Thru-Hull with Non-Return Flap Valve Ø25 mm (Plastic Scupper)

This thru-hull fitting with a non-return valve combines a standard hull drain with a mechanism to stop water from flowing in the reverse direction. Inside the fitting is a flap (a swinging cap or rubber flapper) that opens under the pressure of water flowing out from inside the boat and automatically closes if water from outside tries to come in. Thus, you have a permanently open drain for outflow, which will not allow external water to rush back in – especially useful in rough seas or when the boat comes to a sudden stop.

At Ø25 mm diameter, this fitting suits a wide range of small and mid-sized boats. Its plastic body is lightweight, adding minimal stress to your hull, and of course it will not corrode. It installs like a regular through-hull – on the exterior you’ll see a modest white flange that can be placed low on the transom or hull side. It is secured from the inside with a nut. Once installed, the valve operates automatically with no need for manual control or power.

• Applications: An excellent solution for small boats, fishing vessels, and RIBs where drain outlets are near the waterline. For example, if you have a bilge pump discharge outlet low on the transom, this valve will allow the pump to eject water but prevent waves from pushing water back in through the opening
• Installation: Standard procedure – drill a hole (~33 mm, according to the threaded neck’s outer diameter) at the desired location. Insert the fitting; the flange sits on the outside, and the plastic nut is tightened from the inside. It’s recommended to use marine sealant under the flange for a 100% watertight seal. The flapper valve is built into the tube and requires no adjustment. Ensure the valve’s opening direction is correct – typically oriented to open outward
• Design: The internal flap is designed to operate with minimal pressure – even a small amount of bilge water will push it open and flow out. Conversely, even a slight external wave pressure will snap the flap shut. This is achieved by a lightweight flap construction (usually silicone or rubber) and precise calibration. The flange (54 mm diameter) is wide enough to provide stability but compact enough for small hull surfaces
• Advantages: Unlike completely open drains, this valved drain gives peace of mind in adverse conditions – you won’t see water surging back through the holes with every wave. The plastic non-return fitting won’t rust, and it doesn’t need lubrication or special maintenance. It’s a good idea to occasionally inspect the flapper inside for any debris (like leaves or small trash), especially after long periods of inactivity, to ensure it seals fully
• Safety: Having a one-way valve on your drains improves seakeeping in rough weather – the boat stays more buoyant and stable since it isn’t taking on extra water through the drains. This contributes to the safety and comfort of everyone on board by reducing the risk of sudden cockpit flooding through low-level outlets