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Physics/How personal watercraft works


ruko wrote at 2007-04-23 14:04:18
Dear Expert

I’m not sure if this addition to your answer on PWC propulsion got through so I’m resending it.

After doing some basic research I must disagree with all of your conclusions on the water jet question. Based on my research I find there is no difference between rocket propulsion and water jet propulsion. Both are based on equal and opposite reaction. One uses a gas and the other uses water to accomplish thrust.

Since you mentioned a water hose with a nozzle as a possible way to demonstrate the answer, I devised a demonstration that will disprove your conclusion. This demonstration may seem ridiculous but it is very simple and the results are quite informative.

Please keep in mind that only flow rate and volume(mass) determine thrust.

What you need is a garden hose, a lockable trigger type of nozzle and a washtub full of water. Put the nozzle on the hose, lock it open and turn the water on. You should be able to feel thrust against your hand which will be much lower, of course, than a fire hose that takes three men to control. Now hold on to the hose so the nozzle is about 3 to 4 feet out horizontally from you with the water on and the nozzle pointing down. If your water pressure is high enough you should see the thrust lift the hose and the nozzle to nearly horizontal. It should be quite noticeable if you have enough water pressure. Now with the water on and still holding the hose the same way and observing the thrust, direct the nozzle into the washtub of water so the water is eventually exiting the nozzle under the surface. Please closely observe what happens to the thrust as you approach the water.

Your conclusions would show an increase in thrust even as the nozzle gets nearer to the water and when it is under the water the thrust should increase significantly. Do you see more thrust as the nozzle gets really close to the water? Even as close as an inch? No you don’t. There is no change in thrust until the nozzle actually breaks the surface of the water, and then a most interesting thing happens. As soon as the nozzle breaks the surface it sinks immediately under the surface. Most of the thrust you just had in air has disappeared!

This happens because water speed and volume have been reduced to some fraction of what they were. Water is so much more dense than air resulting in considerably more resistance to flow and remember please this overlooked and most misunderstood fact: It is only flow rate and volume that determine thrust. The only influence a denser medium, such as water compared to air, has on thrust is to reduce it. In order to gain back the thrust you had in air, you would have to increase the water pressure in the hose to get the flow rate back up to the level it was in air. This is Newton’s third law of motion in action.

Try the demo.  

NASA scientists say their rockets develop 10% more thrust in the vacuum of space as compared to sea level air pressure. Is there anything for the gases rushing out the back to push against in a vacuum? Then why is the thrust higher in a vacuum? The reason of course is there is no air to resist or impede the flow of gasses out the back. It’s pure Newton physics in the vacuum of space.

If you visit any of the web sites listed below please visit this one first:

Here is the interesting paragraph:

“Some misunderstandings that people commonly have is that the exit jet must strike the water or that the thrust will be greater if the water jet strikes the water. This is most definitely not the case and the vessel would be propelled even if the water jet exited into the air.” (Which it does in all modern jet boats of the Jetski variety)(My comments).

Web site: Read third paragraph

Web site: : Go to the heading: Principle

Visit this NASA site: (Rocket scientists)

Forth paragraph down:

“There's an important thing to remember about rockets: thrust exists whether or not there's an atmosphere. People used to say that Goddard's ideas about sending rockets into space were silly because in space there's no atmosphere "to push against". But it isn't the atmosphere that's providing the rocket's thrust, it's the flow of material from inside the rocket out to the surroundings (whatever they are) that's providing the thrust. That's why rocket engines work in space!”

Fifth paragraph down:

“I would imagine that dramatically increasing the atmospheric pressure surrounding a rocket engine might actually be detrimental by obstructing the flow of exhaust thereby forcing a completely different optimization at the launch pad than in the air or in space “ (I don’t know why this person is imagining this. It’s a fact!)

Read number 3(III): How rockets work

Web site:

3(111) Some early scientists believed that rocket exhaust needed something to push against (such as the ground or the air) in order to move the rocket. Rockets traveling in the vacuum of space, however, demonstrated that this belief was not true. In fact, rockets produce more thrust in the vacuum of space than on Earth. Air pressure reduces a rocket's thrust by about 10 percent on Earth as compared to the rocket's performance in space.

My Conclusions:

1)  Rocket propulsion is exactly the same principle as water jet propulsion, an equal and opposite reaction. The only difference is one uses gas and the other uses water. This is common sense and agreed with by NASA scientists.

2) Water jets produce more thrust in air than in water with the same horsepower applied. Demo proves it. Rocket scientists say so too. Jet boat designers found this out very early. All jetski type boats have their jet nozzles above the surface at high speed.

3) Jets or rockets do not rely on something external to “push against” to produce thrust. Rocket scientists say this. NASA says this.

4) Jets or rockets exiting into a dense medium have reduced thrust compared to exiting into a less dense medium. My water demo proves it and NASA scientists say this is true.

5) Newton’s third law of motion, action and reaction, applies to all of this and has nothing to do with the external medium the jet is operating in.

6) It is only volume(mass) and velocity of this mass that determine thrust. If either one of these are reduced by restricting flow, such as entering a denser medium, thrust and efficiency are reduced.  

jeff wrote at 2007-11-21 01:12:54
I have a different view of the way jets work, be they water jets or rockets. I've never really understood what flow rate has to do with it. What I'm saying is that any acceleration is caused by an imbalance in energy. For instance, if you draw a box on a piece of paper, and imagine the box as a compressed air tank having say 100 psi of air pressure, the air pressure is pressing equally on all the walls of the tank. This can be illustrated by drawing arrows (from the inside out)towards all four walls of the box. Because the pressure is equal in all  directions, no acceleration occurs. Now erase all or some of  the bottom line of the box, and it can be seen that there is a pressure imbalance between the top wall of the box and  the equal to the imbalance of surface area, in sq. in. multiplied by the psi.It seems to me that the flow rate is meaningless to the fundamental physics of the jet. The same scenario would apply to the garden hose illustration in a slightly different way. It seems that, though the hose has three of the four elements of the box (the open end and the two side walls), it is missing the back wall. Interestingly enough, the pressurized column of water provides, in the energy scenario, the equivalent of the back wall. Similarly when the hose is submerged in the tank, the water in the tank directly in front of the expelling water is pressurized by the force of the water, causing the water to be pressurized. This sets up the energy equivalent of a back wall, and thus closes off the "open  box" scenario, and throws the "jet" back into a state of balance. The importance of the flow rate of a rocket engine is a question of practical physics rather than the fundamentals of how the "reaction" engine works. Most of the complex mathematics of "rocket science" involve the most efficient way of expelling the exhaust gasses out of the nozzle so as to prevent a restriction from  being set up, which would cause an equal reduction in the force of the imbalance.  (For the gasses to be impeded, they have to "push" against something, and that "something" causes the acceleration of the rocket to be decreased proportionally to the decrease of the flow rate of the gasses. Thus, to focus on flow rate, is to focus on what is essentially a symptom of the problem, not the problem itself

al wrote at 2009-02-22 05:48:24
i disagree. if you sit on a raft in water and hold the hose shooting into the air, you won't move. if you put the hose under the water surface, the raft will move forward.

Brandon wrote at 2009-11-05 06:53:31
Ruko is right.. you other guys are wrong.  You have more thrust in a less dense medium such as air or in a vacuum than you do in water.  It may go against what you THINK but you can not argue with the laws of physics.  If you could somehow put wheels on a jetski and drive it on dry land but still expel water at the same volume and force it would still work the same if not better depending on the friction of the wheels compared to the friction of the craft going through water.  Look up water rockets or the Jetlev.  Thrust has nothing to do with having something to push against. If it did you couldn't have thrust in space :)

mymove wrote at 2011-10-12 02:52:44
To understand how jet-ski's or PWC's work, you need to know a little about rocket science, but you don't have to be a rocket scientist.  All you need is a grasp of Newton's 2nd and 3rd laws of motion:  Namely, "F=m*a" and "For every action, there is an equal and opposite reaction".  Without a force a jet-ski will just sit in the water and not move.  A force out the back (action) will cause a force to move the jet-ski forward (reaction).  If you accept this, you accept Newton's third law and the only thing  you need to worry about now is how to provide or maximize this "force out the back".  From Newton's second law, we know that this "force" must equal m*a (mass times acceleration).  Don't concern yourself with the numbers (quantitative analysis), just examine the relationship of the variables in the equation:  If the mass increases, the force increases.  If the acceleration increases, the force increases.  How does this translate into our jet-ski analysis?  If you want more force (action and reaction) you need to accelerate the water out of the discharge nozzle to a higher velocity (greater acceleration) or you need to pump more volume of water (more mass) or some combination of both.

How do we increase the acceleration?  If the characteristics of the pump are fixed, raising the RPM will usually raise the discharge velocity (up to a point).  How do we increase the mass (translation: how do we pump more water)?  If you look at a pump curve (a graph showing the relationship of pump pressure, flow, RPM), you will see that raising the RPM will also have the effect of pumping more water.  Pretty simple so far.

Frame of reference.

To further your understanding, let's contrive a "test cell" to actually measure the force (reaction) of a jet-ski (pump) pumping water.  Suppose the jet-ski was in a tank of water on dry land and was configured such that the water could discharge through the nozzle into the atmosphere (free air).  Also suppose we had a spring scale (like a fish weight scale) connected to the back of the jet-ski to measure the forward force.  At the inlet of the pump, the velocity of the water is nearly zero (the water in the tank and the jet-ski are at the same velocity.  Now let's rev up the pump.  The water at the inlet of the pump is still zero relative to the jet-ski (at least in the horizontal direction).  The water at the nozzle discharge, however, is some positive value (say, 100 feet per second with the throttle wide open).  The pump converted rotational energy into mass flow and accelerated the water out of the discharge nozzle.  By the way, at this point you may want to ask yourself if this same velocity could be attained if the discharge nozzle was below water instead of above.  Note that within the construct of Newton's 2nd and 3rd laws, the concept of "pushing against something" has no meaning.  Elsewhere on the internet you will read about the astronaut in outer space throwing a baseball and the forces associated with that activity.  It's all about accelerating a mass, not pushing against something.  Whoa, you say.  What about paddlewheel boats?  Certainly they won't generate as much forward thrust if the paddles are out of the water.  True, but a paddlewheel boat is not a jet drive and functions by exploiting the concept of INERTIA (see Newton's 1st law) instead of mass acceleration and the associated action/reaction.  If the paddlewheel boat is cruising at a constant speed, virtually nothing is accelerated.

I hope this helps explain the mystery jet-ski physics.  If you've gained some insight into jet-ski operation, you've also gained a little knowledge about rocket science:  It's all about action/reaction and F=ma. After all, a jet-ski is simply a rocket on water  

Problem Child wrote at 2013-07-30 17:50:39
This can also be proved by watching a jet boat running with its divert up.  The exiting water is actually being thrown up into the air not pushing against any water.


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I can help with understanding physics that does not involve eggs. I will NOT help with academic or professional questions, which are NOT limited only to homework. Please do not waste your time by asking a question that comes out of ANY kind of academic, professional, or business matters.


Have been fascinated by physical laws ever since I learned, at age seven, that magnets work under water. My study continued through college and has not ceased even after I retired.

B.A. in Physics (with honors) from University of California at Berkeley.M.A. in Physics (with honors) from University of Texas Austin.

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