Forespar Thru-Hull Fittings

Forespar Marelon® Thru-Hull Fittings: The Corrosion-Free Standard for Marine Plumbing

When it comes to penetrating your hull — one of the most safety-critical tasks in marine plumbing — the material and engineering behind your fittings matter enormously. Forespar has built a decades-long reputation around this exact challenge, and their answer is Marelon®: a proprietary polymer composite that has redefined what a thru-hull fitting can be. Browse the full range of thru-hull fittings to find the right solution for your vessel.

What Is Marelon® and Why Does It Matter?

Marelon® is not ordinary plastic. Marelon thru-hull fittings are made from a proprietary formulation of polymer composite compounds — precision-molded to deliver strength, light weight, and complete freedom from corrosion and electrolysis that can plague traditional bronze hardware over time. For boaters in salt water especially, eliminating galvanic corrosion at through-hull penetrations is a serious safety consideration, and Marelon® addresses this directly.

Forespar's thru-hull connectors are UL (Underwriters Laboratories) and ABYC (American Boat and Yacht Council) approved, and many products also meet ISO marine standards — giving boatbuilders, yards, and owners verifiable assurance that these fittings perform to recognized industry benchmarks.

Series Overview: Flush Head, Mushroom Head, and Anti-Venturi Designs

Forespar offers a thoughtfully organized lineup of thru-hull connector styles to suit different installation needs:

• Series 250 – Flush Head Thru-Hull Connectors: Crafted in Forespar's California factory from high-strength Marelon®, these low-profile fittings are suitable for both above and below waterline use and feature NPSM (National Standard Free-Fitting Straight Mechanical) threads for a reliable, non-tapered seal.
• Series 251 / CF 251 – Mushroom Head Thru-Hull Connectors: The classic mushroom profile offers a broader bearing surface against the hull. These fittings can be paired with optional elbow connectors when a 90° intake angle is required.
• Series 253 – Mushroom Head with Hose Barb: Designed for direct hose attachment, these are a popular choice for wet exhaust and drainage applications above the waterline.
• Anti-Venturi (AV) Thru-Hulls: Engineered to prevent siphoning and unwanted water ingestion at speed, these specialty fittings are available as an option within Forespar's Pro Series valve configurations.

Buttress Threads and the Pro Series System

For below-waterline applications paired with Forespar seacocks, the Pro Series thru-hulls use a specialized "buttress" style thread with flat thread faces that provide superior load-carrying strength — a geometry particularly well-suited to the mechanical properties of polymer composites. This thread design is exclusive to Pro Series valve and thru-hull assemblies, ensuring a matched, engineered system rather than an ad-hoc mix of incompatible hardware.

Forespar's integrated approach is a key differentiator: their seacock and thru-hull assemblies are designed together to eliminate thread-style mismatches, a common source of installation errors and potential leaks when mixing hardware from different manufacturers.

Made in the USA, Backed by Decades of Marine Engineering

Forespar has been manufacturing marine products since the mid-1960s, and their thru-hull fittings are still produced at their California facility. The operating temperature range of Marelon® spans -40°F to +176°F (-40°C to +80°C), making these fittings reliable across all seasons and climates. Installation follows the same techniques and bedding compound practices used with traditional bronze — so switching to Marelon® doesn't require a learning curve.

For boaters upgrading aging bronze hardware or specifying a new build, Forespar's Marelon® thru-hull fittings represent a proven, lightweight, and genuinely corrosion-free alternative backed by one of the marine industry's most trusted names in plumbing innovation.