How To Install A One-Way Scupper Valve

By Dan Armitage

If you find your feet getting wet on your boat's self-bailing deck, something's amiss.

Whether it's worn scupper-valve flaps or the weight of a heavier four-stroke outboard (or crew members!) at the transom of your boat submerging the self-bailing drains, if your deck scuppers are allowing your deck to flood when weight is shifted to the transom, or the boat is backing, consider installing an aftermarket scupper system that allows water to flow in only one direction: off the deck and out of the boat.

Photo of the tools need for the scupper valve installationThe tools needed were minimal — a Phillips-head screwdriver and marine-grade sealant. (Photo: Dan Armitage)

Several styles of one-way scuppers designed for use on self-bailing decks are available: duckbill, flapper valve, and ball style are most common, for which aftermarket versions are available. The ball style is popular among owners of trailerable boats for their effectiveness and relative simplicity, and we selected T-H Marine Supply's Flow-Max Ball Scupper ($14.99 at for our application. The T-H offers a large exit hole for quick draining, and a twist-off feature makes for easy cleaning when leaves or other debris clog the system. The Flow-Max ( meets all ABYC standards, includes a stainless-steel security pin, multiple mounting holes, and a neutral buoyancy ball that won't float up and break the seal. All one-way scupper valves should be checked regularly for possible debris.

Here are the steps we took when installing one of two Flow-Max drains on a center-console boat:

  • Photo of removing the scupper assembly
    Photo of filling original screw-mounting holes with selant

    1. First, remove the original scupper assembly. See tools needed (see above).

    2. Fill the original screw-mounting holes with marine-grade sealant.

  • Photo of fitting the Flow Max base-mounting over the scupper hole
    Photo of securing the scupper bowl assembly to the base

    3. Fit the Flow-Max base-mounting plate over the scupper hole and use the screws supplied to secure to the transom. In most cases, such as this, the plate's holes will align perfectly with those that secured the original scupper-valve mounting plate so no drilling is required.

    4. Secure the scupper bowl assembly to the base with a twist-and-lock motion.

  • Photo of using the locking screws supplied, securing the bowl to the base
    Photo of completed assembly

    5. Using the lock screws supplied, secure the bowl to the base. All that's needed to clean the bowl is removal of the screws and twisting the ball unit free of the mounting plate.

    6. Installation is complete. 

— Published: Fall 2013


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