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SUBJECT: # 26382: Straight-thread seacock sealing question
Submitted by DavidS (204.149.20.50) from TEXAS on 3/2/04 3:43:00 PM
How does a seacock make a watertight seal to its thru-hull
fitting when they're both NPS straight thread? Must we rely
on some ackempucky (thread sealant compound), or does the
seacock have a sealing face at the top of its thread to
mate with the end of the thru-hull? If it seals against the
end of the thru-hull, how do we simultaneously seat the seal
at the bottom of the seacock thread and the seat the flange
securely against the hull? Or is the latter unnecessary/undesirable?
The "usual" pipe thread (e.g. on the other side of the seacock)
is NPT (National Pipe thread, Tapered). The taper allows
clearance to turn the fitting together; when tight, you run
out of clearance and the metal-to-metal interference makes
the seal; the fitting turns up to a specific point, no more and no less (virtually).
The interference between threads makes the seal.
The NPS (National Pipe thread, Straight) on thru-hull fittings
doesn't have this taper. The absence of taper lets you spin on
a nut to whatever depth is needed to secure the fitting against
the hull. It also means there's clearance between the
threads at any position, so they can't seal. Without some
other sealing surfaces, even tight straight threads are a
spiral leak. At low pressures like a thru-hull that may
be a nearly infinitesimal drip, but semi-isolated
seawater-wet metal can't be a good thing from a corrosion
perspective.
- 3/2/04 4:31:00 PM
Submitted by
beachblitz (205.148.63.217) from NEW JERSEY says from West Marine:
got this from the West Marine site:Thru-hull threads are a complicated topic, since you are trying to accomplish different tasks at each end of the seacock. Threads on the thru-hull are untapered (straight) pipe threads, since you don't know how thick the hull is, and you therefore need a locking nut which engages the threads on the entire length of the thru-hull body. This means that the hull side of seacocks have to have straight threads also, since you cannot screw a tapered female thread onto a straight male thread for more than a few turns, and it is guaranteed to leak and lack strength if you do.
The "other" end of the seacock is a different subject, however, since you would commonly thread a tailpiece with a tapered thread into the seacock body. Tapered threads are used here since you want a tight, waterproof seal. In general, you can aim the direction of an ell tailpiece in the right direction with judicious tightening.
This is one of the BEST reasons to NOT use a ball valve, or worse yet, a gate valve, on a thru-hull. While ball valves are great for inline applications, they all use tapered threads, which will not engage the thru-hull threads correctly. Plus, they cannot be bolted to the hull for strength.
- 3/4/04 12:19:00 AM
Submitted by
JackR (216.12.192.74) from TEXAS says Check this out
http://www.boatus.com/boattech/casey/07.htm
- 3/7/04 12:02:00 AM
Submitted by
DavidS (24.174.80.75) from TEXAS says That boattech article makes clear
both that the NPS threads don't seal and that there's no other metal-to-metal seal between a seacock and its thru-hull fitting, so we have to rely on thread sealant and a good fit between the flange and its backing block. I guess it should've been obvious; we already have to rely even more heavily on sealant between the fitting and hull, a thin film of ackempucky sealing straight threads is nothing in comparison.Thanks, guys. That was bothering me. I'm OK now.
