[ruko_ruko]

I have observed the propulsion jet nozzle on a JetSki or personal water craft is completely out of the water at plane or high speed. Why is this? What position produces the most thrust, above the water surface or below the water surface? Assume the same amount of power is applied in both cases.

[FlyYakker]

Not a jet ski expert, but to get the ski up on "the plane' at high speed, you want to have a smooth hull with no protrusions and with minimum contact with the water to limit drag. You do not want the nozzle projecting down into the water.

There is no reason to have the nozzle down in the water anyway as the thrust from the jet comes from the water being accelerated and expelled at a high rate - not due to the water jet working against the water the ski is moving in.

[ruko_ruko]

Not a jet ski expert, but to get the ski up on "the plane' at high speed, you want to have a smooth hull with no protrusions and with minimum contact with the water to limit drag. You do not want the nozzle projecting down into the water.

There is no reason to have the nozzle down in the water anyway as the thrust from the jet comes from the water being accelerated and expelled at a high rate - not due to the water jet working against the water the ski is moving in.

Thank you. I totally agree. After doing some on line research I have come to the conclusion that having the jet under the surface would actually decrease the performance of the system. The reason being it is much more difficult to push water out of the way than air. It is the reason rockets work better in the vacuum of space. According to NASA rockets are 10 to 15 percent more efficient in space because there is no air to resist the flow of gasses out the back.

[FlyYakker]

The reason being it is much more difficult to push water out of the way than air. It is the reason rockets work better in the vacuum of space. According to NASA rockets are 10 to 15 percent more efficient in space because there is no air to resist the flow of gasses out the back.

Again, not the expert, but the improvement of performance of a rocket in space is due to the ability to expand the high pressure exit gas further through use of a larger diameter nozzle cone, thereby generating more thrust due to the imbalance of forces on the added interior surface of the cone versus the exterior surfaces.

I'm pretty sure that expansion is not a factor in the jet ski, if for no other reason that water is essentially incompressible. Jet skis generate thrust via acceleration of the incoming water and forcing it out the back - the equal and opposite force generated results in the ski moving forward. Not sure back pressure really has any effect at all, and even if it did the difference on pressure is inconsequential. Certainly it is once you are at speed. I think reduced drag of the higher nozzle relative to a submerged nozzle is the main (probably only) design driver.

[ruko_ruko]

Again, not the expert, but the improvement of performance of a rocket in space is due to the ability to expand the high pressure exit gas further through use of a larger diameter nozzle cone, thereby generating more thrust due to the imbalance of forces on the added interior surface of the cone versus the exterior surfaces.

I'm pretty sure that expansion is not a factor in the jet ski, if for no other reason that water is essentially incompressible. Jet skis generate thrust via acceleration of the incoming water and forcing it out the back - the equal and opposite force generated results in the ski moving forward. Not sure back pressure really has any effect at all, and even if it did the difference on pressure is inconsequential. Certainly it is once you are at speed. I think reduced drag of the higher nozzle relative to a submerged nozzle is the main (probably only) design driver.

The NASA Q&A site says: “The shuttles main engines develop 375,000 lbs of thrust at sea level and 470,000 lbs in the vacuum of space” According to NASA rocket scientists the reason for this is there is no air in the vacuum of space to choke off and slow down the gasses flying out of the nozzles.

I think we 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. I think we also agree that back pressure in any jet system will only result in reduced thrust. I realize water is uncompressible but it certainly can exhibit back pressure.

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 was lowered beneath the water surface, maintaining the same applied horsepower, you would see a noticeable decrease in thrust available. This would result in a much slower craft speed. Not because of drag, jet nozzles can be streamlined so there is minimal drag, but because the motor will have to work much 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 of the nozzle. With the jet nozzle under the surface, the jet of water will be slowed down significantly compared to being above the surface. This is because it is being choked by the water.

You can demonstrate this principle simply with a garden hose, a five gallon pail full of water and a lockable nozzle for the hose. Put the nozzle on the hose and turn the water on and lock the nozzle open. Now hold the hose so the nozzle is about 2 or 3 feet from you with the water on and the nozzle pointing down. The thrust should lift the hose and the nozzle more or less to horizontal depending on the pressure you have. 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 now exiting the nozzle under the surface.

What happens? Most all of the thrust just disappears and the nozzle sinks into the water. Why does this happen? It happens because the water speed and volume have been reduced to a fraction of what they were and the back pressure in the hose has increased. Why? Because water is so much more dense than air and there is considerably more resistance to flow and, it is only flow rate and volume that determine thrust. In order to gain back the thrust produced in air, you would have to increase the pressure on the hose. 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 much less work required.

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