Groco Thru-Hull Fittings

Groco Thru-Hull Fittings: A Century of Below-the-Waterline Trust

When it comes to fittings that penetrate the hull of your vessel, there is simply no room for compromise. Groco has been engineering below-the-waterline hardware since 1918, and their thru-hull fittings represent some of the most trusted products in that century-long legacy. Founded in Baltimore and now based in Stevensville, Maryland, Groco remains a family-owned, American-made manufacturer whose thru-hull lineup reflects generations of refinement for both recreational and commercial marine applications.

A Series for Every Hull and Application

What sets Groco apart in the thru-hull fittings category is the depth and specificity of their product series. Rather than offering a one-size-fits-all solution, Groco engineers distinct lines for different hull types, installation conditions, and performance demands:

• TH Series (Standard Bronze): The flagship line, cast from C83600 bronze (with 4" and 5" sizes in C84400 bronze), featuring an NPSM/NPT combination thread and available in sizes from 1/2" to 5". These are UL Listed 647B and offered with or without a flanged retaining nut.
• TH-WS Series (Stainless Steel): For boaters who prefer #316 stainless, Groco’s stainless thru-hulls are metallurgically compatible with both bronze and aluminum components, and won’t discolor or corrode engine rooms over time.
• THXL & THXL-WS Series (Extra Long): Designed for thicker hull sections and butt blocks, these extended-shank versions eliminate the need to cut threads in the field on most installations.
• FTH Series (Flush Mount): A low-profile option that sits flush with the hull exterior, minimizing drag — ideal for performance-conscious sailors and powerboat builders.
• STH Series (Scoop Thru-Hull): Directional scoop designs that increase water intake flow for raw water cooling and other intake applications, available in both standard and BSPP thread configurations.
• HSTH & HSTHXL Series (High Speed Pickup): Engineered specifically for increased flow with less drag at speed — a critical consideration for fast powerboats and sportfishers where raw-water intake efficiency directly affects engine cooling.
• TH-CS Series (Bulkhead Fitting): For through-deck and bulkhead plumbing applications where a traditional hull penetration fitting is not appropriate.

Why Material and Certification Matter

Groco’s thru-hull fittings are cast from genuine marine-grade bronze, chosen for its ability to resist saltwater corrosion, freezing, and impact over decades of service. The standard TH Series carries UL Listed 647B certification, an important compliance marker for vessels subject to insurance inspections and ABYC standards. Stainless models offer an aesthetic and corrosion advantage in visible installations, while BSPP-threaded variants serve the international market and European-built vessels.

The NPSM/NPT combination thread found across most Groco thru-hull series is a practical design choice: it allows the fitting to mate reliably with both straight and tapered pipe threads, reducing the risk of cross-threading or seal failure during installation. Combined with the availability of matching TH Series nuts in both bronze and stainless, Groco ensures that every component of the hull penetration comes from a single, engineered system.

Installation Confidence and System Compatibility

Marine surveyors and boat builders consistently recommend pairing Groco thru-hulls with Groco seacocks and backing blocks to create a fully integrated, stress-free hull penetration. The fittings are designed with flange dimensions that align directly with Groco’s valve flanges, eliminating the fit issues that arise when mixing brands. Groco even offers a dedicated thru-hull installation tool (model THT-530) to simplify the process of tightening fittings from inside the hull without damaging the flange or hull surface.

Whether you are repowering a cruising sailboat, outfitting a new build, or replacing aging hardware during a haulout, explore the full range of thru-hull fittings available from Groco to find the right series, size, and material for your vessel.