When the engine is off water can enter through an open exhaust port. On multi-cylinder engines, there will almost always be a port in an open position with the engine off. When the engine is running, exhaust pressure prevents water intrusion. This is a common design on just about all marine engines, but is a much less common problem on outboards, etc., due to the different design profile of these engines and the placement of baffles.
If it were possible to change the water flow to a safer design, I would think that manufacturers would have developed it by now. The same basic design is common to all skis and has been that way for over 30 years. This might be something for a smart engineering type to look into. SeaDoo engines use a closed loop cooling system, but those skis are just as prone to the problem due to the way the exhaust system is cooled. Unless the exhaust manifold is cooled, rubber parts of the exhaust system will melt.........this happens when a hose comes off, etc..
Also keep in mind that on a ski the entire exhaust system is in an enclosed space shared by the engine and the fuel tank. You might be able to design a ski with a totally metal exhaust system that did not have a cooled exhaust, but it would get hot as hell inside the ski, and your ass would be on fire before you went too far.........not to mention the risk of your fuel tank exploding.
Early ski designs experimented with air cooling, but had many problems and eventually dropped attempts to do it that way.
If you own a ski, when running it on a hose it is always: start engine, start water, stop water, stop engine. If you remember that sequence you will never have a problem. You don't have to worry about water intrusion while the engine is running on a healthy system.
2008 Yamaha FX HO Cruiser
2010 Hurricane Sundeck 2100
Last edited by Ernest T; 07-21-2012 at 07:08 AM.