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

The Ugly

Photo of a crumbling brass pipe and hose adapter
This brass pipe adapter crumbled after just a few seasons of use.
Photo of brass threads in raw-water discharge
Brass threads in a raw-water discharge after only a year.

Stainless steel bears mentioning in this listing of seacock alloys. Contrary to popular belief, except in some metal vessel applications, it's a less than ideal choice for seacocks. Generally speaking, stainless steel, at least the 316 variety, is highly corrosion resistant and exceptionally strong. However, stainless steel is susceptible to crevice corrosion, a malady that occurs when the metal is exposed to an oxygen-poor environment such as stagnant water, as is found inside seacocks and marine raw-water plumbing systems. Therefore, it is less than ideal for use in seacock applications aboard fiberglass or wood vessels.

Glass-reinforced nylon represents a viable and reliable alternative to bronze seacocks and thru-hull fittings. Marelon, a proprietary glass fiber-reinforced polymer that's used by Forespar Products, the most popular manufacturer of non-metallic seacocks and seawater fittings is equal to and in some ways exceeds the reliability and durability of bronze. Marelon's chief attribute is its resistance to corrosion. Being nonmetallic, it is entirely immune to both galvanic and stray-current corrosion, as well as lightning-induced discharge issues. Other plastics such as PVC, nonreinforced nylon, acetyl and polypropylene should not be used in seacock or raw-water applications. All lack the necessary tensile strength and flexural modulus of Marelon.

H-27.6.1.2. Threads used in seacock installations shall be compatible (e.g., NPT to NPT, NPS to NPS).

Among the most important, most common, and most insidious deficiencies where seacock installations are concerned is the issue of thread compatibility, and this was the problem with the installation cited at the beginning of this article. Many boat builders, boatyards and do-it-yourselfers inadvertently, and likely unknowingly, select two components, such as a thru-hull fitting and in-line ball valve that are inherently incompatible. This arrangement is so ubiquitous that I'm tempted to refer to it as a tradition, albeit an undesirable one.

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