If you look at the rear of most PWC, you will see the nozzle above the waterline when on plane. This is to reduce drag. If the drive was below the water line, there would be lots of drag created by the part in the water. The jet itself points slightly down so the water jet has the water surface to react against. This down angle also helps keep the nose of the craft from flying.

First and foremost: The force that drives a jet boat forward does NOT come from the water jet pushing on or reacting to water after it leaves the nozzle. The force comes from one of Newton's laws of motion. For every action, there is an equal but opposite reaction. This law is universally misunderstood most of the time. The thrust a jet boat mechanism produces takes place at the impellers of the pump system, not outside the pump, just as rocket thrust takes place inside the combustion chamber. Rocket exhaust just turns out to be neat fireworks. Go to the “ASK NASA” web site http://www.nasa.gov/about/contact/ask_nasa_form.html and ask this question: Do rockets produce more thrust in a vacuum or in air? The answer will be in a vacuum. How can this be since there is nothing to push against in a vacuum? It is because air just chokes off the exhaust resulting in less thrust. The same thing happens with water jets only to a much greater degree because water is considerably more dense than air. I think you will agree that all modern jet boats are designed so that the propulsion water exits the jet nozzle above the surface of the water at plane speed. The reason this is so is the question I will try to answer. Thrust is determined by mass and velocity. That is, the amount of material and the speed of this material as it exits the thrust nozzle. Consequently, if the water jet nozzle is lowered beneath the water surface you will see a noticeable decrease in the performance of the system. Why you ask? Not because of drag, but because the motor will have to work harder to push water out of the way as opposed to air. Jet boat manufacturers learned this very early on. The forward speed of the boat is only determined by how much water and how fast it is sent out the back and not because of what it is pushing or reacting against when it leaves the nozzle. Rocket scientists will tell you this. With the jet nozzle under the surface, the jet of water is slowed down significantly compared to being above the surface. This is because it is being choked or slowed down by the environment it is exiting into, water. You can demonstrate this principle simply with a garden hose and a five gallon pail full of water. Put a nozzle on the hose and turn the water on. Now hold the hose so the nozzle is about 2 to 3 feet from you with the water on and the nozzle pointing down. You should see the thrust try to lift the hose and the nozzle more or less depending on the pressure. Now with the water still on and you still holding the hose the same way, direct the nozzle into the full five gallon pail of water so that the water is exiting the nozzle under the surface of the water. What happens? Does the nozzle immediately jump out of the pail because now the fast moving water has something to react to or push against besides air? No, it doesn't jump out of the pail! In fact, most of the thrust disappears and the nozzle sinks into the water! Why does this happen? It happens because the water speed and volume have been reduced to some fraction of what they were. Why? Because water is so much more dense than air and there is considerably more resistance to flow and remember, it is ONLY flow rate and volume of mass that determine thrust. In order to gain back the thrust you had in air, you would have to increase the pressure on the hose to get the flow rate back up to the level it was in air. The only way to do that is add more horsepower to the system. This is the one and only reason jet boats are designed with the jet exiting above the surface at plane speed. They go faster with less horsepower! Visit the web site http://www.nasa.gov/about/contact/ask_nasa_form.html and you can ask NASA scientists about rockets and even jet boats because the same principles apply. rukovich