How to Install Scuppers on a Self-Bailing Deck

Measure the deck height of your boat above the waterline with the fuel tanks full. If you normally carry several passengers on your outings, enlist some people to stand inside the boat, to simulate that load. If your deck height equals 4 inches or more above the waterline, you are safe to install a scupper or two just above the deck surface in the transom. If your height sits at or below the waterline, think about lightening your load or redistributing your passengers up toward the front when taking the boat out.

Trailer your boat to a convenient work location. Locate the old scupper on your boat, if you need to replace it. Look for screws holding a circular flange, either on the inside or outside of the transom -- different brands vary with configuration. Remove the screws with a screwdriver. Use a paint scraper to pry the flange ring off -- be careful not to force it too hard. Pry evenly around the flange ring until it breaks free. Pull it off the transom.

Pry the other end of the flange lip free and pull the drain tube assembly out. Use the paint scraper to shave off any old marine caulk or silicone on both sides of the transom. Remove the old caulk or silicone from inside the transom hole, making sure the bore is clean and smooth.

Apply new silicone inside the transom bore hole and on the inside and outside of the transom, where the flange and flange lip will mount. Push a new scupper valve assembly into the hole until flush with the transom. Insert the inner (or outer) flange ring and reinsert the screws by hand in their mounting holes.

Tighten the screws with a screwdriver. Note: some scupper valves will have through-bolt screws, so attach the nuts on the opposite side and hold them with an end wrench and tighten the screws. Wipe away excess marine silicone with a rag.

Measure the thickness of your transom with an outside micrometer, from the top over the transom rail. Remember this measurement if you are installing a scupper valve for the first time on your boat. Purchase a scupper valve that has the same tube drain length dimensions as your transom thickness. Select a tube drain diameter of at least 1 inch or more, which will allow an adequate drain flow.

Position yourself on the boat deck next to the transom, and pick a spot just above the deck to drill your drain tube hole. If installing two scupper valves, space them evenly apart from the outside rails. Fit the inside flange ring over your proposed location, to make sure it will fit without hitting the deck. Use a drill and a hole saw bit to drill the hole completely through the transom. Make sure the hole matches the outside diameter of the scupper drain tube.

Apply marine silicone to the inside of the bore hole and to the inside and outside of the transom where the flanges will mount. Push the new scupper valve into the hole until it sits flush, taking care to set it with the arrow facing the rear of the boat, which shows the water flow direction. Place the inside, or outside, flange ring on the scupper valve, seating it flush. Use a drill and small 1/8-inch bit to drill the mounting screw holes through the flange ring, into the transom.

Insert the mounting screws and tighten them evenly with a screwdriver. If the scupper valve has tube guides for a through-bolt design, drill through to the other side of the transom and install the nuts on the threads. Then, tighten the screws from the inside (or outside) with a screwdriver. Wipe away all excess silicone with a rag.