The term "self bailing" has been used for many years but what does it really mean? A short explanation may be enough. Self bailing suggests that the boat at rest will drain by gravity water setting on the floor, sometimes called the deck or even sole. This is normally a single level design with a one piece horizontal floor, much as we would expect in a typical center console design, but not limited to CCs. Now let's move on to the details.
For this to happen the floor must be above the outside water level when the boat is at rest, or floating to be more specific. The drains themselves can be below the outside water level as long as the water enters above the outside water level. Or the drains can be straight out the transom or side of the boat. Either arrangement can drain the floor. The higher the floor is above outside water level, the quicker the water will drain. Closer to water level of course results in a slower drain. With smaller boats that can have very little height of the floor above water level, a hard rain can result in water gathering on the floor since it can come in quicker than it can drain. Weight added to the boat can also affect the rate of draining or even the ability for the floor to drain. For example, take a small 17' center console "self bailing" design. Sitting at the dock with no passengers the floor may be only an inch or so above the outside water level. A rainstorm should drain by gravity, but add a person to that boat and it may displace just enough more water for the self bailing feature to stop. In a small boat that passengers weight can move to the stern and make the draining even worse, or to the front and improve the draining. The smaller the boat the more dramatically passenger or other weight affects its at rest floating depth and attitude.
But shouldn't a self bailing boat, even a small one, always self bail? In a perfect world that would be great, but design limitations and natural physics prevent that from happening. The boat's self bailing capability can be enhanced by raising the floor level in the hull, but that reduces the inside gunnel height, and at some point the boat is uncomfortably shallow. In a small boat that shallow feel happens fast as the floor level is raised. It is all a compromise. Moving up to an ever bigger boat and the self bailing ability is enhanced along the way, with some of the bigger boats to maintain dependable self bailing even with a full load of passengers, fuel, and equipment, while still providing a comfortable interior depth.
Should all compartments in a self bailing boat also drain overboard? Again that would be nice, but there are limitations in design that often prevent that or render it impractical. For example; a fish or storage box under the floor would likely have the drain under the outside water level at rest. This would require a pump to evacuate that water, or a gravity drain into the bilge where a bilge pump could remove it. A sink on the other hand could be plumbed to drain overboard if the drain itself is above the water level. It depends on the level of the compartment's drain and plumbing. Most self bailing designs have some areas that do not drain overboard, and consequently most of these boats do have bilge pumps for that purpose.
