Curious about Jet drives.

I saw the Swirl mentioned in the Military version.
Still would be pretty tough to make it come out correct the first time without a little R&D testing with some kind of ultrasonic testing or tiny photo devices.

But, in one night's posting I learned what I needed to know. Thanks much!

 

Assume the container a 1 meter cube, the hole 10 cm round near the bottom, so 31.4 sq,cm, are missing.
The pressure of the water is ca. 1kg/cm2 on all sides, but the back has 31.4 cm2 with zero pressure, so there is 31.4 kgf less than at the front, causing the container to move.

 

No! Aircrafts, jets drives and propellers are basically just "throwing mass". Jet drives are throwing water into air, not into water. They don't need the "column of water" to produce trust and they are much less efficient, if they throw the water into the water.

They all are basically just like oars, they accelerate water. This acceleration needs force and this force is your thrust. The less you accelerate, the better efficiency you have. Thus you like to use a big amount of water and accelerate it only slightly. This is why big propellers are more efficient and also jet drives need to have a big diameter (unless you are going very fast).

Another way of explaining it is it's like picking stones from the sea bottom and throwing them backwards. This makes you move. The bigger stones you use the lesser power you need throwing them to get the same thrust.

 

No, no no and no!!!! There is no "stacking of gasses" or "column" to push onto, whatsoever. Let's take it down to physics then, since you did pass the basics:

Regard the propulsor (be it an open propeller, a closed waterjet or an oar) a black box. Into this box we have a mass flow with the intensity M kg/sek, with a velocity of v1 m/s, equal to the vessel speed. This flow constitutes an incoming impulse, its dimension is M*v1, i.e. kg*m/s^2, which we identify as the dimension for force in Newton.

Same reasoning to the flow leaving the black box; we did not add any mass flow, but we increased its velocity to, say vj. The leaving impulse is then: M*vj Newton. The resulting thrust is the difference between incoming and leaving impulse. Irl there are unavoidable mixing and turbulence losses that destroy some energy, particularly to the incoming flow, but the equation still holds. If you look at a rocket in outer space, there is no incoming impulse, but the leaving impulse is still providing thrust.

Now to the common misinterpretations of what happens to a jet, exiting from a nozzle and into a recipient. If we have a mass flow of water leaving our jet nozzle into a surrounding stationary volume of water particles, then part of that stationary mass has to be moved away, in order to allow an outflow from the nozzle. This movement equals a pumping effort, driven by the nozzle flow. Since pumping power equals (Pressure differential)*(volume flow), and the pressure diff is proportional to vj*density/2), some of the pressure diff over the nozzle is used for this purpose, resulting in a lower total velocity leaving the nozzle.

If the water is exiting into a low-density fluid, like air, the pumping power used for "making room" for the jet flow is reduced by a factor of about 800, i.e. proportional to the density ratio, meaning that there is a practically uncompromized pressure differential over the nozzle, giving full flow and maximum impulse difference, read maximum thrust.

In addition to the phenomena directly linked to the impulse change of the driving fluid, the flow into, and around the black box will influence the pressure- and velocityfield around the hull itself, but that is a consequence of the propulsor flow.

So this change of impulse is what it is all about, no matter what is inside the black box; there is no column of fluid "to stand on". If we want to delve deeper into the details of propulsion, we must first consider the pumping power necessary to create the basic velocity increase to the mass flow. Again, this is the product of (Pressure difference)*(volume flow), or ((vj^2-v1^2)*density/2)*M/density). When going into real life, the relevant losses in all stages have to be considered, but for clarity, they are left out here.

The theoretical impulse efficiency (regardless of the fluid-mechanical pumping efficiencies of the mechanism inside the black box) is the effective power divided with the pumping power required: v1*thrust/((Pressure diff)*(volume flow)) After some arithmetics, you find that the impulse efficiency is: (2*v1/(vj+v1)). Note that this is NOT the total propulsion efficiency; it just describes how the "global outflow mixing losses" vary with velocity ratio; as you see, this efficiency is increasing with reduced velocity increase. But to keep the impulse (thrust) constant, that means that the mass flow has to be increased. Ergo: high impulse efficiency comes with high flow, low velocity increase, and that goes for any of the possible paraphernalia inside the black box!

 

Yah, so there!

-Tom

 

In my defence the column of water I was referring to in my post about the stator does exist ,in that it is the way in which the "thrown mass" manifests itself behind a jet unit. In my case a column about 6 inches diameter and varying in length with engine revolutions .Given the previous post "column"may have been an unfortunate choice of word.

 

You guys are certainly empowered to speak on the Subject.
At first I was confused, then I thought I understood, and now I'm not sure of anything....
Being snowed in four days is affecting my reasoning.

 

Ya asked for it, didn't ya........!

 

Here is video and details from Hamiltom jet.
http://www.hamjet.co.nz/hamiltonjet_waterjet/waterjet_working

 

Your right Baeck, and I had enough of that deep thinking to put me to sleep for almost 8 hours.
An old friend said: "When I work, I work hard. and When I play, I play Hard and When I think, I just go to sleep!"

Believe it or not, even with all that info I have questions. Amazing eh?

Rivrlivin, that was a pretty good movie. It the Guy drawing all the bubbles would have been a bit less prolific, it'd have been better. Person pretty good at PhotoShop can take those bubbles out.

We got a little more snow last night. When this starts to melt, this is gonna be a Soggy place for a week or two. I'll be a wreck, stuck inside for almost a week.

 

....he who sleeps does not sin......!

Anyway, you are welcome with your questions. Seems we are in a comparable climate zone, though; new powdersnow overnight in addition to the half meter we got before. Can't stay inside, the dogs have to come out; they keep me sane (yep, everything is relative...). They also provide an acceptable escape from some of the "must do's".

 

Dont you have a Volvo with Abba tapes in it for the long cold nights?

 

Hey Frosty ! Long time no smell eh. Beeeg wet hello kiss for you there mate You still have those two Yanmar anchors on the boat ?

Mudsucker... I mean Pudthucker I mean Thudpucker the jet drive thingy is really very simple.

Water gets sucked from below and pumped out the rear. It gets sucked from the bottom because it has least effect opposing the movement of the vessel and fortunatelely the water is usually down below too (you can check). The vessel hardly knows it gets sucked down.

It gets squirted out the back above the water for the most drive, not in the water. Do the hose pipe in the pool or bucket thing. Full blast open tap the force is rather small if the hose end is submerged. As soon as the hose end is pulled from the water you can feel the force increasing substantially. Don't waste water and don't play with the hose too long.

That's probably why loo's are so small, so us guys doesn't get propelled over backwards taking a leak... the door stops you. Fortunately also the forward weight helps us to keep a bit of balance and there's our big feet too. If you push the end in the water you'll be falling over forward. Oh well, try and see...

Wiemen on the other hand have small feet, but their propulsion would be up and not backwards. Eeeesh, what an explanation.

A rough thumb suck estimate of propulsion would be the boat's weight vs the amount of water moved within a period of time. The more water in the shortest span of time is the faster one. If you accellerate the water then the propulsion speed or pushing force increases - but there are limits - the oomph still comes from moving loooots of water asap.

The inside of the drive has a trick, almost like a pop in a prop. I think the stationary one acts like a non return valve when the water is at speed and keeps the water from trying to spin with the prop, so it gets forced out and not turbulated out. So if you increase the power you just get more propulsion power.

You can use a normal water pump as a jet drive.

 

Thanks Fanie.
All you guys seem to under stand the Physics of the drive. I think I do now as well.

20 years ago, almost 30 now, I saw my first jet drive at a boat show. Cutaway (Single Stage) actual size I turned that shaft with my hand and thought to myself: "that's just a water pump. Buy why's that Stator there?"

He had a 3 stage in cutaway too.
I wondered..... if a single had the impeller blades formed to make the water go down the Stator at a certain pitch and rate, wouldn't a 3 stage have to aligned so the water is going through at almost a seamless flow to each stage would have to be adjusted to that end?
Other wise it's just Three water pumps end to end to end. High speed confusion!
But a three stage, tuned well, would make one heckuva power washer!

So I learned a lot from you guys. Thanks so very much.

 

If the stator you refer to is the stationary blade I think without it the vessel would tilt if you rev it up - like the old Fords used to do when you rev it up at a robot when the chicks walks by. Always made me think of a dog lifting it's leg by a tree them Fords