Stainless Steel 316 Thru-Hull (Threaded) – Sizes 3/8" to 2"

A threaded thru-hull is a critical component in a boat’s plumbing system, effectively forming the interface between onboard systems and the water outside. These AISI 316 stainless steel thru-hull fittings are engineered to withstand the constant contact with seawater, galvanic stresses, and mechanical forces that occur at the hull penetration. The outer flange (mushroom head) has a smooth, low-profile dome that sits on the hull exterior, which minimizes drag and prevents catch points for lines or debris. The flange is typically either integrally cast or welded to the threaded neck and often features cross-bar slots or holes, allowing you to use a tool to hold the fitting in place while tightening the retaining nut inside. Installing the thru-hull: You’ll insert the threaded portion from outside the hull and then thread on the included nut (or use a compatible backing nut) on the inside, sandwiching the hull (and a bedding compound/sealant) between the flange and nut for a leak-proof seal. Because they come in sizes from 3/8" to 2", you can equip everything from a small drain (like a sink or baitwell drain with a 3/8" or 1/2" fitting) up to large engine intakes or deck drains (1-1/2" and 2" for high flow). Inside the boat, the male thread will typically connect to a bronze or stainless seacock (ball valve) for safety—especially below the waterline installations—allowing you to shut off water ingress. Alternatively, for above-waterline drains, a hose can be directly clamped using a threaded hose barb adapter on the inside. Each stainless thru-hull is precision-machined to ensure the threads are clean and compatible with standard pipe fittings (most are NPS or NPT threads; check specifications). The strength of stainless steel means these fittings won’t snap or crack under load (for instance, if a hose is accidentally hit or if the boat hull flexes), providing more reliability than plastic thru-hulls. They are also more fire-resistant, which is a consideration in engine compartments. Maintenance is minimal – periodically inspect the exterior flange for marine growth (these can be gently cleaned as needed) and ensure the internal connection remains tight. By using 316 stainless thru-hulls, you add an extra measure of quality and safety to your boat’s plumbing, as they combine a polished, professional appearance with robust performance. These thru-hulls will keep water flowing where it should (or keeping it out, as needed) for many voyages to come.