Newton's Law and Jetskis

[ruko]

I have a question about jet boats or personal water craft or jetskis or what ever you like to call them. It concerns the water jet propulsion unit in the back of the boat from which water exits as a jet stream and moves the boat forward. One of Newton’s laws at work here I believe. My question is: With an equal amount of power supplied to the system, forget about drag etc, is it better to have the water jet propulsion unit above the surface of the water or below it? Another way to put this is: Do you get more measurable thrust with the same amount of power applied with the jet above the water surface or below the water surface?

[Buffy]

Now this is actually a very interesting question. Most people have a hard time with "every action has an equal and opposite reaction" because in space, there's nothing to push against. Well in this instance, the real power providing accelleration comes from pushing against something! You'll definitely get some pure action reaction power out of this, but pushing the water out the back builds up pressure against the water thats behind it, thus pushing the ski forward. Since water has greater density than air, you'll go faster if the water is pushed back into the water than if it was spit out into the air above the water line (you'd still go forward though more than would be accounted by the pure inertial forces because of the fact that the air also forms a pressure buildup). On the other hand, there's so much friction created by having to plow through the water (and the air), that trying to figure this out becomes quite a bit more complicated than the pure F=m*a that you'd be able to do lobbing a space probe to Saturn....

 

Cheers,

Buffy

[alxian]

***applause***

 

linear induction is a similar process

 

The generation of electromotive force in a closed circuit by a varying magnetic flux through the circuit.

 

replace eletric and magnetic with water

 

notice when the craft leaves the water it loses that solid connection with it power source (the water in)

 

then again hydro planing craft require the same drive, water-water but keep the hull from getting all the friction from the bow being in the water.. thus jetskis over come that friction easily but would perform better out of water,the drive requires water-water though as a more dense medium to push against directly as buffy said.

 

i believe there are hydroplaning personal water craft if yu really want that extra performance taking the bow out of the water.

[ruko]

I did some research on this question. Asked a scientist at NASA and I have been enlightened about this subject. My boating friends perhaps will believe, perhaps not. I have devised a simple demonstration to prove some of the following points if anyone is interested.

 

First question I asked:

ASK A NASA SCIENTIST

QUESTION:

Dear NASA Scientists

Why do you say your rockets develop 10% more thrust in the vacuum of space as compared to sea level air pressure?

Thanks.

Ruko

 

ANSWER from Bruce Thompson on Feb 5, 2005:

Good afternoon, Ruko,

A rocket works more efficiently, and develops more thrust, in a vacuum, because the flame roaring from the nozzle doesn't have to expend any energy pushing the air out of the way.

 

It is a common misconception that a rocket or jet must "push" against something in order to work or to work better. In fact, it doesn't. The rocket moves by the reaction to the flame expelled from its nozzle - the flame goes one way, making the rocket move in the opposite direction - look up Newton's Third Law. This is why it works so well in the vacuum of space.

 

All the work - the thrust - is created inside the rocket's combustion chamber and nozzle, and once the flame leaves the nozzle, it does nothing more than make spectacular fireworks. However, the flame loses some of its efficiency if it has to push air out of the way on exiting the nozzle. The less air around the rocket's nozzle, the more efficiently it works which means more thrust.

 

Second question I asked:

 

NQ004309 Thrust in the Vacuum of Space

QUESTION:

Dear Mr. Thompson

 

Thank you very much for answering my question about thrust. Would this then also apply to jet skis or personal water craft that use a jet of water for propulsion? Could you say the overall efficiency of the jet of propulsion water would be greater if it exited the nozzle above the surface as opposed to under the surface? It is obviously much easier to move air out of the way compared to water. Are these correct statements? My boating friends do not agree with this.

 

As I now understand it, jet boat thrust is determined by volume and velocity of water out of the nozzle. If the jet nozzle is below the surface the water would definitely resist this velocity and volume compared to air. In other words you would have to raise the horsepower of the system to get the same thrust with the nozzle under the water as compared to over it. True?

 

Thanks.

 

Ruko

 

ANSWER from Bruce Thompson on Feb 5, 2005:

True!

A boat's jet unit works more efficiently if the discharge nozzle is above water surface. The actual thrust/propulsion is created at the trust faces of the impeller blades and the water needs an unobstructed passage from there. If the discharge nozzle is submerged, the surrounding water will choke the discharge, in the same way that sea-level atmospheric density chokes a rocket nozzle's discharge, and the boat will not perform as well.

 

John Hamilton - the designer of the jet boat unit - discovered this very quickly, which is why small jet boats are planing hulls. This hull allows the water jet, at planing speed, to be above the water surface where it is the most efficient and produces the most thrust with available horsepower.

 

Bruce

 

"If you can't explain something to your Grandmother so that she understands it, you don't understand it yourself"

[Buffy]

Wow, that's facinating. What it sounds like is that the work is done by the sucking (err, lift on those little airfoils in the jet) rather than the spitting! I can still see why sucking water would produce more thrust that sucking air though, since the density is higher and the mass moved is greater (you need a bigger prop/turbine using only air).

 

It begs the question though, how come all "jet" boats don't spit the water out above the waterline into the air? I'm not sure I've ever seen one that does... Can you describe your demonstration?

 

Cheers,

Buffy

[ruko]

Wow, that's facinating. What it sounds like is that the work is done by the sucking (err, lift on those little airfoils in the jet) rather than the spitting! I can still see why sucking water would produce more thrust that sucking air though, since the density is higher and the mass moved is greater (you need a bigger prop/turbine using only air).

 

It begs the question though, how come all "jet" boats don't spit the water out above the waterline into the air? I'm not sure I've ever seen one that does... Can you describe your demonstration?

 

Cheers,

Buffy

 

You ask why all the jet boats you have seen have the jet nozzle under the water. At rest or just floating the jet nozzle is submerged. However at plane speed, perhaps as low as 15 mph, the jet nozzle is mostly above the water surface. These boats are called planeing hulls. It is not easy to see this if you are observing from shore. It is easy to see if you are along side and slightly behind a jet boat at 30 mph or so. All modern personal water craft jet boats operate this way.

 

This is the seemingly ridiculous but straight forward demo I dreamed up to demonstrate thrust change. It works!

 

Materials needed: A garden hose, a lockable trigger pistol type nozzle and a tub of water. There should be at least a foot of water in the tub. The lock-on type nozzle has to be able to produce a fairly heavy fast stream, not a spray. The cheap ones work the best for this, not the ones with the various spray patterns and bells/whistles.

 

Put the nozzle on the hose, lock it on and turn the water on. You should have noticeable thrust present. Now hold on to the hose so the nozzle is about 2 to 3 feet out 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 almost horizontal. City water pressure of 40 PSI is very adequate for this demo. Now with the water on and you still holding the hose the same way and observing the thrust, direct the nozzle into the washtub of water so that the water is now exiting the nozzle under the surface. While you are doing this pay close attention to what happens as the nozzle goes under the surface.

 

What happened? Do you see an increase in thrust as the nozzle end dips below the surface into the denser medium of water? No, you don’t. As soon as the nozzle breaks the surface and goes under, most of the thrust you had in air just disappears and the nozzle sinks rapidly into the water!

 

Why does this happen? It happens because the water speed and volume from the nozzle 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. Please remember 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 pressure on the hose to get the flow rate back up to the level it was in air. The only way to do this of course is to add more pressure/horsepower to the system.

[Buffy]

Now that sounds like a great science fair experiment formy kid! Thanks ruko!

 

I guess my question that obviously planing hulls will get the jet out of the water at speed. I would think that the principle applies when starting or at low speed too: if this is the case, I'd think it would be more efficient (and gee whiz more plain fun with all the spraying water!) to direct the exaust nozzle to a point above the waterline at rest. How come no one seems to do that?

 

Cheers,

Buffy

[ruko]

Now that sounds like a great science fair experiment formy kid! Thanks ruko!

 

I guess my question that obviously planing hulls will get the jet out of the water at speed. I would think that the principle applies when starting or at low speed too: if this is the case, I'd think it would be more efficient (and gee whiz more plain fun with all the spraying water!) to direct the exaust nozzle to a point above the waterline at rest. How come no one seems to do that?

 

Cheers,

Buffy

 

Glad you found the demo interesting.

 

Spraying water water out the back and efficency at low speed:

The Sea-Do RXP has 215 HP and can reach 30 mph in 1.8 seconds. This is above it's plane speed so there is plenty of water blasting/spraying out the back probably less than one second after the throttle is opened. Yes it certainly would be more efficent to start with the nozzle above the surface but with 215 HP on tap I don't think you would notice. The 0 to 60 time for this craft is 5.5 seconds which is almost as fast as the new Pontiac GTO. Must be a blast! I don't know exactly why the boats are designed or built with the jet nozzle submerged at rest but I have an idea. It must have something to do with thrust line and hull design. Some boats do sit a little higher in the water and there is a portion of the nozzle showing, but most don't.

 

Ruko

[Turtle]

Excellent. I have an addition you may ask Bruce. To the outlet nozzle, add a chamber 14" long that is packed with straws(tubes). The chamber shall be the larger in area than the normal outlet nozzle. (A reverse venturi) The packing of tubes begins several inches forward of the inlet & let's say they are 1/4" tubes to get a sense of scale with a real jet boat. Further in the tube chamber, say 10", the tubes terminate, leaving a cavity if you will & the exit from the chamber is a round hole the same area the as the normal outlet. This round hole is in a flat plate, which is the back of the chamber.

What I am describing is a laminar flow nozzle, & I believe this idea is orignal to me to put it on a boat. I also think it would work on a fire extenguisher. I have built such chambers with straws & hose; this is the way they do those jumping fountain displays.

Hey, if you got a jetski, go ahead & try this out. All that patent stuff be darned. Go for it!

[alxian]

All the work - the thrust - is created inside the rocket's combustion chamber and nozzle, and once the flame leaves the nozzle, it does nothing more than make spectacular fireworks. However, the flame loses some of its efficiency if it has to push air out of the way on exiting the nozzle. The less air around the rocket's nozzle, the more efficiently it works which means more thrust.

 

d'uh no bow wake.. puts index parallel to lips and vibrates.. making stupid noise. sometimes context is more important than the facts and setting of the example...

 

A boat's jet unit works more efficiently if the discharge nozzle is above water surface. The actual thrust/propulsion is created at the trust faces of the impeller blades and the water needs an unobstructed passage from there. If the discharge nozzle is submerged, the surrounding water will choke the discharge, in the same way that sea-level atmospheric density chokes a rocket nozzle's discharge, and the boat will not perform as well.

 

this is misleading. as our answer of yes the thrust must occur in the water was right, but he says the water jet should be above the water, think speed boats and the huge rooster tail.

[ruko]

All the work - the thrust - is created inside the rocket's combustion chamber and nozzle, and once the flame leaves the nozzle, it does nothing more than make spectacular fireworks. However, the flame loses some of its efficiency if it has to push air out of the way on exiting the nozzle. The less air around the rocket's nozzle, the more efficiently it works which means more thrust.

 

d'uh no bow wake.. puts index parallel to lips and vibrates.. making stupid noise. sometimes context is more important than the facts and setting of the example...

 

A boat's jet unit works more efficiently if the discharge nozzle is above water surface. The actual thrust/propulsion is created at the trust faces of the impeller blades and the water needs an unobstructed passage from there. If the discharge nozzle is submerged, the surrounding water will choke the discharge, in the same way that sea-level atmospheric density chokes a rocket nozzle's discharge, and the boat will not perform as well.

 

this is misleading. as our answer of yes the thrust must occur in the water was right, but he says the water jet should be above the water, think speed boats and the huge rooster tail.

 

 

No it's not misleading, just a little hard to understand. I think you will get it though. It makes perfect sense to me and to many scientists and especially considering NASA’s remarks about thrust and what a denser medium does to choke off thrust.

 

The answer is no, the thrust must occur above the water for the highest efficiency. Please read on.

 

Think rooster tail? What does a rooster tail have to do with this? Rooster tails come from prop boats I believe. There is a difference between jet boats and prop boats. A prop “screws” through the water, that’s why it is called a screw in ships and a jet boat uses action and equal and opposite reaction of a jet of water to move forward.

 

Try the seemingly ridiculous water tub demo explained in previous submissions to this thread and you will see for your own eyes how thrust from a jet is choked or reduced under water as the NASA scientist says it is. And of course the inventor of the jet boat should be considered too. He discovered very quickly the advantages of over the water jet propulsion. He very quickly found out you need less horsepower to move air out of the way compared to water. Also you might want to look at the back of a jet boat while traveling at above plane speed. You will see the jet is completely out of the water for reasons discussed throughout this whole thread.

 

Rocket or jet thrust is not produced by pushing against anything outside of the output nozzle, hence why rockets produce more thrust in the vacuum of space(according to NASA). There is no air to push out of the way as the gasses leave the nozzle. Newton’s law of equal and opposite reaction is usually highly misunderstood. It is volume and velocity combined that determine thrust, nothing else.

 

Lucky, or maybe not so lucky, for us there are scientists and engineers who truly understand this concept. We might never have gotten to the moon or have those great GPS satellites if there wasn’t. Of course, we also wouldn’t have ICBMs.