What's Below Your Waterline?

Seacocks fall into the 'out of sight, out of mind' category for many boat owners ... until they are needed.

By Steve D'Antonio
Published: April 2014

Photo of NPS and NPT mismatched threads Photo: RC Collins, Compass Marine Inc.

When NPS and NPT threads are mismatched, the seacock (left)
won't have enough threads engaged.

Thru-hull fittings typically utilize national pipe straight or NPS threads. These threads are parallel for the entire length of the fitting, not unlike those of a common machine screw. Nearly all in-line ball valves, however, rely on national pipe taper or NPT threads. As the name implies, these taper or are cone-shaped. NPS and NPT threads are entirely incompatible, and under no circumstances should they ever be mated together. In most cases, the thread engagement is no more than two threads, three at most, compared to a proper seacock's thread engagement, using NPS to NPS hardware, of eight or more threads.

A purpose made seacock or seavalve's internal, female threads are NPS, and thus fully compatible with those of the thru-hull fitting. The two engage completely and make for an exceptionally strong installation. Furthermore, nearly all seacocks incorporate a load-distributing flange into their design, enhancing their integrity. Not only do in-line ball valves not incorporate a flange, the sole means of retention to the vessel's hull is via a gossamer nut that's included with most thru-hull fittings; it typically is a scant three threads deep and is designed to retain the thru-hull fitting alone, not the added leverage of a valve as well, even one whose threads are compatible. Backing blocks further distribute load over an even greater area than the seacock flange itself, and, again, such load distribution is welcomed. Backing blocks are also used to adapt a hull's concave shape to the flat mounting surface of the seacock's flange.

Backing blocks should be roughly a minimum of 1.5 times the diameter of the seacock's flange. Suitable material includes epoxy-encapsulated, marine (void-free) plywood or a fiberreinforced laminate such as GPO3 or G10. If the seacock's flange is equipped with fastener holes, they must be used, either with lag bolts that are screwed into the backing block alone or through bolts that pass completely through the hull. If the latter, the fasteners must be bronze rather than stainless steel.

If you are considering buying a vessel, new or used, carefully scrutinize all seacock installations for thread incompatibility. In many cases, I find the original builder's seacocks are correct, however, after-market installations, including air-conditioning or water makers on new vessels (these are often installed by dealers or subcontractors when a boat is sold), utilize incompatible threads. If your current vessel's fittings are installed with incompatible threads, you have a dilemma. I'm often asked by folks in this position, "Should I change all the seacocks?" The advice I provide my clients encourages their replacement, particularly in the case of offshore-capable, oceangoing vessels. If you opt to leave them in place, you must do so knowing they are dangerously weak.

H-27.6.1 A seacock shall be securely mounted so that the assembly will withstand a 500 pound static force applied for 30 seconds to the inboard end of the assembly, without the assembly failing to stop the ingress of water.

H-27.6.1 involves the durability of the seacock installation. This test will typically separate true seacocks from incompatible-thread thru-hull and in-line ball valve assemblies. The meager thread engagement of the former simply can't stand up to this sort of load. It's also worth noting that the weight for this test must be applied to the most inboard segment of the assembly, which means if the installer has rigidly plumbed a six-inch pipe nipple, a T-fitting and strainer directly to the seacock, the leverage imparted by these components is often enough to cause a failure at well under the specified test load of 500 pounds. Therefore, it is my strong recommendation that installers avoid directly plumbing anything to a seacock other than a pipe-to-hose adapter. Isolating the seacock from other hard plumbing with a suitable J2006-rated hose actually enhances its durability.

While it's easy and not uncommon for a yard or builder to say "That's the way we've always done it," if what they are doing fails to comply with this standard, they have little to fall back on in the event of a failure. From the boat owner's or buyer's perspective, insisting on this compliance is, in my opinion, not only wise, it's mandatory if you wish to have peace of mind regarding what's below your waterline.End of story marker

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A former full service yard manager and longtime technical writer, the author now works with boat builders, owners and others in the industry as, "Steve D'Antonio Marine Consulting Inc". (www.stevedmarineconsulting.com). His book on marine systems will be published by McGraw-Hill later this year.

Continue to Slideshow: Seacock Installation



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