How Do Self-Bailing Boats Work?

Boats, particularly open-cockpit and open-bow designs, are subject to overspray, high wave activity and rain. The excess water accumulates in the deck, cockpit or transom area, adding dangerous weight. The excess water weight reduces the freeboard, which lowers the profile of the hull in the water. Too much water can require extra horsepower and reduce fuel economy. The worst scenario for an overladen boat is swamping -- where the boat settles so low in the water it sinks -- or capsizing, where the boat flips over. Self-bailing systems rank as one of the most important safety features on a boat.

Scupper valves are through-hull fittings, either round or square in design, made of high-grade plastic, stainless steel or bronze. A sleeve, most often two, are permanently hard-fitted from the inside of the transom to the outside, inside a large diameter transom hole. A rubber, plastic or metal flapper valve, or ball check, controls the amount of water allowed to enter the scupper valve from the inside of the transom. Scupper valves are normally located on each side of the transom, just above the water line or higher. Scupper valves come in various diameters and discharge rates, depending upon the size and design of the craft.

True self-bailing scupper valves open when the boat is underway. They perform best when the speed of the boat causes a rush of pressure from water on the deck or in the cockpit to open the one-way valve -- water leaves but can not re-enter the boat. Scupper valves are fitted at precise locations above the waterline, taking into consideration the fully laden weight of the boat with passengers, fuel and gear. Even at plane, the boat's transom holds an angle that promotes water to flow backward via gravity. Scupper valves that sit at or below the waterline can cause a reverse flooding if the valve is defective or clogged.

Transom plugs, also called scupper plugs or deck plugs, function as stoppering devices. They can be plastic or rubber and can be removed when the boat is underway to expel unneeded water through the transom. Another bailing device -- one used only when the boat is out of the water -- is the bilge plug. This drains water that has accumulated in the bilge, the bottom-inside of the boat.

In reality, self-bailing transoms are those found on boats that have little or no transom on the boat's stern. The rear of the boat is primarily open, free of an enclosed transom structure. On-board water is flushed to the rear of the boat when underway, then immediately discharged aft. If the deck or cockpit is sealed off from the bilge, there is no need for scupper valves, although some boat types use both self-bailing designs.