can just swap out the the ball valve without replacing the whole seacock. The flanged adapter doesn’t look like something that’s going to wear out. For price and swap-ability, the flanged adapter arrangement seems superior to a bronze seacock.
Bronze NPT ball valves are available everywhere in the world. Bronze seacocks are not.
