90° Marine Thru-Hull Drain Fitting with Check Valve — Stop Backflow Before It Starts
A drain outlet that lets water flow back into your bilge the moment the pump shuts off is more than an inconvenience — it’s a safety hazard. Whether you’re plumbing a bilge pump discharge, a livewell drain, or a cockpit scupper on a powerboat, sailboat, or pontoon, a properly rated 90° thru-hull drain fitting with a built-in check valve is the right way to do the job. This fitting is built for boaters, marine DIYers, and boat builders who need a reliable, corrosion-resistant elbow outlet that installs cleanly through the hull and stays watertight.
Each unit is a single fitting consisting of a 90° elbow body, a threaded hull-mount shank, a flange nut, and (on check valve variants) an integrated one-way valve. Molded from UV-stabilized ABS plastic. Available in multiple configurations: standard 90° drain outlet, 90° drain with check valve, mirror-finish check valve version, and right-angle pump connector. Hose barb sizes range from 19mm (≈3/4”) to 38mm (≈1-1/2”). Small size: H82mm, flange OD 51mm, barb ID 13mm / OD 19mm, body thread OD 25mm, weight 39.5g. Large size: H98mm, flange OD 65mm, barb ID 21mm / OD 29mm, body thread OD 35mm, weight 68g. Available in black and white.
Typical use cases include bilge pump discharge outlets on powerboats and sailboats, livewell and baitwell drain fittings on fishing boats, cockpit and deck drain scuppers, and waste water outlet connections on marine heads and sinks. The 90° elbow geometry makes this a go-to choice for anyone searching for a marine thru-hull elbow fitting or a boat bilge pump drain outlet with check valve.
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Compatibility Note: Designed for standard hose barb connections. Match the barb OD to your hose inner diameter — 19mm, 20mm, 24mm, 25mm, 29mm, or 38mm. Compatible with SEAFLO and most major marine pump brands. Use marine-grade sealant on hull flange during installation.
✅ Key Features of These 90° Marine Thru-Hull Drain Fittings
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🔄Built-In Check Valve — The integrated one-way valve closes automatically when the pump stops, preventing bilge water or overboard water from siphoning back into the boat — a critical safety feature for at or near-waterline drain installations.
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📐90° Elbow Design — The right-angle geometry routes the discharge hose parallel to the hull interior, saving space in tight bilge compartments and reducing hose stress compared to straight thru-hull fittings.
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🧱UV-Stabilized ABS Body — Molded from corrosion-resistant ABS with UV inhibitors, this marine drain fitting holds up against salt spray, fuel vapors, and sun exposure without cracking, yellowing, or degrading over time.
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✨Mirror Finish Option Available — The polished-flange variant delivers a clean, factory-integrated look on fiberglass and painted hulls — ideal for visible installations where hull aesthetics matter.
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🔧Threaded Shank with Flange Nut — The threaded body passes through the hull and locks with a nut from the exterior, creating a secure, watertight mount. No special tools required — compatible with standard marine installation practices.
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💧Multiple Sizes for Different Hose Diameters — With barb sizes from 19mm to 38mm, these fittings cover the full range of common bilge pump hose sizes, making it straightforward to find the right match for your existing plumbing.
📐 Technical Specifications — 90° Marine Thru-Hull Drain Elbow Fitting
| Product Type |
90° Thru-Hull Drain Fitting / Marine Elbow Outlet |
| Material |
UV-stabilized ABS plastic |
| Check Valve |
Integrated one-way valve (on check valve variants); not included on standard drain outlet |
| Elbow Angle |
90° |
| Small Size — Dimensions |
H 82mm (≈3.2”) / Flange OD 51mm (≈2.0”) / Body Thread OD 25mm / Barb ID 13mm / Barb OD 19mm (≈3/4”) |
| Small Size — Weight |
39.5g |
| Large Size — Dimensions |
H 98mm (≈3.9”) / Flange OD 65mm (≈2.6”) / Body Thread OD 35mm / Barb ID 21mm / Barb OD 29mm (≈1-1/8”) |
| Large Size — Weight |
68g |
| Available Barb Sizes |
19mm, 20mm, 24mm, 25mm, 29mm, 38mm (see variant options) |
| Available Colors |
Black, White |
| Variants Available |
90° Elbow, 90° Drain with Check Valve, Mirror Finish Check Valve, 90° Drain Outlet, Right Angle Pump Connector |
| Mounting Method |
Threaded shank + exterior flange nut (marine sealant recommended) |
| Quantity per Order |
1 piece |
| Recommended Use |
Bilge pump discharge, livewell drain, cockpit scupper, marine head waste outlet |
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Installation Note: Always apply marine-grade sealant (e.g., 3M 5200 or equivalent) around the hull flange before tightening. For below-waterline installations, verify the fitting’s suitability with a qualified marine technician. Hose clamps are recommended to secure the hose to the barb.
❓ Frequently Asked Questions
How do I choose the right size thru-hull drain fitting?
Match the hose barb diameter to your existing hose inner diameter. Available sizes include 19mm (≈3/4”), 20mm, 24mm, 25mm, 29mm (≈1-1/8”), and 38mm (≈1-1/2”). Measure your hose ID before ordering. The hull flange outer diameter is larger and sits flush against the hull exterior — account for this when planning your cutout.
What does the check valve do, and do I need it?
The built-in check valve (one-way valve) prevents water from flowing back into the boat through the drain outlet when the pump is off or when the vessel is heeled. It is generally recommended for any below-waterline or near-waterline drain installation to prevent backflow flooding. If your drain exits well above the waterline, a standard drain outlet may suffice.
What material is this fitting made of, and is it corrosion-resistant?
The fittings are molded from UV-stabilized ABS plastic with anti-corrosion additives. ABS is generally well-suited for marine environments, resisting salt water, fuel vapors, and UV degradation better than standard plastics. It is lighter than bronze or stainless alternatives and will not rust or oxidize.
Can I use this fitting below the waterline?
The check valve variants are designed with backflow prevention in mind, making them suitable for at or near-waterline installations. For installations significantly below the waterline, we recommend consulting a marine plumber and using marine-grade sealant around the hull flange. Always verify the fitting’s pressure rating matches your application before installing.
What is the difference between the standard drain outlet and the mirror finish version?
The Mirror Finish variant features a polished, smooth exterior surface on the hull flange, giving a cleaner aesthetic on the hull exterior. Both versions share the same check valve mechanism and hose barb connection. The mirror finish is typically chosen for visible installations on fiberglass or painted hulls where appearance matters.
How is the fitting installed through the hull?
Drill a hole in the hull matching the threaded body diameter. Insert the fitting from inside the hull, apply marine-grade sealant (such as 3M 5200 or equivalent) around the flange, then thread on the exterior nut to clamp the fitting against the hull. Attach your hose to the barb and secure with a hose clamp. Always follow your boat manufacturer’s guidelines for thru-hull installations.
Is this compatible with SEAFLO bilge pumps?
These fittings are designed to be compatible with standard hose barb connections used by SEAFLO and most other marine pump brands. Match the hose barb size to your pump’s discharge hose diameter to confirm compatibility. The 19mm and 29mm sizes cover the most common SEAFLO pump hose sizes.
Do I need a hose clamp, and what tools are required for installation?
A hose clamp is generally recommended to secure the hose to the barb fitting, especially in bilge pump applications where vibration is common. Installation typically requires a drill, a hole saw matching the shank diameter, a wrench for the flange nut, and a sealant gun. No specialized marine tools are required in most cases.
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