When installing a flanged seacock below the waterline, mounting it directly onto a curved or uneven fiberglass hull is a recipe for disaster. Uneven pressure can crack the hull or distort the seacock flange, leading to critical leaks. Groco Marine Seacock Backing Blocks offer a bulletproof, professional solution that replaces unstable marine plywood or messy hand-laid fiberglass donuts.

Molded from a proprietary, high-density synthetic composite, these backing blocks are completely impervious to water absorption, marine borers, rot, fuel, and bilge acids. They are designed to sit perfectly between your boat's hull and the seacock flange, spreading structural stress evenly across the hull surface.

Each backing block is pre-drilled with a center hole to accommodate your thru-hull fitting and comes equipped with integrated, threaded brass inserts. These inserts match the standard triangular 3-bolt mounting patterns of Groco flanged seacocks, allowing you to bolt your seacock securely to the block from inside the bilge without drilling holes completely through your boat's bottom.

Key Features

• Indestructible Composite Build: Solid synthetic fiber-reinforced construction will never rot, delaminate, swell, or absorb moisture over decades of bilge immersion.

• Threaded Brass Inserts: Equipped with pre-installed threaded inserts that match standardized flanged seacock bolt patterns for fast, secure blind-fastening.

• Structural Stress Distribution: Spreads the mechanical loads of the seacock and connected hoses evenly across the hull to protect against impact and leverage damage.

• Easy Hull Contouring: The durable composite material can be easily sanded, planed, or shaved on the bottom side to match the unique curvature or deadrise of your hull.

• Chemical & Fuel Resistant: Completely unaffected by gasoline, diesel, bilge cleaners, and harsh marine environments.

Specifications