I have a Catalina 27 (1974) with an inboard Atomic 4. The more I think about it, the more I am convinced the exhaust system is ill-designed. I am not quite sure if that design is specific to my boat or if it is the way all Catalinas 27 are set up. I have a hot exhaust elbow, connected to the manifold. Cooling water is injected at the outlet of the elbow, a few inches before the elbow is connected to the hose. The elbow is approximately 10" below the cockpit sole, which is very low. At cruising speed water rises on the transom to a height approximately 10" above the exhaust hole, which is very high. I noticed a few times that the engine would not start after an extended period of cruising under following seas. I realized that I could have a siphoning problem. Under a wave bigger than usual, water would make its way through the exhaust hose, pass the elbow and "drop" into the manifold! I considered several options: 1) Adding a check valve just after the elbow. I am very skeptical though as to the water tightness of such a valve. I think they work well under sudden back pressure but not under constant load such as in the case of following seas. In other words, if you pour water in the "ouptut" side of a check valve, I think water will go through! Does anyone have experience with check valves? 2) Adding a normal valve: the problem with that is the accessibility. Unless you keep the port lazarette empty, closing and opening that valve will be a major pain. Ideally, it should be placed close t the transom but there again, access is impossible. Due to the "wet" lazarette design, I can not cut a hole through the bottom of the transom lazarette. This, by the way, also makes changing the exhaust hose close to impossible. Another problem of a valve is the risk of filling the engine with water if you crank it when the valve is closed. Last but not least, a valve make it absolutely necessary to add an anti-siphon break on the host between the manifold and the ouptput of the elbow. Otherwise, when the boat is heeled to port, water could siphone trough the block, fill the exhaust above the valve until water reaches the top of the elbow and fills the manifold. How are other Catalina 27 exhausts designed??? People who take their boat in seas larger than 4 feet must have had a similar problem and solved it... Anybody has an idea? Thanks and enjoy the rest of the season! Laurent