In some elbow designs, the jacket may hold water at all times, even when the engine isn't running. These are known as jacketed risers, and they're often used to raise the injection point above the vessel's sailing or dynamic waterline. If the outer wall of the jacket develops a leak, seawater spills out, often onto the engine, starter, or transmission, where it may lead to corrosion and other problems. If, however, the inner jacket should develop a leak, the consequences can be more dire. Seawater may leak back into the engine through open exhaust valves, causing severe damage that's frequently irreparable.