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Marine Plumbing & Ventilation - Thru-Hull Fittings
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Marine Thru-Hull Fittings That Keep Water Where It Belongs — Trusted Brands, Every Size You Need
Marine thru-hull fittings are among the most critical components on any boat, forming the sealed connection between your hull and the hoses that carry water in and out for livewells, bilge systems, cockpit drains, and raw-water intakes. This collection brings together fittings from GROCO and Attwood Marine — two names that professional marine installers and serious boaters rely on — spanning a wide range of diameters, materials, and configurations to match virtually any application above or below the waterline.
What Sets These Fittings Apart
- Watertight integrity where it matters most — bronze and stainless steel construction resists corrosion in saltwater and freshwater environments alike, giving you long-term reliability in demanding marine conditions.
- Tight spaces, solved — 90-degree configurations like the GROCO 1-1/2" 90 Degree Hose Thru-Hull Fitting and the GROCO 1-1/4" 90 Degree Hose Thru-Hull Fitting let you route hose lines cleanly when a straight run simply won't fit.
- High-speed applications handled — GROCO's bronze high-speed fittings are designed for performance hulls where water pressure and flow rates are significantly higher than on displacement vessels.
- Extra depth, extra security — GROCO's extra-long series provides the additional thread engagement needed for thick composite or cored hulls where standard-length fittings come up short.
- Budget-conscious builds without cutting corners — the Attwood White Plastic 90 Degree Thru-Hull Connector offers a lightweight, corrosion-proof option well-suited for above-waterline drains and non-structural freshwater applications.
How to Choose the Right Thru-Hull Fitting for Your Boat
Start with location: fittings below the waterline should be bronze or stainless steel — never plastic. For above-waterline cockpit drains or scuppers, high-quality plastic can be appropriate and saves weight. Next, match the inner diameter to your hose size precisely; a mismatch here leads to leaks or restricted flow. Attwood's straight barbed stainless fittings, available in sizes from 5/8" up to 1-1/8", make it straightforward to find the right match for common hose sizes.
Also consider your hull thickness. Cored or heavily glassed hulls often require an extra-long fitting to ensure the nut seats properly on the interior surface. Finally, think about flow angle — if the hose needs to exit at a right angle to the hull, a 90-degree fitting eliminates stress on the hose and produces a cleaner installation. When in doubt, consult a qualified marine technician to confirm the right specification for your vessel.
Frequently Asked Questions about Marine Plumbing & Ventilation - Thru-Hull Fittings
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Thru-hull fittings are marine plumbing components that create a watertight passage through a boat's hull, allowing water to flow in or out for systems like bilge pumps, livewells, and exhaust. Thru-hull fittings are essential for keeping those openings sealed and structurally sound while the vessel is in use.
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Thru-hull fittings are designed for boat owners, marine mechanics, and DIY boaters who need to install or replace hull penetrations for plumbing, ventilation, or drainage systems. Anyone working on a vessel's below-waterline or above-waterline plumbing will need the correct thru-hull fitting for a safe, leak-free connection.
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Start by drilling the correct size hole through the hull at the desired location, then insert the thru-hull fitting from outside the boat and secure it with the included nut from the inside. Apply a marine-grade sealant around the flange before tightening to ensure a watertight seal and prevent water intrusion.
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The right thru-hull fitting size depends on the inner diameter of the hose or pipe you are connecting, with common sizes ranging from 3/4 inch up to 2 inches for most marine plumbing applications. Always match the fitting's inner diameter to your hose size to ensure a proper, leak-free fit.
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A straight thru-hull fitting passes water directly through the hull in a linear path, while a 90 degree thru-hull fitting redirects flow at a right angle, which is useful when space behind the hull is limited or when hose routing requires a sharp turn. Choosing between the two depends on your boat's layout and plumbing configuration.
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Marine thru-hull fittings are commonly made from bronze, stainless steel, or plastic, each offering different levels of corrosion resistance and strength. Bronze and stainless steel thru-hull fittings are preferred for below-waterline installations because of their durability in saltwater and freshwater environments, while plastic options work well for above-waterline use.
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Yes, thru-hull fittings can be used below the waterline, but the fitting must be made from a material rated for submerged marine use, such as bronze or stainless steel. Plastic thru-hull fittings are generally recommended for above-waterline applications only, so always check the manufacturer's guidance before installing below the waterline.
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Because high speed thru-hull fittings are specifically shaped to reduce drag and turbulence when water flows past the hull at higher vessel speeds, making them the better choice for faster boats. Standard thru-hull fittings work well at slower speeds, but a high speed design helps maintain performance and prevents water from being forced back into the fitting.
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Inspect thru-hull fittings at least once a season by checking for corrosion, cracks, loose nuts, and deteriorated sealant around the flange. Clean the fittings with a soft brush and fresh water after saltwater use, and replace any fitting that shows signs of pitting, cracking, or significant corrosion to avoid hull leaks.